Nuclear Tracks, Vol.]2, Nos I-6, pp.5-17, ]986. Int.J.Radiat.Appl. Instrum., Part D Printed in Creat Britain.

0 l 9]-278X/8653.00+. UU Pergamon Journals Ltd.

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS

P.B. Price and M.H. Salamon

Department of Physics and Space Sciences Laboratory University of California, Berkeley, CA g'tTZ0, USA

ABSTRACT

A new source of natural tracks in mica has been discovered--due to recoiling atoms struck by alpha particles from radioactive impurities. These submicron tracks have a radiation-damage rate and thermal stability similar to that expected for a slowly moving monopole-nucleus bound state and are a valuable benchmark in an ongoing search for s|ow, supermassive magnetic monopoles. Plastic track detectors are being used to discover new modes of radioactive decay involving monoenergetic heavy ion emission. Bevalac studies show that plastic track detectors can be used to identify relativistic nuclei with Z/B from -6 to ~!20 with a charge resolution o Z < 0.25e. The charge resolution obtained is comparable to the irreducible limit set by fluctuations in energy loss and is consistent with that expected of a track-formation mode] based on the effects of both K-shell ionization and restricted energy loss. Advances in automation of scanning and measurements are opening up new physics applications requiring high statistics and resolution. Examples include searches for quark-nucleus complexes and anomalous mean free paths of projectile fragments produced in collisions at higher energies than are available at the Bevalac; analysis of thousands of tracks of ultraheavy cosmic ray nuclei to be collected on the NASA LDEF missions; and hybrid experiments such as the University of Chicago Spaclab II CRNE ultrahigh-energy cosmic ray experiment and the EXAH heavy anttnucleus experiment.

KEYWORDS

Mica; monopole; CR-3g; Rodyne polycarbonate; Cronar polyester; quark; anomalon; cosmic rays; antimatter; track detectors; exotic radioactive decay modes.

A NEW SOURCE OF NATURAL TRACKS IN MINERALS

While searching for tracks of magnetic monopoles in large crystals of muscovite mica that had been cleaved, etched, and realigned, Price and Salamon (Ig85a) discovered a population of natural tracks of length intermediate between those due to spontaneous fission of Z58U (total length ~Z0 jJm with the track-in-cleavage technique) and those due to recoil of alpha-decay daughters in the uranium and thorium series (total length ~0.01 ~m). Figure I shows the length distribution of the three types of tracks in a typical crystal. Figure 2 shows ( i) that the etch pits of intermediate length that intersect a cleavage plane are correlated with the densities of fission tracks and alpha- recoil tracks in samples from a variety of locations and (i i) that the source of these etch pits is less resistant to thermal fading than either of the other two types of tracks. We attribute this new class of natural tracks to interactions of 8.8 HeV alpha particles from ZI2Po decay with A1 and Si nuclei In mica. Alpha particles from other daughters in the Z55U, Z58tJ, and Z52Th series have too low an energy to contribute. Our irradiations of muscovite micas with 4 to 10 MeV alpha particles have shown that etch pits with the observed length distribution are produced. Using the excitation function obtained in this experiment, together with recent cross section data of Seamster et el. (1984), and taking into account slowing of 8.8 HeV alphas in mica containing Th and U impurities in the ratio Th/U -- 4, we estimate a ratio of alpha-interaction to fission tracks -0.27 + 0.7 as

6 P.B. PRICE and M. H. SALAMON

10s~

,(i0 q- d_

o z 10 3 p- (_~

1 - - 10-2

[ i ]

ANCIENT TRACKS c<-recoi Is IN MICA

1.3 x 10 6

spont, f iss ion of 23BU .

212po o< + AI, 5i --~CN

177

I 0 - I 1 10

R(jJm) IN MUSCOVITE MICA

Fig. 1. Length distribution of natural tracks Intersecting a cleavage plane In mica.

0.5

0.4

~- 0.3

i ~ 0.2

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cak. vame 1 if no fading : I

0.5 i I I I

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calc. value if no fading

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MEAN FISSION TRACK LENGTH (pro)

F i g . Z.

z(s) Z(b)

Correlatlon of the density of a-lnteractlon tracks (a) with a-recoll tracks and (b) with the mean length of spontaneous fission tricks In muscovite micas.

