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CHAPTER VIII
INITIAL NUCLEAR RADIATION
NATURE OF NUCLEAR RADIATIONS
NEUTRONS AND GAMMA RAYS ries, namely, initial and residual\\\(§ 1.02). The line of demarcation is
8.01 As stated in Chapter I, one of somewhat arbitrary, but it may be takenthe special features of a nuclear explo- as about 1 minute after the explosion,sion is the emission of nuclear radia- for the reasons given in § 2.43. Thetions. These radiations, which are quite initial nuclear radiation, with which thedifferent from the thermal radiation dis- present chapter will be concerned, con-cussed in the preceding chapter, consist sequently refers to the radiation emittedof gamma rays, neutrons, beta particles, within I minute of the detonation. Forand a small proportion of alpha parti- underground or underwater explosions,cles. Most of the neutrons and part of it is less meaningful to separate thethe gamma rays are emitted in the fis- initial from the residual nuclear radia-sion and fusion reactions, i.e., simul- tion \\\(§ 2.82, 2.100), although the dis-taneously with the explosion. The re- tinction may be made if desired.mainder of the gamma rays are 8.03 The ranges of alpha and betaproduced in various secondary nuclear particles are comparatively short andprocesses, including decay of the fission they cannot reach the surface of theproducts. The beta particles are also earth from an air burst. Even when theemitted as the fission products decay. fireball touches the ground, the alphaSome of the alpha particles result from and beta particles are not very impor-the normal radioactive decay of the ura- tanto The initial nuclear radiation maynium or plutonium which has escaped thus be regarded as consisting only offission in the weapon, and others (he- the gamma rays and neutrons producedlium nuclei) are formed in fusion reac- during a period of 1 minute after thetions \\\(§ 1.69). nuclear explosion. Both of these nuclear
8.02 Because of the nature of the radiations, although different in charac-phenomena associated with a nuclear ter, can travel considerable distancesexplosion, either in the air or near the through the air. Further, both gammasurface, it is convenient, for practical rays and neutrons can produce harmfulpurposes, to consider the nuclear radia- effects in livin.g organisms (see Chapter Itions as being divided into two catego- XII). It is the highly injurious nature of
,
324 !
NATURE OF NUCLEAR RADIATIONS 325
these nuclear radiations, combined with not true for gamma rays and neutrons.their long range, that makes them such a For example, at a distance of I milesignificant aspect of nuclear explosions. from a I-megaton explosion, the initialThe energy of the initial gamma rays nuclear radiation would probably proveand neutrons is only about 3 percent of fatal to a large proportion of exposedthe total explosion energy, compared human beings even if surrounded by 24with some 35 to 45 percent appearing as inches of concrete; however, a muchthermal radiation in an air burst, but the lighter shield would provide completenuclear radiations can cause a consider- protection from thermal radiation at theable proportion of the casualties. Nu- same location. The problems of shield-clear radiat~on can also damage certain ing from thermal and nuclear radiationselectronic equipment, as will be seen are thus quite distinct.later in this chapter. 8.06 The effective injury ranges of
8.04 Most of the gamma rays ac- these two kinds of nuclear weapon radi-companing the actual fission process are ations may also differ widely. For ex-absorbed by the weapon materials and plosions of moderate and large energyare thereby converted into other forms yields, thermal radiation can haveof energy. Thus, only a small proportion harmful consequences at appreciably(about I percent) of this gamma radia- greater distances than can the initial nu-tion succeeds in penetrating any dis- clear radiation. Beyond about I1f4 miles,tance from the exploding weapon, but the initial nuclear radiation from a 20-there are several other sources of kiloton air burst, for instance, would notgamma radiation that contribute to the cause observable injury even withoutinitial nuclear radiation. Similarly, protective shielding. However, expo-many of the neutrons produced in fis- sure to thermal radiation at this distancesion and fusion reactions \(§ 1.69) are could produce serious skin bums. Onreduced in energy and captured by the the other hand, when the energy of theweapon residues or by the air through nuclear explosion is relatively small,which they travel. Nevertheless, a suf- e.g., a few kilotons, the initial nuclearficient number of high-energy neutrons radiation has the greater effective range.escape from the explosion region to 8.07 In the discussion of therepresent a significant hazard at consid- characteristics of the initial nuclear ra-erable distances away. diation, it is desirable to consider the
neutrons and the gamma rays sepa-COMPARISON OF NUCLEAR WEAPON rately. Although their ultimate effectsRADIATIONS on living organisms are much the same,
the two kinds of nuclear radiations differ8.05 Although shielding from ther- in many respects. The subject of gamma
mal radiation at distances not too close rays will be considered in the sectionto the point of the explosion of a nuclear which follows, and neutrons will beweapon is a fairly simple matter, this is discussed in § 8.49 et seq.
326 INITIAL NUCLEAR RADIATION
GAMMA RAYS
SOURCES OF GAMMA RAYS excess energy as gamma rays. This typeof interaction of a fast neutron with a
8.08 In addition to the gamma rays nucleus is called "inelastic scattering"that actually accompany the fission and the accompanying radiations are re-process, contributions to the initial nu- ferred to as "inelastic scattering gammaclear radiations are made by gamma rays." I The fast neutrons produced
rays from other sources. Of the neutrons during the fission and fusion reactionsproduced in fission, some serve to sus- can undergo inelastic scattering reac-tain the fission chain reaction, others tions with atomic nuclei in the air asescape, and a large number are inevit- well as with nuclei of weapon materials.ably captured by nonfissionable nuclei. 8.10 During the fission process,Similar interactions occur for the neu- certain of the fission products andtrons produced by fusion. As a result of weapon products are formed asneutron capture, the nucleus is con- isomers.2 Some of the isomers decayverted into a new species known as a initially by emitting a gamma ray. This"compound nucleus," which is in a is generally followed by emission of ahigh-energy (or excited) state. The ex- beta particle that mayor may not becess energy may then be emitted, almost accompanied by additional gamma rays.instantaneously, as gamma radiations. The initial gamma rays emitted by suchThese are called' 'capture gamma isomers may be considered an indepen-rays," because they are the result of the dent source of gamma rays. Thosecapture of a neutron by a nucleus. The gamma rays that may be emitted sub-process is correspondingly referred to as sequently are generally considered to be"radiative capture." part of the fission product decay.
8.09 The interaction of weapon 8.11 Neutrons produced during theneutrons with certain atomic nuclei pro- fission and fusion processes can undergovides another source of gamma rays. radiative capture reactions with nucleiWhen a "fast" neutron, i.e., one hav- of nitrogen in the surrounding atmos- .
ing a large amount of kinetic energy, phere as well as with nuclei of variouscollides with such a nucleus, the neutron materials present in the weapon. Thesemay transfer some of its energy to the reactions are accompanied by (secon-nucleus, leaving the latter in an excited dary) gamma rays which form part of(high-energy) state. The excited nucleus the initial nuclear radiation. The in-can then return to its normal energy (or teraction with nitrogen nuclei is of par-ground) state by the emission of the ticular importance, since some of the
I The term" scattering" (cf. § 7. 10) is used because, after interacting with the nucleus. the neutron of
lower energy generally moves off in a direction different from that in which the original neutron wastraveling before the collision.
'In an isomer of a particular nuclear species the nuclei are in a high-energy (or excited) state with anappreciable half-life. The isomers of interest here are those that decay rapidly, with a half-life of aboutone thousandth of a second or less. by the emission of the excess (or excitation) energy as gammaradiation.
GAMMA RAYS 327
gamma rays thereby prQduced have very hypothetical nuclear weapon is shown inhigh energies and are, consequently, Fig. 8.14. The energy rate is expressedmuch less readily attenuated than the in terms of million electron voltsother components of the initial gamma (§ 1.43) per second per kiloton of ex-radiation. plosion energy. The gamma rays that
8.12 The gamma rays produced result from neutron capture in nitrogenduring fission and as a result of neutron occur at late times relative to some ofinteractions with weapon materials form the other sources because the probabilitya pulse of extremely short duration, of capture is much greater for low-en-much less than a microsecond (§ 1.54 ergy neutrons, i.e., those that have lostfootnote). For this reason, the radiations energy by multiple scattering reactions.from these sources are known as the The dashed lines in Fig. 8.14 show the"prompt" or "instantaneous" gamma gamma-ray source as it would exist in arays. vacuum, e.g., from an explosion above
8.13 The fission fragments and the normal atmosphere. The gammamany of their decay products are radio- rays that result from inelastic scatteringactive species, i.e., radionuclides of neutrons by nuclei of air atoms and(§ 1.30), which emit gamma radiations capture in nitrogen would be absent(see Chapter I). The half-lives of these from such an explosion.radioactive species range from a fraction 8.15 The instantaneous gamma raysof a second to many years. Neverthe- and the portion of the delayed gammaless, since the decay of the fission frag- rays included in the initial radiation arements commences at the instant of fis- produced in nearly equal amounts, butsion and since, in fact, their rate of they are by no me~s equal fractions ofdecay is greatest at the beginning, there the initial nuclear radiation escapingwill be an appreciable liberation of from the exploding weapon. The in-gamma radiation from these radionu- stantaneous gamma rays are producedclides during the first minute after the almost entirely before the weapon hasexplosion. In other words, the gamma completely blown apart. They are,rays emitted by the fission products therefore, strongly absorbed by themake a significant contribution to the dense weapon materials, and only ~initial nuclear radiation. However, since small proportion actually emerges. Thethe radioactive decay process is a con- delayed gamma rays, on the other hand,tinuing (or gradual) one, spread over a are mostly emitted at a later stage in theperiod of time which is long compared explosion, after the weapon materialsto that in which the instantaneous radia- have vaporized and expanded to form ation is produced, the resulting gamma tenuous gas. These radiations thus suf-radiations, together with part of the fer little or no absorption before emerg-gamma radiations that arise from initial ing into the air. The net result is that, atisomeric decays and interactions of a distance from an air (or surface) burst,neutrons with nuclei of the air, are re- the delayed gamma rays, together withferred to as "delayed" gamma rays. those produced by the radiative capture
8.14 The calculated time depen- of neutrons by the nitrogen in the at-dence of the gamma-ray output of a mosphere, contribute about a hundred
328 INITIAL NUCLEAR RADIATION
1030
1029
1028
27 \10 \ INELASTIC SCATTERING OF NEUTRONS
\ BY NUCLEI OF AIR ATOMS
\1026 \
\t::: '--~ --" 1025~ DECAYSVI"i:s 024~ 1
uJ NEUTRON CAPTUREI- 23 IN NITROGENcX 10a:>-~5 1022ZuJ
1021
1020
1019
1018
1017
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 102
TIME (SEC)
Figure 8.14. Calculated time dependence of the gamma-ray energy output per kilotonenergy yield from a hypothetical nuclear explosion. The dashed line refers to
an explosion at very high altitude.
