Published by the Association Pro ISSI

No 11, November 2003

INTERNATIONAL SPACESCIENCEINSTITUTE

Cosmic Rays

In a deep cave some people havebeen caught since their early child-hood. They are chained down in a way that they even cannot turntheir heads around. They can onlysee shadows on the walls of theirinconvenient shelter, which arecast by a f ire blazing in the back-ground. The shadows stem fromobjects of unknown form and ma-terial carried by some servants.During the years they have giventhe shadows names and they inter-pret them as the reality.

One of these cave dwellers is ableto shake-off his chains and leavethe cave. His eyes get dazzled bythe light of the Sun at f irst, butafter a while he becomes able tosee all the wonderful objects thatcast the shadows.

And again he names them all andcalls them the reality. But, upon hisreturn to his pitiable colleagues,he is far from being welcome: hisview of reality has been revolu-tionised by his stunning experi-ence outside and has nothing moreto do with the prisoners’ view ofreality.

So far Plato’s cave parable. It tellsus about the relation between ourideas and the objects behind them.We are in a similar situation likethe cave dwellers when it comes toexploring the universe. What wecan see with the best of our sophis-ticated technical means are noth-ing more than shadows of the real-ity out there and, like the prisonersin the cave, we have to contentourselves with the images on thewall. But – in contrast to Plato’sparable – an unexpected second

fire lights up in the backgroundcasting additional shadows fromthe unknown objects onto the wallof our cave. This second fire arethe cosmic ray particles that reachus from the depth of the universe.Apart from the electromagneticspectrum, where astronomical ob-servations have taken place sincemankind started looking at thestars, the cosmic rays are inde-pendent and complementary mes-sengers from violent processes inthe universe. That is why cosmicrays are such a fascinating topic,which is still in our days rich ofmysteries.

This issue of Spatium is a shortsummary of articles edited by theInternational Space Science Insti-tute in several of its fascinatingpublications as outlined on theback-cover. We are convinced thatour readers will enjoy these in-sights into the mysteries of cosmicrays.

Hansjörg SchlaepferZürich, October 2003

2SPAT IUM 11

Impressum

SPATIUMPublished by theAssociation Pro ISSItwice a year

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President Prof.Heinrich Leutwyler,University of BernPublisherDr.Hansjörg Schlaepfer,legenda schläpfer wort & bild,CH-8185 WinkelLayoutMarcel Künzi,marketing · kom-munikation, CH-8483 KollbrunnPrinting Druckerei Peter + Co dpcCH-8037 Zurich

Front Cover: The remnants ofthe N 49 supernova in the LargeMagellanic Cloud, a source of cos-mic ray particles (credit: HubbleHeritage Team [STScI /AURA],Y.Chu [UIUC] et al., NASA). Thelower part shows data from theClimax Neutron Monitor, seeFigure 6.

INTERNATIONAL SPACESCIENCEINSTITUTE

Editorial

3SPAT IUM 11

The EarlyYears1

Cosmic ray studies now span anepoch of almost exactly 100 years.At the close of the nineteenthcentury, scientists using gold-leafelectroscopes to study the conduc-tivity of gases discovered that nomatter how carefully they isolatedtheir electroscopes from possiblesources of radiation they still dis-charged at a slow rate. In 1901 twogroups investigated this phenome-non, J. Elster and H.Geitel in Ger-many, and C.T. R.Wilson in Eng-land. Both groups concluded thatsome unknown source of ionisingradiation existed.Wilson even sug-gested that the ionisation might be“due to radiation from sourcesoutside our atmosphere, possiblyradiation like Roentgen rays or likecathode rays, but of enormouslygreater penetrating power.” A yearlater two groups in Canada, ErnstRutherford and H. Lester Cooke at McGill University, and J. C.McLennan and E.F.Burton, at theUniversity of Toronto showed that5 cm of lead reduced this mysteri-ous radiation by 30%. An addi-tional 5 tonnes of pig lead failed to reduce the radiation further.