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS /

Indicated by the horizontal arrow in Ftg, 2, A measured ratio less than -0 . I Indicates significant thermal fading has occurred, Crozaz et al. (1969) showed that alpha particles could produce short interaction tracks tn lunar or meteoritic samples but did not realize that they could also be produced as a by-product of alpha decay In terrestrial minerals,

SEARCH FOR SUPERMASSIVE MAC.g~ETIC MONOPOLES USING MICA

The survival of the Galactic magnetic field sets an astrophysical upper bound of -10-15 cm-Z sr-I s-I on the flux of monopoles, which are predicted by Grand Unified Theories to exist and tc have enormous mass (>IO 16 OeV/c2). Currently planned searches with large arrays of scintil lators or CR-3g track detectors wtll require years of operation to significantly exceed this l imit. Using the scheme shown In Fig. 3, Price et al. (1984) were able to set a l imit -50 times lower on the flux of monopolea, and we have recently reached a new limit ~1000 times lower (Price and Sslamon, Ig85b). Although the radiation damage rate of a monopole with the expected speed of ~0.OOIc (slow because of Its gigantic mass) would Be far too small to produce an etchable track In mica, a monopole should pick up a nucleus of ZTA1 with a mean free path of a few km in earth, and such a bound system wil l produce an etchable track due to elastic collisions of the A! nucleus with atoms in mica, subject only to the requirement that the system remain bound.

6ARE MONOPOLE ~ ~

VF D TH

~"3~ " / ~jkMONOPOLE ATTACHES TO <%/ " ~ HEAVY NUCLEUS,

• t~lgA IS CLEAVED AND ETCHED WITH CONC HF I ~ " ~

I • MONOPOLE-NUCLEUS TRAJECTORY REQUIRES >/4-FOLD COINCIDENCE LATTICE DAMAGED

AS M-N COMPOSITE [ ETCHPITS TRAVERSES

t, IUSCOVITE MICA

Fig. 3. Mica technique for searching for slowly moving supermassive magnetic monopoles predicted by Grand Unified Theories.

Figure 4 shows a simulated track of a monopole-nucleus bound state in mica. With a long etch time, a string of shallow etch pits on successive surfaces, four in this case, can easily be seen. (These etch pits delineate the trajectory of a nucleus with Z " 82 produced in the projectile fragmentation of a U nucleus at the Bevalac. ) We have scanned nearly 300 cmZ of large, transparent mica crystals with fission track ages >500 Hyr without seeing any examples of multiply aligned shallow etch pits such as in Fig. 4. The presence of alpha-interaction tracks In the ratio pa_lnt/PSF > O.l was a requirement we Imposed on each crystal In order to Insure that a track of a monopole-nuc]eus bound state would have survived.

NT 12:1/6-C

8 P.B. PRICE and M. H. SALAMON

Fig. 4. Simulated track of a monopole-nucleus bound state with v ~ O.OOic, photographed st five different depths on successive surfaces of mica cleaved into four thin sheets, etched, and reassembled.

Figure 5 compares the flux limits based on the mica scans with the best of several experiments of other types, with the Parker limit based on survival of the Galactic field, and with the missing mass limit based on the gravitational effects of supermassive monopoles on the expansion of the universe. It is true that the mica limit would be invalid if the proximity of a monopole to a nucleus were to result in catalyzed baryon decay (an effect predicted by Rubakov and Callan for Grand Unified Theories that predict proton decay), since the release of -938 HeV of energy would unbind the system. However, in that case monopoles could catalyze baryon decay inside neutron stars and white dwarf stars and increase their surface temperatures. The flux limit based on measured temperatures of neutrons stars and white dwarfs is so severe that it appears that either baryon decay is not catalyzed by Grand-Unified monopoles or that the monopole flux ls orders of magnitude lower than can be detected either by mtce or by reel-time detectors. Thus, the combination of the mice limit plus the neutron star/white dwarf temperature argument leaves l i t t le room for real-time detectors except as an independent check.

NEW RADIOACTIVE DECAY MODES INVOLVING HONOENERGETIC HEAVY ION EHISSION

Unaware of the predictions by Sandulescu et al. (1980) that radioactive decay processes Intermediate between alpha decay and spontaneous fission might occur among nuclei with Z z 88, Rose and Jones (1984) recently reported that ZZ3RI emits -30 HeY 14(: nuclei with a branching ratio -6 x IO-IO relative to alpha decay. Degradation of their detector due to the intense alpha background discouraged them and others from using conventional silicon particle telescope technology to search for other examples, all of which are predicted to have even smaller branching ratios than ZZ3Ra.