r
GAMMA RAYS 329
times more energy than the prompt passage through matter, as described ingamma rays to the total nuclear radia- § 1.38. In simple terms, a roentgen istion received during the first minute the quantity of gamma radiation (or Xafter detonation (§ 8.47). rays) that will give rise to the formation
8.16 There is another possible of 2.08 x 109 ion pairs per cubic centi-source of gamma rays which may be meter of dry air at S.T.P., i.e., atmentioned. If a nuclear explosion standard temperature (O°C) and pressureoccurs near the earth's surface, the (1 atmosphere). This is equivalent to theemitted neutrons can cause what is release of about 88 ergs of energy whencalled "induced radioactivity" in the 1 gram of dry air under S.T.P. condi-materials present in the ground (or tions is exposed to I roentgen of gammawater). This may be accompanied by the radiation. 3
emission of gamma rays which will 8.18 The roentgen is a measure ofcommence at the time of the explosion exposure to gamma rays (or X rays).and will continue thereafter. However, The effect on a biological system, suchexcept near ground zero, where the in- as the whole body or a particular organ,tensity of gamma rays from other or on a material, e.g., in electronicsources is very high in any event, the equipment, however, is related to thecontribution of induced radioactivity to amount of energy absorbed as a result ofthe initial gamma radiation is small. exposure to radiation. The unit of en-Consequently, the radioactivity induced ergy absorption, which applies to allin the earth's surface by neutrons will be kinds of nuclear radiations, includingtreated in the next chapter as an aspect alpha and beta particles and neutrons asof the residual nuclear radiation (§ 9.31 well as gamma rays, is the "rad." Theet. seq.). rad represents the deposition of 100 ergs
of radiation energy per gram of the ab-RADIATION DOSE AND DOSE RATE sorbing material. In stating the quantity
(or dose) of a particular radiation in8.17 Gamma rays are electromag- rads, the absorbing material must be
netic radiations analogous to X rays, specified since the extent of energy de- .but, generally of shorter wave length or position depends on the nature of thehigher photon energy (§ 1.74). A mea- material. In tissue at or near the surfacesurement unit that is used specifically of the body, the gamma (or X-ray) ex-for gamma rays (and X rays) is called posure of 1 roentgen results in an ab-the "roentgen." It is based on the abil- sorption of approximately 1 rad,4 butityof these radiations to cause ioniza- this rough equivalence does not neces-tion and produce ion pairs, i.e., sepa- sarily apply to other materials. Further-rated electrons and positive ions, in their more, the relationship does not hold for
, According to the official definition, I roentgen produces electrons (in ion pairs) with a total charge of258 x 10-- coulomb in I kilogram of dry air.
-The rough equivalence between a gamma (or X-ray) exposure of I roentgen and the absorption inbody tissue of I rad holds for photons of intermediate energies (0.3 to 3 MeV). For photon energiesoutside the range from 0.3 to 3 MeV, the exposure in roentgens is no longer simply related to the
absorption in rads.
330 INITIAL NUCLEAR RADIATION
absorption in tissue in the interior of the for the detection and measurement ofbody. However, in describing the bio- various nuclear radiations. Some of thelogical effects of nuclear radiations in instruments described below respond tothis book, the energy absorption (in neutrons (to a certain extent) as well asrads) refers to that in tissue at (or close to gamma rays. For gamma-ray mea-to) the body surface nearest to the ex- surement, the instrument would have toplosion (§ 12.108). be shielded from neutrons. The basic
8.19 There are two basic types of operating principles of the instrumentsnuclear radiation measurement both of are described below and their use forwhich are important for biological ef- determining either doses or dose rates isfects and damage to materials. One is indicated in §§ 8.29, 8.30.the total "exposure" in roentgens of 8.21 Normally a gas will not con-gamma rays or the total absorbed duct electricity to any appreciable ex-"dose" in rads of any radiation accu- tent, but as a result of the formation ofmulated over a period of time. The other ion pairs, by the passage of nuclear (oris the "exposure rate" or the "dose ionizing) radiations, e.g., alpha parti-rate", respectively; the rate is the ex- cles, beta particles or gamma rays, theposure or the absorbed dose received gas becomes a reasonably good con-per unit time. Exposure rates may be ductor. Several types of ionization in-expressed in roentgens per hour or, for struments, e.g., the Geiger counter andlower rates, in milliroentgens per hour, the pocket chamber (or dosimeter), forwhere 1 milliroentgen is one thousandth the measurement of gamma (and other)part of a roentgen. Absorbed dose rates radiations, are based on the formation ofcan be given correspondingly in rads per electrically charged ion pairs in a gashour or millirads per hour. In connec- and its consequent ability to conducttion with damage to electronic equip- electricity.ment, the exposure rates are generally 8.22 Semiconductor (solid-state)stated in roentgens per second and the detectors depend on ionization in a solidabsorbed dose rates in rads per second. rather than in a gas. These detectors
consist of three regions: one is the n (forMEASUREMENT OF GAMMA negative) region, so called because itRADIATION has an excess of electrons available for
conducting electricity, the second is the8.20 Thermal radiation from a nu- p (for positive) region which has a defi-
clear explosion can be felt (as heat), and ciencyof such electrons, and the third isthe portion in the visible region of the neutral. In the detector, the neutral re-spectrum can be seen as light. The gion is located between the nand phuman senses, however, do not respond regions. A voltage from a battery isto nuclear radiations except at very high applied across the detector to balanceintensities (or dose rates), when itching the normal difference of potential be-and tingling of the skin are experienced. tween the outer regions and there is noSpecial instrumental methods, based on net flow of current. When exposed tothe interaction of these radiations with nuclear radiation, ionization occurs inmatter, have therefore been developed the neutral region and there is a pulse of
GAMMA RAYS 331
current proportional to the radiation in- stance commonly used in radiation do-tensity. Semiconductor detectors for simeters is lithium fluoride containing aoperation at normal temperature are small quantity of manganese. The totalmade of silicon which is either pure light emission from radio-photolumine-(neutral region) or contains regulated scent and thermoluminescent dosimetersamounts of impurities, e.g., arsenic or is a measure of the absorbed dose in theantimony (n region) or boron or ,alumi- sensitive material.num (p region). 8.25 In most materials, the energy
8.23 Another type of interaction of of the absorbed radiation ultimately ap-nuclear radiations with matter, either pears in the form of heat. Thus, the heatsolid, liquid, or gas, called "excita- generated by the passage of radiation istion," is also used in radiation mea- a measure of the absorbed dose. Thissurement. Instead of the electron being fact is utilized in a special calorimeterremoved completely from an atom, as it dosimeter consisting of a thin sample ofis in ionization, it acquires an additional absorbing material. The energy depo-amount of energy. As a result, the atom sited by the radiation can then be deter;.is converted into a high-energy (or ex- mined from the measured temperaturecited) electronic state. When an atom rise and the known heat capacity of this(or molecule) becomes electronically material.excited, it will generally give off the 8.26 Indirect effects of nuclear ra-excess (or excitation) energy within diations, notably chemical changes,about one-millionth of a second. Certain have also been used for measurementmaterials, usually in the solid or liquid purposes. One example is the blacken-state, are able to lose their electronic ing (or fogging) of photographic filmexcitation energy in the form of visible which appears after it is developed. Filmflashes of light or scintillations. In scin- badges for the measurement of nucleartillation detectors, the scintillations are radiations generally contain two or threecounted by means of a photomultiplier pieces of film, similar to those used bytube and associated electronic devices. dentists for taking X rays. They are
8.24 In radio-photoluminescent do- wrapped in paper (or other thin material)simeters, irradiation produces stable which is opaque to light but is readilyfluorescence centers which can be sti- penetrated by gamma rays. The films aremulated by subsequent ultraviolet illu- developed and the degree of foggingmination to emit visible light. For ex- observed is a measure of the gamma-rayample, after exposure to gamma (or X) exposure.rays, a silver metaphosphate glass rod or 8.27 Other optical density dosi-plate system emits a phosphorescent meters depend on the production by ra-glow when subjected to ultraviolet light; diation of stable color centers whichthe glow can be measured by means of a absorb light at a certain wavelength. Anphotoelectric detector. In thermolumin- example is a device that measures radi-escent dosimeters metastable centers are ation by a change in the transmission ofproduced by radiation, and these centers light through a cobalt-glass chip. A leadcan be induced to emit light by heating borate glass containing bismuth has alsothe material. A thermoluminescent sub- been developed for the measurement of
i
i332 INITIAL NUCLEAR RADIATION
high levels of radiation, specifically for in pulses of very short duration as welluse in mixed gamma-neutron environ- as of the total dose.ments, Other materials that are utilized 8.30 Dose (or exposure) rates arein instruments for the measurement of usually determined by what are calledradiation by color changes include dyed "survey meters." They may be ionplastics, such as blue cellophane and chambers, Geiger-Mueller tubes, or"cinemoid" film, i.e., a celluloid-like scintillation detectors, together with as-film containing a red dye. sociated electronic counting circuitry. In
...general, these survey meters are port-8.28 In practice, measuring mstru-
bl d b tt d Th d t.a e an aery powere, e ose-ra ements do not determine the exposure m t be t d . t thmeasuremen may conver e m 0 e
roentgens or the absorbed dose in rads t t I d. t. d b It ' I .th.0 a ra la Ion ose y mu IP ymg e
dIrectly. One or other of several ob-I d d t b th t t Iproper y average ose ra eye 0 a
servable effects, such as current pulses. f11 .time 0 exposure.produced by IOniZation, scmtl atlons,changes in optical response, or temper-ature rise, serves as the basis for the GAMMA-RAY DOSE DEPENDENCE ONactual determination, The instruments YIELD AND DISTANCE
can indicate the exposure in roentgens 8 31 Th b ' I . I ff t f.,. .e 10 oglca e ec s 0
or the dose m rads after beIng calIbrated, d. t. d .11 be.various gamma-ra la Ion oses WI
wIth a standard gamma-ray source, . d d f II . Ch t XII, .consl ere more u y m ap er .
usually a known quantity of a radloac- H . d t .d 'owever, m or er 0 provi e some m-
tive material that emits a gamma ray of d ., f th ..fi f thIcatlon 0 e slgnl cance 0 e
the appropriate energy at a known rate. be , bel 't be t t dnum rs given ow, I may s a e
8.29 Some instruments can record that a single absorbed dose of gammaboth the total radiation dose (or expo- rays of less than 25 rads (in body tissue)sure) and the dose (or exposure) rate, will produce no detectable clinical ef-but most radiation measuring devices fects in humans. Larger doses have in-are designed to indicate either the total creasingly more serious consequencesor the rate. Total radiation doses (or and whole-body doses of 1,000 radsexposures) are measured by personnel would probably prove fatal in nearly alldosimeters worn by individuals who cases, although death would not occurmay be exposed to unusual amounts of until a few days later,nuclear radiation in the course of their 8.32 As is to be expected, thework. Examples of such instruments are gamma-ray dose at a particular location,pocket ion chambers, optical density resulting from a nuclear explosion, isdevices (especially film badges), and less the farther that location is from thephololuminescent, thermoluminescent, point of burst, The relationship of theand color-change dosimeters, Calori- radiation dose to the distance is depen-meters also measure total radition doses. dent upon two factors, analogous toThe charge collection time in semicon- those which apply to thermal radiation.ductor detectors is so short that these There is, first, the general decrease dueinstruments lend themselves to the to the spread of the radiation over largermeasurement of gamma-ray dose rates and larger areas as it travels away from
GAMMA RAYS 333
3,000
2,500
2,000
II)0a:~>-
'"~ 1,500z~a:I-z~..JII)
1,000
500
0I 2 5 10 20
EXPLOSION YIELD (KT)
Figure 8.33a. Slant ranges for specified gamma-ray doses for targets near the ground as afunction of energy yield of air-burst fission weapons, based on 0.9 sea-level
air density. (Reliability factor from 0.5 to 2 for most fission weapons.)