In 1907 Father Theodore Wulf ofthe Institute of Physics of IgnatusCollege in Valkenburg, Holland,invented a new electroscope.Wulf ’selectroscope enabled scientists tocarry the search for the origin ofthe mysterious radiation out of thelaboratory, into the mountains,atop the Eiffel Tower and, ulti-mately, aloft in balloons.Assuming

that the radiation came from theEarth, they expected to f ind a rapiddecrease in the radiation as theymoved away from the surface.They did not f ind the decreasethey expected and in some casesthere seemed to be evidence that

the radiation actually increased.Intrigued by the conf licting re-sults obtained by Wulf and his col-leagues a young Austrian nuclearphysicist, Viktor Hess, obtainedsupport from the Austrian Imperi-al Academy of Sciences and the

Cosmic Rays

Figure 1Viktor Hess after a balloon landing in 1912

1) See reference list at the end of this article

Royal Austrian Aero Club to con-duct a series of balloon f lights tostudy the radiation. Hess obtaineda license to pilot balloons in orderto reduce the size of the crew andthereby increase the altitude towhich he could carry his electro-scopes. On 12 August 1912, usingthe hydrogen-filled Böhmen, Hessreached an altitude of 5,350 m.Carrying two hermetically sealedion chambers, he found that theionisation rate initially decreased,but that at about 1500 m it beganto rise, until at 5,000 m it was overtwice the surface rate. Hess con-cluded that the results of these ob-servations can best be explainedby the assumption that radiation ofa very high penetrating powerfrom above enters into the atmos-phere and partially causes, even atthe lower atmospheric layers, ioni-sation in the enclosed instruments.

On a voyage from Amsterdam toJava, Clay observed in 1927 a vari-ation in cosmic ray intensity withlatitude with a lower intensitynear the equator, thus establishingthat before entering the Earth’smagnetic f ield, the bulk of the primary cosmic rays were chargedparticles. In 1930 Bruno Rossishowed that if the cosmic rayswere predominantly of one chargeor the other there should be aneast–west effect. In the spring of 1933 two American groups,Thomas H. Johnson of the BartolResearch Foundation and LuisAlvarez and Arthur H. Comptonof the University of Chicago,simultaneously and independentlymeasured the east–west effect. Itshowed the cosmic radiation to bepredominantly positively charged.

In a series of balloon f lights in thelate 1930s, M. Schein and his co-workers used Geiger counter tele-scopes interspersed with lead ab-sorbers to determine that most ofthe primary particles were notelectrons, and hence protons weremost plausibly the dominant con-stituent.

In 1948 research groups from theUniversity of Minnesota and theUniversity of Rochester f lew nu-clear emulsions and cloud cham-bers on the same high-altitudeSkyhook balloon f light and dis-covered the presence of heavy nu-clei in the primary cosmic radia-tion. Further studies by manyother groups soon established thatessentially all of the elements be-tween H and Fe were present inthe cosmic radiation near the topof the atmosphere – including anoverabundance of the light ele-ments Li, Be, and B. Then in 1950 it was found that a significant frac-tion of the cosmic radio emissionwas synchrotron radiation – indi-cating the presence of highly rela-tivistic electrons throughout ourGalaxy including some discretesources as well as extragalacticsources. However, because of theirsmall abundance (1% of the inten-sity of cosmic ray nuclei) electronswere not directly detected in theprimary cosmic radiation until1962. These discoveries made itpossible to begin constructing re-alistic models of the origin andinterstellar transport of galacticcosmic rays.

As early as 1934, Baade andZwicky linked the appearance ofsupernovae with neutron star for-

mation and cosmic ray generation.Fermi in 1949 regarded cosmicrays as a gas of relativistic chargedparticles moving in interstellarmagnetic f ields. His paper laid thegroundwork for the modern theo-ry of cosmic ray acceleration andtransport. The close link betweenradio astronomy and cosmic rayswas conclusively established at thetime of the Paris Symposium onRadioastronomy in 1958. Thismarked the birth of cosmic rayastrophysics. The basic model ofthe origin of galactic cosmic rayswas developed by Ginzburg andSyrovatskii in 1964.