Soon after learning of the Rose and Jones discovery, several members of our group began using plastic track detectors with great success to discover new cases of radioactive decay with branching ratios as low as 10-12 relative to alpha decay. With Rodyne polycarbonate detectors and

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS 9

It, ~ "T

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0 t-

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10-11 i

10-12

10-13

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.......'"~ [Old mica limit :

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10 -5 10 -4 10 -3 10 -2 10 -1 1.0 Monopole velocity, #rn

Fig. 5 Negative results of mica searches for monopoles compared with other upper l imits.

sources of short- l ived radium isotopes produced st CERN's ISOLDE faci l i ty, in collaboration with Helge Ravn, Price et el. (1985) found that both ZZZRa and ZZ4Ra emit 14C. Barwick et ai, (1985) then discovered that Z:.3ZU emits Z4Ne ions with a branching ratio ~lO-IZ relative to alpha decay. In this work they exploited the lower sensitivity of Cronar, which tolerates a much higher background of alpha part icles than Rodyne ~dthout losing its ability to record Ne Ions. Figure 6 shows an example of a track of a 56 I'leV Ne ion from Z3ZU collected In Cronar. Efforts are In progress to detect heavy ion decay modes including emission of ions as heavy as 4BAr (using silica glass detectors) in a number of actinide nuclei,

Fig, 6. Photomicrograph showing one etch pit due to a 56 HeY Z4Ne ton striking a Cronar detector nearly head-on. About 3 x 106 alpha particles from 232U decay passed through this field of view.

l O P. B. PRICE and M. H. SALAd(ON

A group at Dubna has used a Melinex polyester track detector to detect Z4Ne emission from Z31Pa (Sandulescu et a l . , 1985) and from Z53U (Tretyekova et e l . , 1985).

It is very grat i fy ing to see how rapidly the solid state nuclear track detector techniques have taken over and extended this new area of nuclear physics. The results to date support the semi- quantitative models of Poenaru et al. (1985) and 5hi and 5wlateckl (1985), both of which view alpha decay, f ission, and heavy ion emission as s imi lar phenomena.

CHARGE RESOLUT ION OF PLASTIC TRACK DETECTORS: IMPL ICAT IONS FOR TRACK-FORMAT ION MECHANISMS

Under ideal conditions such as at an accelerator, plastic track detectors now rout inely I t ta in high charge resolut ion. Salamon et al. (1985) have demonstrated the excel lent separation of charges of re lat iv is t ic nuclei based on measurements of diameters of etch pits of tracks of normal ly incident nuclei. Figure 7 shows new data for fragmentation of 800 MeV/nucleon Au nuclei in Cronar polyester f i lms. The charge standard deviation for a single etch pit in Cronar is o Z '= 0.5e, almost a factor two better than in Rodyne. Figure 8 shows the response curves as a function of Z~/B for three detectors. Rodyne and Cronar show no var iat ion with depth of position in a given batch. With careful attention to the cure cycle, monomer pur i ty , mold condition, and addition of suitable compounds such as dioctyl phthalate and traces of anti-oxidants, CR-39 also shows superior performance.

FRAGMENTATION OF 800 MeV/N Au

69 71 73 75 77 ~ l J I i - - , ,

10

H.- Z

5

0 ~ 15

IN CRONAR

!

i 20 DIAMETER (Jam) 25

Fig. 7. Resolution of charges of project i le fragments from interactions of 800 MeV/nucleon Au nuclei in Cronar detectors.

2° F 1 0 -

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>~

Ir .2 I-

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, . ~ N E / / / , T / r'

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FRAGMENTS OF 0 0.8 A GeV Au

20 50 z * / ~

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100

Fig. 8 . Reduced etch rate as a function of Z*/B for several types of plastic track detectors. Only the most recent data are expl ic i t ly shown.