the explosion center. As with thermal have also been performed of the trans-radiation (§ 7.07), the dose received is port of gamma rays through the air.inversely proportional to the square of These calculations have been correlatedthe distance from the burst point, so that with measurements of the gamma-rayit is said to be governed by the' 'inverse transport from known sources and withsquare" law. Second, there is an atten- observations made at nuclear explo-uation factor to allow for the decrease in sions. The results obtained for air burstsintensity due to absorption and scatter- are summarized in the form of twoing of gamma rays by the intervening graphs: the first (Fig. 8.33a) shows theatmosphere. relation between yield and slant range
8.33 The gamma-radiation doses at for various absorbed gamma-ray dosesknown distances from explosions of (in tissue near the body surface, seedifferent energy yields have been mea- § 8.18) for fission weapons; the secondsured at a number of nuclear weapons (Fig. 8.33b) gives similar informationtests. Extensive computer calculations for thermonuclear weapons with 50
_lJH;.~Jjj,;c:
334 INITIAL NUCLEAR RADIATION
7,000
6,000
5,<xx>
U)0It:
~ 4,000
wCJZ<IIt:
~ 3,000<I-'U)
2,000
1,000
0102 2 5 10 2 5 10 2
EXPLOSION YIELD (KT)
Figure 8.33b. Slant ranges for specified gamma-ray doses for targets near the ground as afunction of energy yield of air-burst thermonuclear weapons with 50 percentfission yield, based on 0.9 sea-level air density. (Reliability factor from 0.25to 1.5 for most thermonuclear weapons.)
percent of their yield from fiss- within a factor of 0.5 to 2 for most.ion(§ 1.72). The data are based on an fission weapons, whereas the reliabilityaverage density of the air in the trans- factor for Fig. 8.33b is from 0.25 to 1.5mission path between the burst point for most thermonuclear weapons. Inter-and the target of 0.9 of the normal pol at ion may be used for doses othersea-level density.5 Because of variations than those shown on the figures.in weapon design and for other reasons 8.34 The use of the gamma-ray(§ 8.127), the gamma-ray doses calcu- dose curves may be illustrated by deter-lated from Figs. 8.33a and b are not mining the absorbed dose received at aexact for all situations that may arise. distance of 2,000 yards from a 50-kilo-Figure 8.33a is considered to be reliable ton low air burst of a fission weapon.
'The density referred to here (and subsequently) is that of the air before it is disturbed by the explosion(cf.§ 8.36).
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336 INITIAL NUCLEAR RADIATION
SHIELDING AGAINST GAMMA RAYS location where the tissue dose is 500rads, e.g., of initial gamma radiation,
8.38 Gamma rays are absorbed (or with no shielding, the introduction ofattenuated) to some extent in the course the appropriate tenth-value thickness ofof their passage through any material. any substance would decrease the doseAs a rough rule, the decrease in the to (approximately) 50 rads. The additionradiation intensity is dependent upon the of a second tenth-value thickness wouldmass (per unit area) of material that result in another decrease by a factor ofintervenes between the source of the ten, so that the dose received would berays and the point of observation. This (approximately) 5 rads. Each succeed-means that it would require a greater ing tenth-value thickness would bringthickness of a substance of low density, about a further reduction by a factor ofe.g., water, than one of high density, ten. Thus, one tenth-value thicknesse.g., iron, to attenuate the radiations by decreases the radiation dose by a factora specified amount. Strictly speaking, it of (approximately) 10; two tenth-valueis pot possible to absorb gamma rays thicknesses by a factor of 10 x 10, i.e.,completely. Nevertheless, if a sufficient 100; three tenth-value thicknesses by athickness of matter is interposed be- factor of 10 x 10 x 10, i.e., 1,000; andtween the radiation source, such as an so on.6exploding nuclear weapon, and an indi- 8.40 In shielding against gammavidual, the dose received can be reduced radiations from a nuclear explosion theto negligible proportions. conditions leading to the tenth-value
8.39 The simplest case of gamma- thickness concept do not exist. In theray attenuation is that of a narrow beam first place, the gamma-ray energiesof monoenergetic radiation, i.e., radia- cover a wide range, the radiations aretion having a single energy, passing spread over a large area, and thickthrough a relatively thin layer of shield- shields are necessary in regions of in-ing material. In these special (and hy- terest. Evaluation of the effectiveness ofpothetical) circumstances, theoretical a given shield material is then a complexconsiderations lead to the concept of a problem, but calculations have been"tenth-value" thickness as a measure of made with the aid of electronic com-the effectiveness of the material in at- puters. It has been found that, beyondtenuating gamma rays of a given energy the first few inches of a shielding mate-(cf. § 8.95 et seq.). A tenth-value rial, the radiation attenuation can bethickness is defined as the thickness of expressed with fair accuracy in manythe specified material which reduces the cases in terms of an effective tenth-valueradiation dose (or dose rate) to one tenth thickness. This useful result apparentlyof that falling upon it; in other words, arises from the fortuitous cancellation ofone tenth-value thickness of the material factors which have opposing effects onwould decrease the radiation by a factor the simple situation considered in ,of ten. Thus, if a person were in a § 8.39. In the first few inches of the
"The .'half-value thickness" is sometimes used; it is defined as the thickness of a given material whichreduces the dose of impinging radiation to (approximately) one half. Two such thicknesses decrease thedose to one fourth; three thicknesses to one eighth. etc.
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NEUTRONS 345
or plutonium. is exposed to neutrons. cannot be correct for all situations thatThe fission products formed are highly might arise. It is with this limitation inradioactive, emitting beta particles and mind that the curves in Figs. 8.64a andgamma rays. By measuring the radioac- b are presented; the former is for fissiontivity produced in this manner, the weapons and the latter for thermonu-amount of fission and. hence. the neu- clear weapons with 50 percent of theirtron fluence to which the fissionable yield from fission. The estimated reli-material was exposed can be deter- ability factors are the same as given inmined. § 8.33 for Figs 8.33a and b, respec-
tively. The curves give absorbed neu-NEUTRON DOSE DEPENDENCE ON tron doses in tissue close to the surfaceYIELD AND DISTANCE of the body received near the ground for
low air bursts. The data are based on an8.63 A basic difficulty in expressing average air density in the transmission
the relation between the neutron dose, path of 0.9 of the normal sea-level den-yield, and the distance from a nuclear sity. If the actual average air density isexplosion is the fact that the results vary higher or lower than this. the neutronsignificantly with changes in the dose will be decreased or increased.
characteristics of the weapon. The ma- respectively.terials, for example, have a considerable 8.65 When comparing or combin-influence on the extent of neutron cap- ing neutron doses with those fromture and. consequently. on the number gamma rays (Figs. 8.33a and b), itand energy distribution of the fission should be noted that the biological ef-neutrons that succeed in escaping into fects of a certain number of rads ofthe air. Further. the thermonuclear re- neutrons are often greater than for theaction between deuterium and tritium is same number of rads of gamma raysaccompanied by the liberation of neu- absorbed in a given tissue (§ 12.97). Astrons of high energy (§ 8.57). Hence, it for gamma rays, the neutron dose de-is to be expected that, for an explosion creases with distance from the explosionin which part of the energy yield arises as a result of the inverse square law andfrom thermonuclear (fusion) processes. attenuation by absorption and scatteringthere will be a larger proportion of in the atmosphere. However, since thehigh-energy (fast) neutrons than from a prompt neutrons are emitted during apurely fission explosion. short time (§ 8.51). and since those of
8.64 In view of these considera- major biological significance traveltions, it is evident that the actual number much faster than the blast wave, there isof neutrons emitted per kiloton of ex- no hydrodynamic enhancement of theplosion energy yield, as well as their (prompt) neutrons dose as there is forenergy distribution. may differ not only fission product gamma rays. This is onefor weapons of different types, i.e.. fis- reason why the garnma-ray dose in-sion and fusiQn. but also for weapons of creases more rapidly with the energythe same kind. Hence, any curve which yield than does the neutron dose. Thepurports to indicate the variation of data in Figs. 8.64a and b may be re-neutron dose with yield and distance garded as applying to air bursts. For
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TRANSIENT-RADIATION EFFECTS ON ELECTRONICS (TREE) 351
fast-neutron fluences as small as lOll or Vacuum Tubes and Thyratronsas large as 1015 (or more) neutrons/cm2.(Fast-neutron fluences referred to in this 8.81 The principal transient effectsection are fission neutrons with ener- in vacuum tubes arises from thegies exceeding 10 keY, i.e., 0.01 (Compton) electrons ejected by gammaMeV).lo The structure of the device has rays (§ 8.89) from the structural parts ofan important influence on the radiation the tube into the evacuated region.resistance of a transistor. As a general These electrons are too energetic to berule, a thin base, as in high-frequency significantly influenced by the electricdevices, and a small junction area favor fields in the tube. However, their impactradiation resistance. For example, dif- on the interior surfaces of the tube pro-fuse-junction transistors are signifi- duces low-energy secondary electronscantly more resistant than alloy-junction that can be affected by the existingdevices because of the smaller junction electric fields, and as a result thearea. Junction and especially thin-film operating characteristics of the tube canfield-effect transistors can be made that be altered temporarily. The grid is par-are quite resistant to radiation. Certain ticularly sensitive to this phenomenon;types of the latter have remained opera- if it suffers a net loss of electrons, itstional after exposure to a fast-neutron voltage will become more positive andftuence of 1015 neutrons/cm2. there is a transient increase in the plate
8.80 Damage in MaS (metal-oxide current. Large ftuences of thermal neu-semiconductor) field-effect transistors is trons, e.g., 1016 neutrons/cm2, can causecaused primarily by gamma radiation permanent damage to vacuum tubes as arather than by neutrons; hence, the ef- consequence of mechanical failure offects are reported in terms of the dose in the glass envelope. But at distancesrads (silicon). The most sensitive pa- from a nuclear explosion at which suchrameter to radiation in these devices is ftuences might be experienced, blast andthe threshold voltage, i.e., the value of fire damage would be dominant.the gate voltage for which current just 8.82 Gas-filled tubes (thyratrons)starts to flow between the drain and the exposed to gamma radiation exhibit asource. In general, gradual degradation, transient, spurious firing due to partiali.e., a shift of about 0.5 volt in the ionization of the gas, usually xenon.threshold voltage, begins at about 104 Additional ionization is caused by colli-rads (silicon) and proceeds rapidly at sions between ions and neutral mole-higher doses. The sensitivity of MaS cules in the gas. As with vacuum tubes,transistors to radiation is, however, de- large ftuences of thermal neutrons canpendent on the impurities in the gate cause thyratrons to become useless as aoxide. With improvements in the tech- result of breakage of the glass envelopenique for producing the oxide, the de- or failure of glass-to-metal seals.vices are expected to survive doses of1()6 rads (silicon).
'0 For the dependence of neutron fluences at various energies on the energy yield and distance from anuclear explosion';- see Figs. 8.ll7a and b.