4SPAT IUM 11

The Nature of Cosmic Rays

Cosmic rays consist of electrons,neutrons and atomic nuclei, whichhave been accelerated to very highspeed. Their elemental composi-tion provides information onchemical fractionation in thesource region as well as some in-sight into the nature of this regionand of the propagation of cosmicrays in interstellar space. Cosmicray isotopes probe more deeply thenature of the source region andthe timescales of the injection and initial acceleration. Radioac-tive isotopes such as 10Be, 26Al, and36Cl reveal the temporal history ofcosmic rays in the disk and halo re-gions. The variation of the chargeand mass composition with energy– their energy spectra – can be re-lated to the acceleration processand to particle transport in theGalaxy. When improved measure-ments are available at ultrahighenergies it should be possible todetermine whether these particlesare of galactic or extragalactic ori-gin. At the highest energies thecosmic ray arrival direction mayalso indicate the approximate di-rection of the most powerfulsources.

The most remarkable feature ofcosmic rays is their energy spec-trum (Figure 2). From �109 eV to�1020 eV these spectra, over some10 orders of magnitude variationin intensity show a relatively fea-tureless power-law distribution.

At energies below a few GeV theinf luence of solar modulation be-comes important with significanttemporal variations at 1AU relatedto the 11- and 22-year solar andheliomagnetic cycles. At energiesof less than 40 MeV the oxygenspectra in Figure 2 show the pres-ence of so-called anomalous cos-mic rays as discussed later. Those

are partially ionised interstellaratoms accelerated at the solar windtermination shock. Near 10 MeVthere is a highly variable turn-upin the ion spectrum produced byparticles of solar/ interplanetaryorigin although the accelerationup to energies more than tens ofGeV was registered in some solarf lare events.

5SPAT IUM 11

Log

[dj/

dT

(flu

x/m

2 se

c st

MeV

)]

Log[T(eV)]5 10 15 20

Knee

Oxygen

All Particles

T–2.6

Low-EnergyTurnup

Anom-alousOxygen

Galactic U-High T

0

–10

–20

–30

Figure 2Energy spectrum of cosmic rays measured at the Earth.

The experimental limitation im-posed by the size of current detec-tor systems does not allow meas-urements of the cosmic-ray in-

tensity at energies greater than 3 x10 20 eV.Above 1012 eV the compo-sition is not well known; this is theregion where acceleration by su-

pernova shocks becomes diff icult.It is generally assumed that thecosmic ray “knee” at �1015 eV mayref lect a gradual transition in thecomposition to particles of in-creasingly higher charge. At ener-gies �1019 eV questions of galacticmagnetic confinement lead to theassumption that these particles areof extragalactic origin. At energiesabove 4 x 1019 eV even protonsshould experience significant de-celeration by the 3 K blackbodyradiation (the Greisen–Zatsepin–Kuzmjn effect). At energies of1012 –1014 eV there are smallanisotropies of � 0.1%, which arethought to be due to local effects.At this time there are no meaning-ful anisotropies observed at higherenergies except the ultra-high en-ergies �1018 eV.

Cosmic ray particles are mostlyhydrogen (87%), and some helium(12%) with diminishing amountsof carbon, oxygen, etc and of heav-ier elements (Figure 3). All are fullyionised. Electrons account for ap-prox. 1% of the cosmic rays. With a few exceptions, their chemicalcomposition corresponds to the el-emental abundances in our solarsystem. The exceptions (H, He areunder-abundant, Li, Be, B are over-abundant) yield important infor-mation about the matter traversedby cosmic rays. Isotopic abun-dances of galactic cosmic rays arevery similar to the isotopic compo-sition of the interstellar gas. Animportant exception is the largerratio of 22Ne/20Ne. The isotopiccomposition provides key infor-mation about the origin, acceler-ation and transport mechanisms of cosmic rays in our galaxy.

6SPAT IUM 11

He

CO

Ne

Mg Si

SN

NaAl

A CaTi Cr

Fe

Ni

P

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ClK

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V

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ativ

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bun

dan

ce

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01

10–1

10–2

10–3

10–4

10–5

2 4 6 8 10 12 14Neutrons

16 18 20 22 24 26 28

Be

Cosmic Rays

Solar SystemSolar System

Figure 3The relative abundance of He to Ni in cosmic rays (red line), and in the Solar System(blue line).