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS 1]

Salamon at el. (Ig84) have shown that, because of the large charge of the actinide nuclei (Z z 89), the physics of electron capture and loss dictates a charge state distribution in which a significant fraction of acttnlde nuclei are not ful ly stripped, even at relativistic velocities. By Introducing Cu stripper foils between plastic sheets, not only Is the ful ly stripped fraction increased but also the Ion charge states are completely uncorreleted between sheets, allowing the successful application of a " l /n" resolution-narrowing technique. They showed that the variance in etch pit diameter improved as I/n (where n is the number of etch pits whose diameters are included in the averaged value) tf 0.13 mm Cu foils are Inserted between 0.25 mm polycsrbonete detectors, whereas the variance Improves less rapidly than l /n for a pure plastic stack. When both radiative and non- radiative electron capture are taken into account, Cu is superior to a htgher-Z material like Pb.

Oartyge et el. (1981) have inferred from their data on small-angle x-ray scattering from latent tracks in silicate minerals that latent tracks are composed of "point" defects and "extended" defects and that the track etch rate Is a function of the linear densities of these defects. Tombrello (1984) has suggested that extended defects are generated by atomic K-shell excitations in the heavier elements of the mineral. The Auger decay of an Inner shell vacancy produces s multiply-charged Ion whose Coulomb interaction with the surrounding Ionized medium causes the atomic motion that produces the observed defect. This model explains both the morphology and frequency of extended defects.

In contrast to minerals, only semi-phenomenological models with limited predictive power exist for polymer detectors. The track etch rate is usually taken to be a monotonic function of restricted energy loss rate or simply of Z*/JB, but the fit is not particularly good when taken over a very wide range of values of Z and B. 5alamon et el. (1985) have recently shown that measured fluctuations of track etch rate can provide a test of models of track formation that bypasses the complex chemistry of polymer etching. The Ides is that energy loss (or restricted energy loss, K-shell ionization rate, etc.) is proportional to ZZ to f irst order, so that the fluctuation in energy loss depends on observed charge resolution simply as OE/AE = Zoz/Z, where o E Is the standard deviation of the energy loss distribution whose mean is AE. By measuring o Z for a great many values of Z and 8, and by correcting for measurement error and other factors contributing to a finite charge resolution, they were able to place constraints on oE, whose value depends on the model of track formation. Salamon et el. found that their data rule out models in which track formation depends upon total energy loss or solely upon K-shell excitation rates (the calculated charge standard deviations are too large) and also rule out pure restricted energy loss models with a reasonable value for the cutoff energy (the calculated charge standard deviations are too small). They conclude that their measured charge resolutions are consistent with a model in which Auger- decayed Inner-shell vacancies of C and 0 atoms interact with the surrounding outer-shell-ionized medium to produce defects of enhanced etch rate. In this respect, tracks in plastic and in minerals are similar.

AUTOHATED 5CANNING AND HEASUR£HENT

Heinrich and his co-workers (Trakowskl e t a l . , lg84) have pioneered in the development of an automated system that can be left unattended to locate and measure elliptical etch pit mouths at the top surface of a plastic detector. In a recent study of the fragmentation of 1.7 6eV/nucleon Fe nuclei normally Incident on a stack of 150 CR-3g sheets interleaved with 150 Ag foils, Helnrlch e ta l . (I985) used their automated system to locate the points of interaction of Fe nuclei that lost at least ! charge unit in collisions. The purpose of their experiment was to investigate with high statistics whether the interaction mean free path of s freshly produced fragment depended upon its age or distance downstream from the Interaction point, as had previously been reported by Friedlander et el. (1980) and other groups. Their null result in a study of 7500 charge-changing interactions provides strong negative evidence for the existence of "anomalons" and is a most impressive demonstration of the success of their automated equipment.

Price and Kriacher (1985) have developed a method using a commercial image processor to measure the three-dimensional structure of etched tracks with arbitrar i ly small cone angle, even if the tips from opposite surfaces have etched together to form a cylinder. Their method exploits the fact that in transmitted light any portion of the Image of a track is in best focus when it is darkest. A

12 P. B. PRICE and M. H. SALAMON

minimization algorithm produces a two-dimensional projected image of the three-dimensional etched track, as i l lustrated in Fig. g. The locus of points of maximum gradients in the image leads to a closed contour of the projected image. From the gradient imago the profile, length and width of the conical etched track are automatically determined. Both the Vlcom and DeAnza Image Processors can produce a new image consisting of the pixelwise minimum of two digitized images taken at two different focus settings in 1/30 second. Wtth a stepping motor to change the focus from the top to the bottom surface, a composite minimized image can be completed In l i t t le more than 1 second, if the focus Is reset every 5 pm or so.