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TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION I:! 353
terial. These can usually be identified by changes in the electrical properties oftheir gradual disappearance (saturation) the insulating materials. When suchafter repeated exposures and by their damage becomes appreciable, e.g.,reappearance after additional exposures when the resistance is reduced severely,in which there is a considerable change electrical characteristics may be af-in the applied voltage, e.g., it is re- fected. The extent of the damage tomoved or reversed. insulating materials increases with the
8.87 Nuclear radiation can have neutron fluence (or gamma-ray dose),both temporary and permanent effects humidity, and irradiation temperature.on the insulating material of cables. If Certain types of insulation are quite su-ionization occurs in the material, the sceptible to permanent damage. For ex-free electrons produced contribute to its ample, silicon rubber is severelyconductivity. Hence, insulators are ex- cracked and powdered by a fluence ofpected to have a temporary enhancement 2 x 1015 fast neutrons/cm2. The ap-of conductivity in an ionizing radiation proximate gamma-radiation damageenvironment. Conduction in the insula- thresholds for three common types oftor is frequently characterized by two cable insulation are: polyethylene,components: (I) for very short radiation I X 107 rads (carbon); Teflon TFE,pulses, a prompt component whose I x 1()4 rads (carbon); and Teflon FEB,magnitude is a function of only the in- 2 x 1()6 rads (carbon). On the otherstantaneous exposure rate, and (2) fre- hand, some irradiated polyolefins arequently at the end of the short radiation capable of withstanding up to 5 x 109exposure, a delayed component having rads (carbon). A considerable degree ofapproximately exponential decay, i.e., recovery has been observed with respectrapid at first and then more and more to insulation resistance; this implies theslowly. possibility of adequate electrical servi-
8.88 Permanent damage effects in ceability after moderate physical dam-cables and wiring are apparent as age.
"
~TECHNICAL ASPECfS OF INITIAL NUCLEAR RADIATION 12
INTERACTION OF GAMMA RAYS first of, ~he the.se!s calle~ the' 'ComptonWITH MATfER effect. In thIS Interaction, the gamma- ;,
ray (primary) photon collides with an ~8.89 There are three important electron and some of the energy of the ~
types of interaction of gamma rays with photon is transferred to the electron. ~matter, as a result of which the photons Another (secondary) photon, with less !(§ 1.74) are scattered or absorbed. The energy, then moves off in a new direc- I
~"The remaining sections of this chapter may be omitted without loss of continuity. ~J
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354 INITIAL NUCLEAR RADIATION
tion at an angle to the direction of mo- "pair production." When a gamma-raytion of the primary photon. Conse- photon with energy in excess of 1.02quently, Compton interaction results in Me V passes near the nucleus of ana change of direction (or scattering) of atom, the photon may be converted intothe gamma-ray photon and a degrada- matter with the formation of a pair oftion in its energy. The electron which, particles, namely, a positive and a neg-after colliding with the primary photon, ative electron. As with the photoelectricrecoils in such a manner as to conserve effect, pair production results in theenergy and momentum is called a disappearance of the gamma-ray photonCompton (recoil) electron. concerned. However, the positive elec-
8.90 The total extent of Compton tron soon interacts with a negative elec-scattering per atom of the material with tron with the formation of two photonswhich the radiation interacts is propor- of lower energy than the original one.tional to the number of electrons in the The occurrence of pair production peratom, i.e., to the atomic number atom, as with the other interactions,(§ 1.09). It is, consequently, greater per increases with the atomic number of theatom for an element of high atomic material, but it also increases with thenumber than for one of low atomic energy of the photon in excess of 1.02number. The Compton scattering de- MeV.creases with increasing energy of the 8.93 In reviewing the three types ofgamma radiation for all materials, irre- interaction described above, it is seenspective of the atomic number. that, in all cases, the magnitude per
8.91 The second type of interaction atom increases with increasing atomic !of gamma rays and matter is by the number (or atomic weight) of the mate-"photoelectric effect." A photon, with rial through which the gamma rays pass.energy somewhat greater than the bind- Each effect, too, is accompanied by ei-ing energy of an electron in an atom, ther the complete removal of photons ortransfers all its energy to the electron a decrease in their energy. The net resultwhich is consequently ejected from the is some attenuation of the gamma-rayatom. Since the photon involved in the intensity or dose rate. Since there is anphotoelectric effect loses all of its en- approximate parallelism between atomicergy, it ceases to exist. In this respect, it weight and density, the number ofdiffers from the Compton effect, in atoms per unit volume does not varywhich a photon still remains after the greatly from one substance to another.interaction, although with decreased Hence, a given volume (or thickness) ofenergy. The magnitude of the photo- a material containing elements of highelectric effect per atom, like that of the atomic weight ("heavy elements") willCompton effect, increases with the be more effective as a gamma-ray shieldatomic number of the material through than the same volume (or thickness) ofwhich the gamma rays pass, and de- one consisting only of elements of lowcreases with increasing energy of the atomic weight ("light elements"). An
photon. illustration of this difference in behavior8.92 Gamma radiation can interact will be given below.
with matter in a third manner, called 8.94 Another important point is that
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TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 355 ~
the probabilities of the Compton and without having undergone any interac-photoelectric effects (per atom) both tions can be represented by the equationdecrease with increasing energy of the I L 5.= e-"x. (8.9.1)13gamma-ray photon. However, pall pro- 0
duction, which starts at 1.02 MeV, in- where ~ is called the "linear attenuationcreases with the energy beyond this coefficient. " The distance x is usually
value. Combination of the various at- 'expressed in centimeters, so that thetenuating effects, two of which decrease corresponding units for ~ are reciprocalwhereas one increases with increasing centimeters (cm-I). It can be seen fromphoton energy, means that, at some en- the equation (8.95.1) that, for a givenergy in excess of 1.02 MeV, the ab- thickness x of material, the intensity 1sorption of gamma radiation by a par- of the emerging gamma rays will be lessticular material should be a minimum. the larger is the value of JA,. In otherThat such minima do exist will be seen words, the linear attenuation coefficientshortly. is a measure of the shielding ability of a
definite thickness, e.g., I cm, I foot, orother thickness, or any material for a
GAMMA-RAY An"ENUATION collimated beam of monoenergetic
COEFFICIENTS gamma rays.8.96 The value of JA" under any
8.95 When a narrow (or collimated) given conditions, can be obtained withbeam of gamma rays passes through a the aid of equation (8.95.1) by deter-material, photons are removed as a re- mining the gamma-ray intensity beforesuit of the Compton scattering interac- (/0) and after (I) passage through ation as well as by the photoelectric and known thickness, x, of material. Somepair-production interactions. In other of the data obtained in this manner, forwords, the scattered photons are re- monoenergetic gamma rays with ener-garded as being lost from the beam, gies ranging from 0.5 MeV to 10 MeV,although only part of their energy will are recorded in Table 8.96. The valueshave been deposited in the material. If given for concrete apply to the commonsuch a collimated beam of gamma rays form with a density of 2.3 grams perof a specific energy, having an initial cubic centimeter (144 pounds per cubicintensity (or flux) of 10 photons per foot). For special heavy concretes, con-square centimeter per second, traverses taining iron, iron oxide, or barytes, thea thickness of x of a given material, the coefficients are increased roughly inintensity, I, of the rays which emerge proportion to the density.
"In this equation, the intensity is the number of (uncollided) photons per square centimeter persecond. A similar equation, with the "linear energy absorption coefficient" replacing the linearattenuation coefficient, is applicable when the intensity is expressed in terms of the total energy of the
photons per square centimeter per second.
-"~- --~--
356 INITIAL NUCLEAR RADIATION
Table 8.96,-
LINEAR ATTENUATION COEFFICIENTS FOR GAMMA RAYS i
Linear Attenuation Coefficient (/L) in cm-1
Gamma-ray Energy(MeV) Air Water Concrete Iron) Lead
-
0.5 1.11 x 10-4 0.097 0.22 0.66 1.641.0 0.81 x 10-4 0.071 0.15 0.47 0.802.0 0.57 x 10-4 0.049 0.11 0.33 0.523.0 0.46 x 10-4 0.040 0.088 0.28 0.474.0 0.41 x 10-4 0.034 0.078 0.26 0.485.0 0.35 x 10-4 0.030 0.071 0.25 0.52
10 0.26 x 10-4 0.022 "0.060 0.23 0.55
8.97 By suitable measurements and cess of 1.02 MeV, pair productiontheoretical calculations, it is possible to begins to make an increasingly signifi-determine the separate contributions of cant contribution. Therefore, at suffi-the Compton effect (JLc)' of the photo- ciently high energies the attenuation co-electric effect (JLpe)' and of pair produc- efficient begins to increase after passingtion (JLpp) to the total linear attenuation through a minimum. This is apparent incoefficient as functions of the gamma- Fig. 8.97a, as well as in the last columnray energy. The results for lead, a typi- of Table 8.96, for lead. For elements ofcal heavy element (high atomic number) lower atomic weight, the increase doeswith a large attenuation coefficient, are not set in until very high gamma-raygiven in Fig. 8.97a and those for air, a energies are attained, e.g., about 17mixture of light elements (low atomic Me V for concrete and 50 Me V fornumber) with a small attenuation coef -water.ficient, in Fig. 8.97b. Except at ex- 8.98 The fact that the attenuationtremely low energies, the photoelectric coefficient decreases as the gamma-rayeffect in air is negligible, and hence is energy increases, and may pass throughnot shown in the figure. At the lower a minimum, has an important bearing ongamma-ray energies, the linear attenua- the problem of shielding. For example,tion coefficients in both lead and air a shield intended to attenuate gammadecrease with increasing energy because rays of I MeV energy will be much lessof the decrease in the Compton and effective for radiations of 10 MeV en-photoelectric effects. At energies in ex- ergy because of the lower value of the
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 357
2.0
I
Ec.>
.: 1.5I-zw
u~ 1.0w0uz0
'4 0.5::>zwl-I-<l
00 2 4 6 8 10
GAMMA RADIATION ENERGY (MEV)
Figure 8.97a. Linear attenuation coefficient of lead as function of gamma-ray energy.
~ 2.0
'Ec.>
~0
x 1.5
:l
I-ZW
U 1.0l!.l!.W0U
Z0 0.5 ;1i= ~<l::>z~ "pp -
I- 0<l 0 2 4 6 B 10
GAMMA RADIATION ENERGY (MEV)
Figure 8.97b. Linear attenuation coefficient of air as function of gamma-ray energy.
~}I:;~ --
358 INITIAL NUCLEAR RADIATION
attentuation coefficient, irrespective of for iron is about 0.027 in the same unitsthe material of which the shield is com- (g/cm 2).posed. 8.101 If the symbol p is used for the
8.99 An examination of Table 8.96 density of the shield material, thenshows that, for any particular energy equation (8.95.1) can be rewritten in thevalue, the linear attenuation coefficients formincrease from left to right, that is, with
II...1 = e-!'z = e-(!'/p) (pz) (8 101 1)IncreaSIng density of the material. Thus, '0 ' ..
a given thickness of a denser substance where IIIo is the transmission factor ofwill attenuate the gamma radiation more the shield of thickness x cm, and IJip is,than the same thickness of a less dense by definition, the mass attenuation co-material. This is in agreement with the efficient. Taking ~/p to be 0.023 g/cm2qualitative concept that a small thick- for initial gamma rays, it follows fromness of a substance of high density will equation (8.101.1) thatmake as effective a gamma-ray shield as T .. f.ransmlsslon actor =a greater thickness of one of lower den-
.e-°O23pz = 10-oolpz. (8.101.2)
slty (§ 8.38 et seq.).In the absence of better information, thisexpression may be used to provide a
MASS ATfENUATION COEFFICIENT rough idea of the dose transmission fac-tor, as defined in § 8.72, of a thickness
8.100 As a very rough approxima- of x centimeters of any material (oftion, it has been found that the linear known density) of low or moderateattenuation coefficient for gamma rays atomic weight.of a particular energy is proportional to 8.102 The simple tenth-valuethe density of the absorbing (shield) thickness concept described in § 8.39 ismaterial. That is to say, the linear at- based on equation (8.95.1). For such atenuation coefficient divided by the thickness the transmission factor is 0.1density, giving what is called the' 'mass and if the thickness is represented byattenuation coefficient," is approxi- ~ I' it follows that
mate.ly the same for all substances for a 0.1 = e-!'ZOIspecified gamma-ray energy. This isespecially true for elements of low and or
medium atomic weight, up to that ofiron (about 56), where the Compton XO1 =2~ cm. (8.102.1)effect makes the major contribution to ~
the attenuation coefficient for energiesup to a few million electron volts (cf. If ~/p is taken to be 0.023 g/cm2 for theFig. 8.97b). For the initial gamma rays initial gamma radiation of higher energyof higher energy, the effective mass at- then, as a "rule-of-thumb" approxima-tenuation coefficient (§ 8.104) is close tion,to 0.023 for water, wood, concrete, andearth, with the densities expressed in x (cm) = 100grams per cubic centimeter. The value 0 I p(g/cm3)
~ ~~~ "~
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 359
or the equivalent and the energy of the impinging radia-tion; thus, equation (8.95.1) is now
T(ft) = ~ (8.102.2) written as
I = I B(x)c,.'where, as in § 8.42, Tis the tenth-value O'
thickness in feet and D is the density of Values of the buildup factor for a varietythe material in Ib/ft3. It follows, there- of conditions have been calculated for afore, that for the less-dense materials, number of elements from a theoreticalfor which Jj./p is close to 0.023 g/cm2, consideration of the scattering of pho-the product D x T should be equal to tons by electrons. The fact that theseabout 200 Ib/ft2 for gamma rays of values are frequently in the range fromhigher energy. This is in agreement with 10 to 100 shows that serious errorsthe values in the last column of Table could arise if equation (8.95.1) is used8.41 for nitrogen capture (secondary) to determine the attenuation of gammagamma rays. The D x T for iron (or rays by thick shields.steel) is smaller than for the other mate- 8.104 It will be apparent, therefore,rials because Jj./p is larger, namely about that equation (8.95.1) and others0.027 g/cm2, for the gamma rays of derived from it, such as equations
higher energy. (8.101.2) and (8.102.1), as well as thesimple tenth-value thickness concept,
THICK SHIELDS: BUILDUP FACTOR will apply only to monoenergetic radia-tions and thin shields, for which the
8.103 Equation (8.95.1) is strictly buildup factor is unity. By taking theapplicable only to cases in which the mass attenuation coefficient to be 0.023photons scattered in Compton interac- for less dense materials (or 0.027 fortions may be regarded as having been iron), as given above, an approximateremoved from the gamma-ray beam. (empirical) allowance has been made forThis situation holds reasonably well for both the polyenergetic nature of thenarrow beams or for shields of moderate gamma radiations from a nuclear explo-thickness, but it fails for broad beams or sion and the buildup factors due to mul-thick shields. In the latter circum- tiple scattering of the photons. The re-stances, the photon may be scattered suIts are, at best, applicable only toseveral times before emerging from the shields with simple (slab) geometries.shield. For broad radiation beams and Furthermore, practical radiation shieldsthick shields, such as are of interest in must absorb neutrons as well as gammashielding from nuclear explosions, the rays, and the gamma radiation producedvalue of /, the intensity (or dose) of the in the shield by inelastic scattering andemerging radiation, is larger than that radiative capture of the neutrons maygiven by equation (8.95.1). Allowance produce a greater intensity inside thefor the multiple scattering of the radia- shield than the incident gamma radia-tion is made by including a "buildup tion. Consequently, any problem in-factor," represented by B(x), the value volving gamma radiation shielding,of which depends upon the thickness x especially in the presence of neutrons, isof the shield, the nature of the material, complex, even for relatively simple
~=-~-
360 INITIAL NUCLEAR RADIATION
structures; appropriate computer codes determine the gamma-ray spectrum, areare thus necessary to obtain approxima- complex, it is possible to calculate thetions of the attenuation. In the absence spectrum at various distances from aof better information, however, the ef- nuclear explosion. Computations of thisfective tenth-value thicknesses, as given kind have been used to estimatein Table 8.41 or derived from equation gamma-ray doses, as will be seen later(8.102.2), can be used to provide a (§ 8.125 et seq.). As an example, Fig.rough indication of gamma-ray shield- 8. 106 shows the spectrum of the initial
ing. gamma radiation received at a distanceof 2,000 yards from the explosion of a
THE INITIAL GAMMA-RAY fission weapon with an energy yield ofSPECTRUM 20 kilotons. At this range, some 70
percent of the gamma-ray photons have8.105 The major proportion of the energies less than 0.75 MeV. It should
initial gamma radiation received at a be remembered, however, that the pho-distance from a nuclear explosion arises tons of high energy are the most haz-from the interaction of neutrons with ardous and also are the most difficult tonuclei, especially nitrogen, in the at- attenuate.mosphere and from the fission productsduring the first minute after the burst. INTERACTIONS OF NEUTRONS WITHGamma rays from inelastic scattering MATTERand neutron capture by nitrogen haveeffective energies ranging up to 7.5 8.107 The modes of interaction ofMeV (or more) and those from the fis- neutrons with matter are quite differentsion products are mainly in the 1 to 2 from those experienced by gamma-rayMeV range. After passage through a photons. Unlike photons, neutrons aredistance in air, some of the photons will little affected by electrons, but they dohave been removed by photoelectric and interact in various ways with the nucleipair-production effects and others will of atoms present in all forms of matter.have had their energies decreased as a These neutron-nucleus interactions areresult of successive Compton scatter- of two main types, namely, scatteringings. There will consequently be a and absorption. As already seen, scat-change in the gamma-ray energy distri- tering reactions can be either inelastic (§bution, i.e., in the spectrum. 8.09) or elastic (§ 8.52). In inelastic
8.106 Information concerning the scattering part of the kinetic energy ofgamma-ray spectrum of the initial radi- the neutron is converted into internal (oration is important because the suscepti- excitation) energy of the struck nucleus;bilityof living organisms and of various this energy is then emitted as gammaelectronics components, the attenuation radiation. For inelastic scattering toproperties of air and shielding materials, occur, the neutron must initially haveand the response of radiation detectors sufficient energy to raise the nucleus toare dependent upon it. Although the an excited state. The magnitude of thisinteractions of both neutrons and energy depends on the nature of thegamma rays with the atmosphere, which nucleus and varies greatly from one el-
-i~tt4~ti,(i;:-
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 361
100 I
U)z0~ 80:I:0-
>- .<1 !
~ 60 ~ I
~ I(!) ~~ 40 --
W(!) I<1~~ 20ua:w0-
00 0_75 2 45 8 12
GAMMA-RAY ENERGY (MEV)Figure 8.106. Spectrum of initial gamma radiation 2.000 yards from a 20-kiloton explo-
sion.
ement to another. However. a rough with a nucleus is equivalent to a colli-general rule is that for many, but not all, sion between two billiard balls; kineticheavy or moderately heavy nuclei, e.g., energy is conserved and is merelyiron and uranium, inelastic scattering transferred from one particle to themay occur for neutrons with energies other. None of the neutron energy isranging from a few tenths Me V to as transformed into excitation energy oflow as a few tens of keV. For lighter the nucleus and there is no accompany-nuclei, inelastic scattering is possible ing gamma radiation. In contrast withonly when the neutrons have higher en- inelastic scattering, elastic scatteringergies. Significant inelastic scattering can take place with neutrons of all en-occurs only for neutron energies above ergies and any nucleus. For a givenabout 1.6 MeV for nitrogen and about 6 angle of impact, the fraction of the ki-MeV for oxygen. Neutrons with ener- netic energy of the neutron that is trans-gies below the appropriate threshold ferred to the nucleus in a collision isvalues for the nuclei present in the me- dependent only on the mass of the latter.dium cannot undergo inelastic scatter- The smaller the mass of the nucleus theing. greater is the fraction of the neutron
8.108 When elastic scattering energy it can remove. Theoretically, theoccurs, the interaction of the neutron whole of the kinetic energy of a neutron
-
362 INITIAL NUCLEAR RADIATION
could be transferred to a hydrogen nu- emits the excess energy as a gamma-raycleus (proton) in a single head-on colli- photon. This type of reaction usuallysion, In fact, hydrogen, the lightest ele- occurs with light nuclei and fast neu-ment, offers the best means for rapidly trons, although there are a few in-degrading fast neutrons with energies stances, e.g., lithium-6 and boron-lO,less than about 0.5 MeV. It is for this where it also takes place with slow neu-reason that hydrogen, e.g., as water, is trons. Nitrogen interacts with fast neu-an important constituent of neutron trons in at least two ways in whichshields (§ 8.67). For neutrons of higher charged particles are emitted (§§ 9.34,energy than 0.5 MeV, it is better to take 9.44); one leads to the formation ofadvantage of inelastic scattering to slow radioactive carbon-14 (plus a proton)down the neutrons. The heavy element and the other to tritium, the radioactivein the special concretes described in § isotope of hydrogen (plus stable car-8.69 serves this purpose. bon-12).
8.109 The second fundamental type 8.111 Fission, is of course, also aof interaction of neutrons with matter form of interaction between neutronsinvolves complete removal of the neu- and matter. But since it is restricted to atron by capture. Radiative capture (§ small number of nuclear species and has8.08) is the most common kind of cap- been considered in detail in Chapter I, itture reaction; it occurs to some extent, will not be discussed further here.at least, with nearly all nuclei. The 8.112 The rate of interaction ofprobability of capture is greater for slow neutrons with nuclei can .be describedneutrons than for those of high energy. quantitatively in terms of the concept ofMost light nuclei, e.g., carbon and ox- nuclear "cross sections." The crossygen, have little tendency to undergo section may be regarded as the effectivethe radiative capture reaction with neu- target area of a particular type of nu-trons. With nitrogen, however, the ten- cleus for a specific reaction and is adency is significant (§ 8. I I), but not measure of the probability that this re-great. For other nuclei, especially some action will occur between a neutron, ofof medium or high mass, e.g., cad- given energy, and that nucleus. Thus,mium, the radiative capture reaction each nuclear species has a specific scat-occurs very readily. In certain cases, the tering cross section, a capture crossreaction product is radioactive (§ 8.61); section, and so on, for a given neutronthis is of importance in some aspects of energy; the total cross section for thatweapons effects, as will be seen in energy is the sum of the specific crossChapter IX. sections for the individual interactions.