Fig. g. Image analysis for a 1 6eV/nucleon U lon track at a 54" angle In a CR-?3 detector, using s 7X objective.

The remaining sections of this paper wil l deal with applications of plastic track detectors that will benefit from one or both of these automation techniques.

SEARCH FOR FREE QUARKS ATTACHED TO PROJECT ILE FRAGMENTS

A number of theorists have suggested that quark confinement may not be absolute. DeRujula et el. (1978) developed s detailed model of broken QCI) In which a deconftned quark would bind up to ten or more nucleons to i tself , and others have pointed out that the best conditions for pulling apart bound quarks would occur in s high energy collision of two heavy nuclei. Figure 10 shows the result of s negative search by Price et el. (1983), who used a stack of CR-3g(DOP) sheets to look for non- integrally charged fragments produced tn collisions of 1.85 GeWnucleon Ar projectiles within the stack. By measuring 16 successive etch ptt diameters for each of~he -1000 fragments it was possible to reduce the charge standard deviation from the single etch pit value of oz = 0.Z3e to oz = 0.ZSe/~/n = 0.06e and with this resolution to rule out the occurrence of charges differing by as much as 0.3 from an integer.

In an extension of the theory of OeRuJula et al, 6yulassy (198Z) has shown that quark-nucleus complexes could most llkoIy be produced only in nucleus-nucleus col l is ions at much higher energies than available at the Bevalac. We, as well as S.P. Ahlen and 6. Tarle', Intend to perform such a search at 8rookhaven next year using a beam of 20 GaY/nucleon S ions. Further, in collaboration with 0. Vsnderhaeghe, we plan s search at CERN using s beam of 250 6eV/nucleon S Ions. In both experiments we wil l interleave Pb target foi ls. Using an automated scanning and measuring technique it should be possible to expand the search for nonintegral charges by s factor of 100 to 1000.

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS 13

50

40

~ 3O Z U_I

> 20 W

I0

0

' ' 1 ' ' 1 ' ' 1 ' ' 1 ' ' 1 . . . . I ' ' l ' ' l ' '

I I I ,

9 I0 i I I I I I I i I I I I I

I 12 13 14 15 16 17 18 19

NORMALIZED CHARGE

Fig. I 0. Charge distribution of projecti le fragments from 1.85 6eV/nucleon Ar interactions in CR-3g(DOP) measured within 1.7 cm of their points of interaction.

ULTRAHIGH ENERGY COSMIC RAY DETECTOR ON SPACELAB II

In July, 1985, P. Meyer and D. Muller exposed a 5 mZsr instrument on Spacelab II for the purpose of measuring the energy spectra of the cosmic ray nuclei with charges from Z = 6 to Z6 at very high energies, from about 50 GeV/nucleon to several TeV/nucleon. Their instrument is described by Lamport et al. ( lg7g) . They used plastic scint i l lators for charge determination, and gas Cerenkov and transit ion radiation detectors for energy measurements. To improve the charge resolution, especially for the rarer elements for which the statistics wi l l be poor, we placed three layers of CR-39(DOP) at the top and three layers at the bottom of their instrument. Events wi l l be matched in the electronic and visual detectors by use of signals in mult iwire proportional chambera~ which provide crude position information. In order to insure correct matching and reject slow background particles it wi l l be necessary to measure essential ly al l tracks in the CR-3g. This task, which we plan to undertake in collaboration with W. Heinrich, wi l l require an automated scanning system.

SEARCH FOR EXTRAGALACT IC ANT IMATTER

Although Grand Unified Theories account for the apparent absence of antimatter in the universe in a natural way, their prediction that the proton is unstable has not yet been borne out by experiment, and one should not regard the apparent baryon-asymmetry of the universe as support for the correctness of Grand Unified Theories until much better l imits on the ratio of antlnuclet to nuclei in the cosmic radiation are set. Along with S.P. Ahlen (Boston University) and 6. Tarle" (University of Michigan), we are in the final year of construction of a very large, hybrid detector that wi l l use three types of p last ic - -sc in t i l la tors , Cerenkov radiators, and CR-3g(DOP) track detectors-- together with a dr i f t - tube hodoscope for t rajectory determination, to search for anti- iron and other heavy antinuclei in the cosmic rays. Assuming we f l y the instrument several times in balloons for a total t ime of 120 hours, we should collect enough events to establish whether antinuclei are present even at a level below 10-6 relat ive to normal nuclei. The instrument is described in detail by Ahlen et al. ( Ig8Z). To locate and measure tracks of nearly 107 Fe and other heavy nuclei wi l l require automated techniques.