8.110 Another type of reaction is Both specific and total cross sectionsthat in which the incident neutron enters vary with the energy of the neutron,the target nucleus and the compound often in a very complex manner.nucleus so formed has enough excitation 8.113 The nuclear cross sections forenergy to permit the expulsion of an- neutron-nucleus interactions are analo-other (charged) particle, e.g., a proton, gous to the linear attenuation coeffi-deuteron, or alpha particle. The residual cients (for gamma rays) divided by thenucleus is often in an excited state and number of nuclei in unit volume of the
,c-
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 363
medium. In fact, an expression similar was, therefore, made to measurementsto equation (8.95.1) can be employed to of neutron flux within a few specifieddescribe the attenuation of a narrow energy ranges; from the results a generalbeam of monoenergetic neutrons in their idea of the spectrum was obtained.passage through matter. However, be- Measurements of this kind were madecause the neutrons in the initial nuclear by the use of threshold detectors ofradiation are far from monoenergetic activated foil or fission foil type (§§and the cross sections are so highly 8.61,8.62).dependent on the neutron energy, the 8.116 Neutrons are liberated duringequivalent of equation (8.95.1) must not the fission and fusion processes, but thebe used to calculate neutron attenuation neutrons of interest here are those thatfor shielding purposes. Shielding calcu- escape from the exploding weapon.lations can be made by utilizing cross Both the total number of neutrons andsections, the neutron energy distribution their spectrum are altered during transitin space and direction, and other data, through the weapon materials. Outputbut the calculations require the use of spectra that might be considered illus-computer codes. Such calculations are trative of fission and thermonucleartoo complicated to be described here. weapons are shown in Figs. 8.116a and
b, respectively. As mentioned pre-THE NEUTRON ENERGY SPECfRUM viously, the neutron source can be de-
fined properly only by considering the8.114 The energies of the neutrons actual design of a specific weapon.
received at some distance from a nuclear Hence, the spectra in these figures areexplosion cover a very wide range, from presented only as examples and shouldseveral millions down to a fraction of an not be taken to be generally applicable.electron volt. The determination of the 8.117 Passage of the neutronscomplete energy spectrum (§ 8.53), ei- through the air, from the explodingther by experiment or by calculation, is weapon to a distant point, is accompan-very difficult. However, it is possible to ied by interactions with nuclei that resultdivide the spectrum into a finite number in attenuation and energy changes.of energy groups and to calculate the Hence, the neutron spectrum at a dis-neutron flux in each energy group at tance may differ from the output spec-various distances from the explosion trum of the weapon. Extensive results ofpoint. These calculations can then be computer calculations of neutronchecked by measuring the variation of fluences at (or near) the earth's surfaceflux with distance from known neutron are now available and these have beensources that are representative of each used to plot the curves in Figs. 8.117aenergy group. and b, for fission and thermonuclear
8.IIS Prior to the cessation of at- weapons, respectively. The figuresmospheric testing of nuclear weapons, show the neutron fluences per kiloton ofneither the extremely large and fast energy release for a number of energycomputers nor the sophisticated mea- groups as a function of slant range. Thesurement instruments that are now uppermost curve in each case gives theavailable were in existence. Recourse total fluence (per kiloton) of neutrons
-c'...,~--,;,;v--
364 INITIAL NUCLEAR RADIATION
1023
5
2
-1022I-~"-(/)z0 5ocI-:)WZ
~ 2:)OCI-UWa.. 21(/) 10
5
2
2010
0.1 0.2 0.5 I 2 5 10 20
ENERGY INTERVAL (MEV)
Figure 8.116a. Neutron spectrum for a fission weapon per kiloton total energy yield.
with energies greater than 0.0033 MeV, means that the ftuence in each energyi.e., 3.3 keV. group decreases with increasing dis-
8.118 It is apparent from Fig. tance from the explosion, but the pro-8.117a that for a fission weapon the portions in the various groups do notcurves for the different energy groups all change very much; that is to say, thehave roughly the same slope. This neutron spectrum does not vary signifi-
:~,
~A>;:c,::
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 365
1023
5
2
22..-10I-~"-InZ0 5a:I-::>wz
~::> 2a:I-UWa.. 21In 10
5
2
10200.1 0.2 0.5 I 2 5 10 20
ENERGY INTERVAL (MEV)
Figure 8.116b. Neutron spectrum for a thermonuclear weapon per kiloton total energy
yield.
cantly with distance. Furthermore, al- Fig. 8.116a. This accounts for the equi-though it is not immediately apparent librium neutron spectrum from a fissionfrom Fig. 8.117a, the spectrum is al- explosion mentioned in § 8.55. Themost the same as the source spectrum in spectrum does change at much lower-'
,i:f?,.i;;\W!1
366 INITIAL NUCLEAR RADIATION
1013
5
2
CURVE MeV
1012I 8.1 B to 10.02 6.36 B. I B
53 4.06 6.364 2.35 4.065 1.1 j 2.35
2 6 0.111 1.11
II 7 0.0033 0.11110 B Total above 3.3 keY
5
I-~" 2..~~ 1010V)Z00::l-=>W 2Z
: 109uZW=>J!A-
10'
107
..
10S0 400 800 1200 1600 2000 2400
SLANT RANGE (YARDS)
Figure 8.117a. Neutron ftuence per kiloton energy yield incident on a target located on ornear the earth's surface from the fission spectrum shown in Fig. 8.116a.
-",~ ~;;~,...;;c .~,~ -
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 367
1014
5
2
13 CURVE MeV10
I 12-2 to 15.05 2 10.0 12.2
3 8.18 10.04 6.36 8. 18
2 5 4.06 6.366 2_35 4.06
1012 7 1.11 2.35
8 0.111 1.119 0.0033 0.111
5 10 Totol obove 3.3 keV
..-~ 2'00.
eo~~ lOll(/)
~ 5(l:..-::>!oj 2Z
::: 10'°uz!oj 5::>-JIJ..
2
109
5
lOB
lOT0 400 800 1200 1600 2000 2400
SLANT RANGE (YARDS)Figure 8. 117b. Neutron ftuence per kiloton energy yield incident on a target located on or
near the earth's surface from the thermonuclear spectrum shown in Fig.8.116b.
~---~- ---*'""
368 INITIAL NUCLEAR RADIATION
neutron energies, but this is not impor- compared to fluences at correspondingtanto distances in an infinite air medium. For
8.119 Examination of Fig. 8.117b source-target separation distances lessfor thermonuclear weapons reveals a than about a relaxation length,'4 local-different behavior. Curves 5 through 9, ized reflection from the ground gener-i.e., for neutron energies from 0.0033 to ally tends to increase the intensity of6.36 MeV, are almost parallel, so that in high-energy neutrons; however, at suchthis range the spectrum does not change short distances, the initial nuclear radi-much with distance. But at higher ener- ation is of interest only for very lowgies, especially from 8.18 to 15 Me V, yields, since for higher yields otherthe slopes of the curves are quite dif- weapon effects will normally be domi-ferent. Of the neutrons in groups I, 2, nant (cf. § 8.06). At longer distances,and 3, those in group I, which have the the high-energy neutron intensity mayhighest energies, predominate at a slant be reduced by a factor of five or morerange of 400 yards, but they are present compared to infinite air when both thein the smallest proportion at 1,600 source and the target are at or near theyards. During their passage through the ground surface, e.g. a surface or near-air, the fastest neutrons are degraded in surface burst. These effects have beenenergy and their relative abundance is included in the calculations from whichdecreased whereas the proportions of the figures given above were derived.
the somewhat less energic neutrons isincreased. The neutron spectrum thus INITIAL RADIATION DOSE IN TISSUEchanges with distance, especially in thehigh-energy range. The peak that exists 8.121 Simplified, but reasonablyat 12 to 14 MeV of the source spectrum accurate, methods have been developedin Fig. 8.116b becomes lower and the to predict the initial radiation dose tovalley between about 6 and 12 Me V persons located on or near the surface ofdisappears with increasing slant range. the earth. These methods are describedAt very long ranges, when the high-en- separately for neutrons, secondaryergy neutrons have lost much of their gamma rays from radiative capture andenergy, an equilibrium spectrum would inelastic scattering in the atmosphere (§be approached. 8.11), and fission product gamma rays.
8.120 Figs. 8.117a and b provide The contribution of the primary gammaestimates of neutron fluences and spec- rays from fission to the radiation dose attra from low air bursts for targets on or a distance is small enough to neglect (§near the surface of the ground. As a 8.04). In all cases, the data are based onresult of reflections and absorption by the assumption that the average densitythe ground, an air-ground interface can of the air in the transmission path, be-increase or decrease the neutron tween the burst point and the target, isfluences by as much as a factor of ten 0.9 of the normal sea-level density.
14 A relaxation length may be taken as the distance in which the radiation intensity in a specified
material is decreased by a factor of e, where e is the base of the natural logarithms (about 2.718). Therelaxation length in a given material depends on the neutron energy and on whether the direct fluenceonly or the total (direct plus scattered) fluence is being considered.
c,;"
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 369
Initial Neutron Absorbed Dose fission weapons, there are two curves in8.122 With spectra such as those each part; they do not necessarily rep-
shown in Figs. 8. I 16a and b to serve as resent the extremes in neutron dose thatsources, a neutron transport computer might result from different weapon de-code may be used to calculate the neu- signs, but the dose from most fissiontron dose resulting from a nuclear ex- weapons should fall between the twoplosion in a specified geometry, i.e., curves. It is suggested that the upperburst height, target height, and air den- curve of each pair in Fig. 8. 123a besity. The latter is taken to be the average used to obtain a conservative estimate ofdensity of the air between the burst and the neutron dose from fission weaponsthe target before disturbance of the air for defensive purposes and that theby the blast wave, since the neutrons of lower curve be used for a conservativeinterest depart from the region of the estimate for offensive purposes.explosion before formation of the blast 8.124 In order to determine thewave and are deposited at the target neutron dose received from an air burstprior to its arrival. of Wkilotons energy yield, the dose for
8.123 Results of such calculations, the given distance as obtained from Fig.which have been corroborated by test 8. 123a or b is multiplied by W. For adata, are given in Figs. 8. 123a and b, contact surface burst, the values fromfor fission and thermonuclear weapons, Figs. 8. 123a and b should be multipliedrespectively. In each case the absorbed byO.5. For explosions above the surfaceneutron dose (in tissue) received by a but below about 300 feet, an approx-target on or near the surface of the earth imate value of the neutron dose may beis shown as a function of slant range per obtained by linear interpolation betweenkiloton energy yield for explosions at a the values for a contact surface burst andheight of about 300 feet or more. For one at 300 feet or above. The "de-convenience of representation, the fense" curve of Fig. 8. 123a was used tocurves are shown in two parts; the left generate the data for Fig. 8.64a, and theordinate scale is for shorter ranges and curve in Fig. 8. 123b was used for Fig.the right is for longer distances. For 8.64b.
(Text continued on page 373.)
,.
370 INITIAL NUCLEAR RADIATION
The curves in Figs. 8.123a and b Exampleshow the neutron dose in tissue per O. A 10 KT fi .. ...Iyen: sslon weapon IS
kiloton Yield as a function of slant range I d d h .h f 300 f.exp 0 e at a elg t 0 eet.
from a burst at a height of 300 feet ormore for fission weapons and thermo- Find: The neutron dose at a slantnuclear weapons, respectively. range of 1,500 yards that is conservative
S I . I d I h d .from the defensive standpoint.ca mg. n or er to app y t e ata m
Figs. 8.123a and b to an explosion of Solution: Since the height of burst isany energy, W kilotons, multiply the 300 feet, no height correction is neces-value for the given distance as obtained sary. From the upper ("defense") curvefrom Fig. 8.123a or b by W. For a in Fig. 8.123a, the neutron dose percontact surface burst, multiply the dose kiloton yield at a slant range of 1,500obtained from Fig. 8.123a or b by 0.5. yards from an explosion is 16 rads. TheFor bursts between the surface and corresponding dose, D., from a 10 KTabout 300 feet, an approximate value of explosion is
the neutron dose may be obtained by D - 10 16 -160 d AI .. I .-x -ra s. nswermear mterpo atlon between a surface .
burst and one at 300 feet or above.
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 371
105
5
2
-I
104
5
~ 2
I-~ 3 -2~ 10W::IIIIIII-5I-
III0«~ 2WIII 2 -30 100
Z0It: 5I-::Il&JZ
2
-4
10
5
2
1 10-5
0 1,000 2,000 :3,000 4,000
SLANT RANGE FROM EXPLOSION (YARDS)
Figure 8.123a. Initial neutron dose per kiloton total yield as a function of slant range fromfission weapon air bursts, based on 0.9 normal sea-level air density.
372 INITIAL NUCLEAR RADIATION
10~ 1
I~ ~
2 2
104 10-1
~ ~
~ 2:::::.w~ 103 -2
(/1
E(/1 ~C<[It:
~ 20C
~ 102 -0-3It:I-::>w ~Z
2 -
10 0-4
~
2 -2
1 10-50 1,000 2,000 3.000 4,000
SLANT RANGE FROM EXPLOSION (YARDS)
Figure 8.123b. Initial neutron dose per kiloton total yield as a function of slant range fromthermonuclear weapon air bursts, based on 0.9 normal sea-level air density.
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 373
Secondary Gamma-Ray Absorbed Dose the secondary gamma rays reach a dis-
8.125 The secondary (or air-secon- tant target before the blast wave hasdary) gamma rays, i.e., the gamma rays traveled very far (see Fig. 8.14).produced by various interactions of 8.127 Some of the results of calcu-neutrons with atmospheric nuclei, must lations of secondary absorbed gamma-be considered separately from the fis- ray doses (in tissue), obtained in thesion product gamma rays to provide a manner indicated above, are shown ingeneralized prediction scheme since the Figs. 8.127a and b, which correspond torelative importance of the two depends the neutron dose curves in Figs. 8.123aon several factors, including the total and b, respectively. The conditions ofyield, the fraction of the total yield applicability of the figures, such as airderived from fission, the height of burst, burst or contact surface burst, targetand the slant range from the explosion to location, offensive or defensive use,the target. Since measurements at at- etc., are the same as given in §§ 8.123,mospheric tests have provided only the 8.124. In order to be consistent, if eithertotal gamma radiation dose as a function the "offense" or "defense" curve inof distance from the source, computer Fig. 8.123a is used to obtain the neutroncalculations have been used to obtain dose for a given situation, the corre- '!the doses from the two individual sponding curve in Fig. 8.127a should be :!gamma-ray sources. The results of the used for the secondary gamma-ray dose.calculations of air-secondary gamma-ray doses (and the total doses) predicted 1:",.' P d G R Ab b d ;{ .rlSSlon ro uct amma- ay sor e ~by the calculations have been compared D .
osewith measurements performed at nu-clear weapon tests. For bursts in the S.128 In order to estimate thelower atmosphere, the gamma rays from gamma-ray dose from fission products,isomeric decay provide such a small the radiation transport computer codefraction of the total gamma-ray energy must be supplemented with a code thatthat they can be neglected in the cal- describes the evolution and rise of theculation of total dose in tissue.15 radioactive cloud containing the fission
8.126 By using neutron spectra, products. Since the fission product radi-such as those shown in Figs. 8 .116a and ation is emitted over a sufficiently longb, the secondary gamma-ray source can period of time, the hydrodynamic effectbe calculated. The latter is then utilized (§ 8.36) of the blast wave on the aver-to compute the secondary gamma-ray age air density between the source anddose resulting from a nuclear explosion the target must be considered. The hy-in a specified geometry. As is the case drodynamic enhancement becomesfor neutrons, the air density is taken to more important at high energy yieldsbe the average density of the air between and also at greater ranges because of thethe burst and the target before distur- larger volume of low-density air behindbance of the air by the blast wave, since the shock front.
"It should be noted that the ordinates in Fig. 8.14 are the energy emission rates; the total energywould then be obtained by integration over the effective emission time. This time is very much shorter forisomeric decay gamma rays than for fission products.
374 INITIAL NUCLEAR RADIATION
The curves in Figs. 8.127a and b Exampleshow the secondary gamma-ray dose in. ... k ' l . Id f t . f GIven: A 20 KT fission weapon IStissue per I oton Yle as a unc Ion 0
h . h f exploded on the surface.slant range from a burst at a elg t 0300 feet or more for fission weapons and Find: The secondary gamma-ray dosethermonuclear weapons, respectively. at a slant range of 1,000 yards that is
S I . I d t I th d t .conservative from the offensive stand-ca mg. n or er 0 app yea a In .Figs. 8.127a and b to an explosion of point.
any energy, W kilotons, multiply the Solution: Since this is a contact sur-value for the given distance as obtained face burst, a correction factor of 0.5from Fig. 8.127a or b by W. In the case must be applied to the value obtainedof a contact surface burst, multiply the from Fig. 8.127a. From the lowerdose obtained from Fig. 8.127a or b by ("offense") curve in Fig. 8.127a, the0.5. For bursts between the surface and secondary gamma-ray dose per kilotonabout 300 feet, an approximate value of yield at a slant range of 1,000 yardsthe secondary gamma-ray dose may be from an explosion at or above 300 feet isobtained by linear interpolation. 30 rads. The corresponding dose, D"ys'
from a surface burst 20 KT explosion is
D = 20 x 0.5 x 30"yS
= 300 rads. Answer
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 375
10
5
2
-I10
j:: 5~~W::>In 2In
E3 -2
In 100c{0:
W 5In00
>-c{ 20:1
c{~ 2 -3~ 10
c{(.:I
>- 50:c{0Z0~ 2In
-410
5
2
1 10"5
0 1,000 2,(XX) 3,(XX) 4,(XX)
SLANT RANGE FROM EXPLOSION (YARDS)
Figure 8.127a. Air-secondary gamma-ray component of the initial nuclear radiation doseper kiloton yield as a function of slant range from fission weapon air bursts,
based on 0.9 normal sea-level air density.
376 INITIAL NUCLEAR RADIATION
IV
5
-2
t-~"-W 10.:>InIn
E 5
Ina<tIr~ 2
l&JIna 3 -Ia 10
>-<tIr, 5<t~~<t(:>>- 2Ir<ta 2 -2Z 10 00Ul&JIn
5
2
10 10-30 1,000 2,000 3,000 4,000
SLANT RANGE FROM EXPLOSION (YARDS)
Figure 8.127b. Air-secondary gamma-ray component of the initial nuclear radiation doseper kiloton yield as a function of slant range from thermonuclear weapon air
bursts, based on 0.9 normal sea-level air density.
8.129 With minimal hydrodynamic reduce the dose from fission productenhancement, as is the case for very gamma rays relative to that from secon-low-yield weapons, the fission product dary gamma rays with increasing dis-gamma rays and the secondary gamma tance from low-yield explosions. Forrays contribute approximately equal explosions of higher yield, however,doses at slant ranges up to about 3,000 hydrodynamic enhancement may causeyards. However, the average energy of the fission product gamma-ray dose tothe former gamma rays is considerably exceed the secondary gamma-ray dose,less than that of the latter, and the an- particularly at longer ranges.gular distribution of the fission product 8.130 The calculated fission prod-gamma rays is diffused by the rise of the uct gamma-ray dose in tissue per kilotoncloud. Each of these factors tends to of fission energy yield received by a
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 377
target on or near the surface of the earth amic enhancement depend on the totalas a function of slant range from a energy release. Interpolation may benuclear explosion is shown in Fig. employed to obtain the effective yield8 .130a. In order to determine the fission for slant ranges that are not shown. Theproduct gamma-ray dose received in the curves in Fig. 8.130b were calculatedinitial radiation from an air burst of a for a scaled height of burst of 200 WO4fission weapon of W kilotons energy feet, where W is the total weapon en-yield, the value for the given distance as ergy yield in kilotons. For a given slantobtained from Fig. 8.130a is multiplied range the curve is terminated at the yieldby the "effective" yield, determined at which the height of burst is equal tofrom Fig. 8.130b. The use of the effec- that slant range.tive yield instead of the actual yield 8.132 The data for the effectiveprovides the necessary corrections for yields in Fig. 8.132 are similar to thosethe differences in cloud rise velocity and in Fig. 8.130b but are applicable tothe hydrodynamic enhancement, each of contact surface bursts. There is no sim-which is a function of total energy yield. pie method to interpolate or extrapolate
these curves for fission-product gamma8.131 For thermonuclear weapons, rays to other heights of burst; however,
the dose for a given distance as obtained Fig. 8.130b may be taken to be reason-from Fig. 8.130a must be multiplied by ably accurate for most low air bursts,the fraction of the total yield that results and Fig. 8.132 may be applied to near-from fission, e.g., 0.5 for a weapon with surface as well as to contact surface50 percent fission yield, prior to multi- bursts. The results presented in Fig.plying by the effective yield as obtained 8.33a are based on the upper curves infrom Fig. 8.130b. It should be noted Fig. 8.127a (for secondary gamma rays)that Fig. 8.130b is always entered with and the curves in Figs. 8.130a and b.the total energy yield of the weapon to The results in Fig. 8.33b are based cor-obtain the effective yield, since the respondingly on the curves in Fig.cloud rise velocity and the hydrodyn- 8.127b and those in Figs. 8.130a and b.
(Text continued on page 383.)
/"
~ ,
378 INITIAL NUCLEAR RADIATION
The curves in Fig. 8. 130a show the Example 2initial radiation, fission product G.
A I MT th I..Iven: ermonuc eargamma-ray dose per kiloton fissIon YIeld . th 50 t f . tweapon WI percen 0 I S energy
as a function of slant range from a . Id d . d f fi .. lod d tYle enve rom sslon IS exp e anuclear explosion. a height of 3,200 feet.
Scaling. In order to apply the data in r;o..d Th t t I .. t. I I d..rln: e 0 a 101 la nuc ear ra la-
Fig. 8.130a to a fission explosIon of any t.d t I t f 4 000 d..Ion ose a a s an range 0, yar s.
energy, W kIlotons, multIply the value
for the given distance as obtained from Solution: The total initial nuclear ra-Fig. 8.130a by the effective yield, Welf diation dose is the sum of the initialkilotons, from Fig. 8.130b for a low air neutron dose, the secondary gamma-rayburst or from Fig. 8.132 for a surface dose, and the fission product gamma-rayburst. For a thermonuclear weapon, the dose. From Fig. 8.123b, the neutronvalue obtained from Fig. 8.130a should dose per kiloton yield, is 1.2 x 10-4 radbe multiplied by the fraction of the yield at a slant range of 4,000 yards from athat results from fission as well as by low air burst. The corresponding doseWelf for the total yield. from a I MT explosion is
Example 1 D. = 1.2 X 10-4 x 103 = 0.12 rad.
Given: A 20 KT fission weapon is From Fig. 8.127b, the secondaryexploded on the surface. gamma-ray dose per kiloton yield is 1.8
..x 10-3 rad at a slant range of 4,000FInd: The fissIon product gamma-ray d f I . b t Thyar s rom a ow aIr urs. e corre-
dose at a slant range of 1,000 yards. d. d f I MT I .. spon 109 ose rom a exp oslon IS
.Solutio~: From .Fig. 8.130a, the ini- D = 1.8 X 10-3 x 103 = 1.8 rads.tlal radiatIon, fissIon product gamma- "IS
ray dose per kiloton yield at a slant From Fig. 8.130a, the fission productrange of 1,000 yards is 75 rads. From gamma-ray dose per kiloton fission yieldFig. 8.132, the effective yield at a slant at a slant range of 4,000 yards from therange of 1,000 yards from a 20 KT explosion is 3.2 x 10-4 rad. The heightexplosion on the surface is 45 KT. The of burst, 3,200 feet, is sufficiently closefission product gamma-ray dose for the to the scaled height of 200 WO4, i.e.,desired conditions is therefore 3,170 feet, that Fig. 8.130b should pro-D = 75 45 = 3 375 dA vide an accurate value of the effective
f x , ra s. nswer . Id F F. 8 130b h ff ."I Yle .rom Ig.. , tee ectlve
(This is more than ten times the secon- yield at a slant range of 4,000 yardsdary gamma-ray dose determined pre- from a low air burst I MT explosion is 4viously for the same conditions, but the x IQ4 KT (or 40 MT). Since only 50relative values will change with varia- percent of the total yield is derived fromtions in total and fission yields and fission, a correction factor of 0.5 mustheight of burst.) be applied. The fission product gamma-
ray dose is
';: -
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 379
10 0-2
5
2
-310 0
...~;::-. 5W:)(/)(/)
E 2(/)a~ 10 0-4
W(/)0 5 5a>-<tCt:
~ 2~~;3 I 0-5...U:)a 50Ct:a.zQ 2(/)(/)
l1: 16' 0-6
5
2 2
102 10-70 2.000 4,000 6,000 7,000
SLANT RANGE FROM EXPLOSION (YARDS)
Figure 8. 130a. Fission product gamma-ray component of the initial nuclear radiation doseper kiloton fission yield as a function of slant range from a nuclear
explosion, based on 0.9 normal sea-level air density.