14 P. B. PRICE and M. H. SAL~MON

LDEF HEAVY NUCLEI COLLECTOR (HNC)

Two large electronic detectors have recently been flown in space for over e year to study the ultraheavy cosmic rays. The HEAO-3 experiment resolved everr-A charge peaks up to Z ~ 56 and reported -60 events with Z • 70, Including one event that may have had Z • 90. Although the Ar ie l - 6 experiment had slightly poorer resolution, its charge distribution agreed rather well with that obtained on HEAO-3. It collected three possible acttntde (Z z 90) events. An Irish experiment on the currently orbiting Long-Duration Exposure Facility (LDEF) has recorded tracka tn Lexan of a somewhat larger sample of ultraheavy events, also with resolution not adequate to identify individual charges. To take the next big etap toward understanding the origin of cosmic rays, the HNC, to be carried on an LDEF reflight in May, 1987, has the goals of greatly Increased collecting power (~40 actinides) and charge resolution (o Z • 0.25e for Z up to -g6), which will provide abundances of all the charges 40 ¢ Z ¢ 96 and permit sensitive searches for hypothetical particles such as monopoles, superheavy elements, and "nuggets" of quark matter.

After the LDEF is retrieved and refurbished, it will embark on a new mission, carrying 45 trays of plastic detectors (geometry factor ~80 m2sr) Into a 57" orbit at Z45 n.ml. for a Z.5 year exposure at T < -10" C. Our group, advised by a steering committee, is in charge of the scientific design. 41 of the trays are optimized for Identification of nuclei with Z J~ 70 and ,t are optimized for :30 ¢ Z 70.

To achieve o z < 0.25e in detectors collecting tracks of particles entering from all directions and with all energies over a 2.5 year period is much more challenging than dealing with projectile fragments with a single velocity and angle of entry. Detector response depends on temperature and oxygen pressure, and stability of the latent track against fading demands a low average detector temperature. Thus, each stack will be sealed in 0.3 bar of air in a thermally isolated canister shielded from space with multllayer insulation and passive thermal coating, and each stack will contain an event thermometer that enables the temperature of that stack at the time of passage of each heavy nucleus to be determined by measuring the displacement of a sliding plastic track- recording sheet with respect to the stack. The sliding sheet is driven by a plunger actuated by a silicone liquid whose volume depends on temperature.

Each of the 41 actintde stacks will contain Z? sheets of Z50 I~m CU, 56 sheets of 100 ~m Cronar, 44 sheets of 250 j4m Rodyne, and 14 sheets of 250 ~m CR--39(DOP). Use of copper, with its high Z, in place of plastic of equivalent miss thickness reduces the fraction of events that fragment within the stack. As mentioned earl ier, Cu also serves to strip electrons from the heaviest nuclei. The Cronar will be the primary instrument for identifying acttnide nuclei. Bevalac calibrations show that at Z/B = gz it achieved a charge standard deviation o z = 0.Se for a single etch plt, compared with oz ~ O.ge for Rodyne. Referring to Flg. 8, one sees that it can be used to identify charges Z 76. Up to values of Z/B = I IZ one needs only to measure etch pit mouth dimensions rather than cone lengths. Another advantage of Croner wil l be seen tn the next section In which we discuss aging of latent tracks. Rodyne will be used to study the Pt- and Pb-group nuclei ( for the identification of which, etch pit mouth dimensions suffice) and to provide additional information about the actintdes. CR-39(DOP) will be used in the actlnlde stacks primari ly to identify fragments from the fission of actinlde nuclei. The 4 mld-Z stacks will contain Z8 Cu sheets, Z7 Cronar, Z7 Rodyne, and 47 CR- 3g(DOP). Here the CR-39 will play the major role In identifying the mid-Z nuclei. The task of measuring the tracks of some 104 nuclei with Z • 60 that pass through as many as ~100 sheets of plastic will be formidable. Obviously some degree of automation will be essential. Honte Carlo simulations (Drach et e l . , 1985) indicate that, barring some unknown effect not taken into account, the present st ick will permit the goal of o Z < 0.25e to be attained. During the next year NASA plans to select investigators to analyze data when the HNC mission returns in about 1990.

CHANGE OF CHEMICAL REACTIVITY DURING TRACK A61NG

For many years it has been known (Paterson, 1969;, Hanks et e l . , 1970) that the chemical reactivity of a latent track In texan Increases roughly logarithmically with the time delay between Irradiation and etching, which would cause "old" tracks to appear to have been produced by more highly ionizing particles than "young" tracks. This "aging" effect was usually taken into account by delaying etching

ADVANCES IN SOLID STATE NUCLEAR TRACK DETECTORS ]5

for several days to weeks after an experiment tn which high charge resolution was demanded. With the 2.5 year HNC mission in mind, we have re-examined the aging effect and conclude that it wil l seriously degrade the resolution of Rodyne unless etching is delayed for a time st least of the order of the mission duration. Figures !! and IZ show some of our recent data on the dependence of reduced track etch rate on aging time for Rodyne and Cronar, along with the original data of Henke et al. for LexIn. In Rodyne we find (1) that the effect Is greatest for particles with the highest Ionization rate, (Z) that maintenance of the samples in an Ar atmosphere during Irradiation and aging greatly reduces or eliminates the rate of etch rate increase, and (3) that the rate of etch rate increase is reduced if the irradiation and aging are done at low temperature (-16"C ~n our experiment).

Fortunately, in Cronar (Figs. I I and 1Z) and in CR-3g(DOP) we find no evidence for an increase in track etch rate with times up to ~IOZ days.

To study particles with Z/B between about 60 and 80, which includes those of the Pt and Pb group with 13 = I, we must use Rodyne. At an ambient temperature of -16"C, which will be typical for the HNC mission, Fig. 12 shows that particles with Z/B = gO wil l appear to differ in charge by ~l unit per factor ten in time when studied In Rodyne. Assuming that our ongoing aging studies show that the reactivity of latent tracks in Rodyne aged in air continues to grow logarithmically for times up to 2.5 years, we plan for the f i rst two years after the mission to concentrate on etching and measurements of tracks In the Cronar and CR-3g(DOP) sheets. After a two-year delay all tracks wil l have ages between 2 and 4.5 years, which translates for Rodyne st -16"C into a spread of only ~0.25e in charge. For the Irish experiment st i l l tn space on LDEF, the ambient temperature iS closer to ZO'C, and the spread in charge due to differences in track ages alone will be ~O.Se.

3O

T 2C ©

>

>

< C12

2 2 (..) b -

i i C.) D

rr-

, ] i / RODYNE A " /

C ] / ~ etched 70"C , ~ etched 40"C A / 440 MeV/N U

j~ .A~ / - - D r a c h e t al . 8 0 0 N e V / N Au

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9 MeV/N Ar

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4 CRONAR

800 MeV/N Au ~ ~ etched 70"C • ~ elched 40"C --~--~--~

0 j I

0.1 1 10 10 2

TIME BETWEEN IRRADIATION

AND ETCHING (DAYS)

I

10-3

Fig. 11. Reduced track etch rate as a function of time between irradiation and etching for Rodyne and Cronar samples aged in air at room temperature. Data are from Drach et al. (lg85b) and Hanks et al. ( lg70).

6 P.B. PRICE and M. H. SALAMON

"T ©

< rc i tJ F--

C~ l,r t_J

D~ rr-

4 -

_

i 1 l

J j l J

l ....... I

_ _ -+-+ -+- -

RODYNE

Drach (Au)

[ 22"C ~L air l,.- 1 6" C Z-~ [ 22"C @ AP ~.-16"C C%

0 l 10 10 2

-t

Henke t (At) +

X

10 3

TIME BETWEEN IRRADIATION AND ETCHING (DAYS)

Fig. t2. Reduced track etch rate as a function of time between irradiation and etching for Rodyne and Cronar samples Irradiated and aged in slr and tn argon at 2Z'C and at -16"C. Data for 800 MeV/nucleon Au are from Drach eta!. (1985b). Data for 9 HeV/nucleon Ar are from Hanks et el. (1970).

ACKNOWLEDGHENT

Our research Is supported in part by NSF, DOE, and NASA.

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