--
380 INITIAL NUCLEAR RADIATION
D." = 0.5 x 3.2 x 10-4 X 4 X 104 In adding the doses, it should be re-= 6.4 rads. called that I rad of neutrons may not be
biologically equivalent to I rad ofThe total InitIal nuclear radIatIon dose IS
(§ 8 64)gamma rays ..D=D +D +D ,..,. .,= 0.12 + 1.8 + 6.4= 8.3 rads. Answer
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 381
109
5
2
108
5
2
107
5
2
106
~ 5u>Z
~ 20::! 105~
() 5~W>= 2
~ 104f=
U 5WlL.lL.W 2
103
5
2
102
5
2
10
5
2
I I 2 5 10 2 5 lif 2 5 10 2 5 104 2 X 104
ACTUAL EXPLOSION YIELD (KILOTONS)
Figure 8.130b. Effective yield as a function of actual yield for the fission productgamma-ray dose from a low air burst, based on 0.9 normal sea-level air
density.
~\;;j,,:---
382 INITIAL NUCLEAR RADIATION
I
5
2
1
5
2
10
5
2
10
5~
U)Z 2
~0 1~
:= 5
0~~ 2>-W 104>i= 5UW~~ 2W
103
5
2
10
5
2
10
5
2
1
1 2 5 10 2 5 K> 2 5 K> 2 5 104 2 X 104
ACTUAL EXPLOSION YIELD (KILOTONS)Figure 8.132. Effective yield as a function of actual yield for the fission product gamma-ray
dose from a contact surface burst, based on 0.9 normal sea-level air density.
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 383
MECHANISMS IN TREE: IONIZATION an adjacent material, the former will
8.133 Two basic interactions of nu- acquire a positive charge and the latter aclear radiation with matter are important negative charge. Consequently, a dif-in connection with the transient-radia- ference of potential will exist betweention effects on electronics (TREE); they the two materials. The most obviousare ionization and atomic displacement effect of this potential difference is a(§ 8.76). The charged particl.es, i.e., flow of current through an electrical cir-electrons and ions, produced by ioniza- cuit connecting the two materials, andtion eventually combine but the accom- this current will produce electric andpanying changes in materials may be magnetic fields. If there is matter in themore or less permanent. Some aspects space between the two materials, theof TREE depend on the relative dura- charge transfer may cause ionizationtions of the radiation pulse and the re- and hence conduction if there are localcovery time. If the pulse duration is the electric fields. Finally, if the charge ei-longer, the effect is observed promptly. ther originates or embeds itself in anThe magnitude of the effect is usually a insulator, a long-lived local spacefunction of the density of charged par- charge may result. The effects of chargeticles created by ionization and this is transfer may thus be temporary ordetermined by the rate of energy ab- semipermanent.sorption, i.e., by the dose rate. On the 8.136 The free charge carriers pro-other hand, if the pulse length is short duced during ionization respond to anrelative to the recovery time, the effect applied electric field by causing a netwill be delayed. The amount of damage drift current; there is consequently ais then usually a function of the total transient increase in conductivity. Thisenergy absorbed, i.e., the dose. Thus, effect is particularly important for capa-both absorbed dose and dose rate must citors, since the ability to retain or re-be considered in assessing the effects of store electrical charge is dependent onnuclear radiation on electronics; in the low conductivity of the dielectric. Inmany cases, the dose rate is the deter- an ionizing environment the increase inmining factor. The persistence of the the bulk conductivity results in a de-effect is related, in general, to the re- crease of the stored charge in a capaci-covery time. tor.
8.134 The chief manifestations of 8.137 In semiconductor devices,ionization include (1) charge transfers, such as transistors and diodes, there are(2) bulk conductivity increase, (3) ex- both positive (holes) and negativecess minority-carrier generation, (4) (electron) charge carriers, either ofcharge trapping, and (5) chemical which may be in the minority. The ef-change. These effects will be examined fect of ionization in producing addi-in turn in the following paragraphs. tional minority carriers is of prime con-
8.135 Charge transfer results from cern in many semiconductors and isthe escape of some electrons produced usually the most important manifesta-by ionization from the surface of the tion of ionization in TREE. Some of theionized material. If the net flow of these characteristics of semiconductor deviceselectrons is from the ionized material to depends upon the instantaneous con-
~384 INITIAL NUCLEAR RADIATION
centration of minority carriers in various material. However, the radiation doseregions of the device. Since ionizing required to produce a significant chemi-radiation creates large (and equal) cal effect is larger than would normallynumbers of positive and negative charge be encountered at a distance from acarriers, there is a large relative increase nuclear explosion where the equipmentin the concentration of minority carriers. would survive blast and fire damage.The electrical operation of the devicemay thus be seriously affected. The MECHANISMS IN TREE: ATOMICcurrent pulse observed in a semicon- DISPLACEMENTductor detector (§ 8.22) when exposedto radiation is an example of the effect 8.140 Another potential damageof excess minority carriers, although in mechanism of nuclear radiation in elec-this case it is turned to advantage. tronic systems involves the movement
8.138 When free charge carriers are of electrically neutral atoms. Such dis-created in insulating materials and are placement of atoms from their usualtrapped at impurity sites, sometimes sites in a crystal lattice produces latticepresent in such materials, many may not defects. A common type of defect arisesundergo recombination with the oppo- from the displacement of an atom from asitely charged carriers, which may be normal lattice position to an "intersti-trapped elsewhere. In these cases, the tial" position between two occupiedproperties of the material may be altered normal positions. The displaced atomsemipermanently, even though there is leaves behind an unoccupied normalno net charge in the material. This ion- lattice position (or "vacancy"), possi-ization effect is known as charge trap- bly some distance away. At least part ofping. Trapped charge can change the the damage to a crystalline materialoptical properties of some substances, caused in this manner is permanent.e.g., F (color) centers in alkali halides Since many electronic devices containand coloration of glasses. The trapped crystalline semiconductor materials,carriers may be released thermally, ei- usually silicon or germanium, displace-ther at the temperature of irradiation or ment damage is of special concern forby increasing the temperature. In either TREE.case, the resultant creation of free carri. 8.141 Fast neutrons, in particular,ers is manifested by an increase in con- are very effective in causing atomic dis-ductivity and sometimes by the emission placement. The total number of defectsof light (§ 8.24). (temporary and permanent) generated
8.139 As a result of the recombina- by a neutron depends on its energy.tion of electrical charges, sufficient en- Thus, a 14-MeV neutron (from a ther-ergy may be released to disrupt chemi- monuclear weapon) produces about 2.5cal bonds. The material may thus suffer times as many defects as a I-MeV neu-a chemical change which persists long tron (roughly the average energy from aafter the charged particles have disap- fission weapon). For neutrons of a givenpeared. This chemical change may be energy (or energy spectrum) the numberaccompanied by permanent changes in of defects is determined by the neutronthe electrical and other properties of the ftuence, and the changes in the proper-
TECHNICAL ASPECTS OF INITIAL NUCLEAR RADIATION 385
ties of a semiconductor material are di- (number of defects present) to the dam-rectly related to the total number of age remaining after a long time is calleddefectso Thus, the neutron fluence is an the "annealing factor"; it depends onimportant consideration in assessing the observation time as well as on thedamage to a semiconductor caused by temperature and the electrical conditionatomic displacemento of the material 0 The maximum number
8 142 S f h d f d of defects created at early times follow-.orne 0 tee ects produce ..
0 mg a fast-neutron burst IS frequentlyby the dIsplacement process are perma- .
t t t th f f IImpor an 0 e per ormance 0 e ec-nent but others are temporary 0 The tem- t . t Th .
I0 0 romcs sys ems. e maxImum annea -porary defects are annIhIlated by re- . f t f t . d.
t th..0 0 0 mg ac or 0 a componen mIca es ecombInatIon of the vacancY-InterstItIal
k d th t t be t I t d.0 0 pea amage a mus 0 era epairs, loe., by the movement of an m- b th t d .
f th t00 a ove e permanen amage I aterstltlal atom into a vacancy, by com- t . t to t f t o
...0 0 .componen IS 0 con mue 0 unc Ion.
bmatlon wIth pre-exIsting lattice de- 8 144 Th I tt.d d b.e a Ice amage cause y
fects, or they may eventually escape t o d'
I t d d th Ia omlc ISp acemen egra es e e ec-from a free surface of the material The, 0 .0
d I d o f f tncal characterIstIcs of semIconductorsgra ua Isappearance 0 some de ects .00 I d .by IncreasIng the number of centers forIS cale "annealing" and the rate of 0 .0 .
I ., ..trappIng, scatterIng, and recombInatIonannea mg can be Increased by raIsIng f h 0 Th 0 ,
tT f 0 0 c arge camerso e Increase m rap-
the temperature. he degree 0 dls- .0pIng centers results m removal of charge
placement damage in a crystalline 0d th b d th..0 carrIers an ere y ecreases e cur-
semIconductor Increases rapIdly with t fl Th dd OtO I tt .ren ow 0 e a Ilona sca enng
time, reaches a peak, and then decreases t d th bOI Ot f th..0 cen ers re uce e capa I I Y 0 e
as annealIng becomes IncreasIngly ef- h .t th h thc arge carrIers 0 move roug e
fective. The annealing process may lead 0 d t t . I FoII th0 0 semlcon uc or ma ena 0 ma y, e
to eIther an Improvement or further dd ' tO I bo t 't d0 .0 a Ilona recom ma Ion cen ers e-
degradation of the irradIated material, th t . d o h . h th 0.crease e Ime unng w IC e mmor-
because m some cases thermally stable ot h .01 bl fI Y c arge carrIers are aval a e or
defects may result. These defects may I t . I d t. Th I t ff t0 e ec rIca con uc 10no e as e ec,be more or less effective than the unst- . th d d I.f t. f th .o
t.0 0 I.e., ere uce lelmeo emmonyable ones m changIng a partIcular prop- 0 0 th t . rt t f t . carrIers, IS e mos Impo an ac or m
erty, d .0 h f f .
etermlmng t e per ormance 0 a seml-
8.143 Annealing processes fall conductor device in an environment ofroughly into two time frames. Rapid (or radiation that can cause atomic dis-short-term) annealing occurs in hun- placement. The minority carrier lifetimedredths of a second, whereas long-term is very roughly inversely proportional toannealing continues for times of the the neutron fluence at large values of theorder of tens of seconds 0 At ambient fluence that are likely to cause damagetemperature, annealing of temporary to semiconductors 0 At sufficiently largedamage will be essentially complete fluences, the lifetime becomes too shortwithin about half an hour. The ratio of for the semiconductor device to functionthe damage observed at early times properly 0
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