The Origins of Cosmic Rays

Cosmic ray particles arrive evenlyfrom all directions of the sky, butthis does not necessarily mean thattheir sources are evenly spreadaround us. More likely, they areconstantly def lected and scatteredby magnetic f ields in the galaxy,until any trace of their originalmotion is lost.

There are three categories of cos-mic ray particles according to theirenergy content:

Galactic cosmic rays come from out-side the solar system but generallyfrom within our Milky Way. Theyhave probably been acceleratedwithin the past few million years,and have travelled many timesacross the galaxy, trapped by the

galactic magnetic f ield; they havebeen accelerated to nearly the speed of light, probably by super-novae explosions. A supernova is astar of several times the mass ofour Sun, that has run out of the“nuclear fuel” of light elements,especially hydrogen, needed tokeep it shining. Its nuclear burninggradually converts the light ele-ments into heavier ones, and theheat it produces keeps it from col-lapsing under its own immenseweight. When the star can nolonger produce nuclear heat, itsuddenly collapses to a small vol-ume, releasing enormous amountsof gravitational energy. Much ofthat energy is spent in a grandexplosion as shown in Figure 4,

blowing the star’s outer layers out to space and creating therebya huge expanding shock front. It is this shock front which is be-lieved to accelerate the cosmic ray particles to nearly the speedof light.

Anomalous Cosmic Rays

The discovery of a totally newcomponent of nuclear radiation inthe heliosphere – now called an-omalous cosmic rays (ACR) – hasgreatly expanded our understand-ing of pickup ions, particle acceler-ation and our knowledge of neu-tral atomic abundances in the localinterstellar medium. The story be-gins with satellite measurementsat 1 AU of the proton and heliumspectra as the heliospheric modu-lation of galactic cosmic rays de-clined towards solar minimumcondition in 1971–1972. A spec-trum of helium nuclei with ener-gies below �100 MeV per nucle-on appeared which could not beaccounted for by either the solarmodulation of helium of galacticorigin or its isotopic composition.

It soon became clear that this an-omalous helium component wasalso accompanied by anomalousnitrogen and oxygen. More recent-ly, there has appeared evidence forC, Ne, Ar and H. Garcia-Munozand collaborators showed that theanomalous component of He ismodulated 4He with no spallation3He and, therefore, must have a“local” origin. Fisk, Koslovskyand Ramaty proposed the mostsuccessful model to account for the anomalous components. Assketched in Figure 5, interstellarneutral atoms with high ionisationpotentials enter the heliosphere,undergo single ionisation by solarultra-violet radiation in the innersolar system and–by charged parti-cle pickup in the solar wind – arecarried outward to the solar windtermination shock, where they are

7SPAT IUM 11

Energy level [eV ] Cosmic ray type;origin and acceleration mechanism

Below the knee Galactic cosmic rays I (GCR I) ;E � 3�1015 eV

galactic origin, diffuse shock acceleration in the shock waves of supernova remnants (SNR)

Above the “knee” Galactic cosmic rays II (GCR II) ;3 �1015 � E � �1018 eV

galactic origin, second stage acceleration of GCR I by shocks

Above Extragalactic cosmic rays;E � �1018 eV

acceleration in extragalactic shocks?

accelerated. Some of these acceler-ated nuclei – now possessing highmagnetic rigidities since they aresingly charged – propagate inwardto undergo solar modulation, alongwith the galactic low energy cos-mic ray nuclei.

Solar Cosmic Rays

Most of the time the cosmic raysarriving at Earth are of galactic oreven extragalactic origin. Howev-er, from time to time, the Sun isalso a source of cosmic rays. Evi-dence for the acceleration of ener-getic protons, ions and electrons inclose association with solar f laresand coronal mass ejections is pro-

vided by direct observations of theenergetic particles in interplane-tary space or at Earth, and by thedetection of various neutral radia-tions produced in interactions ofthe accelerated particles with thesolar atmosphere and the solarmagnetic f ield. Solar protons, ionsand electrons of energies extend-ing into the 100 MeV region aredirectly measured by space borneinstruments. Solar protons withenergies above �500 MeV/nucle-on become increasingly rare andtherefore must be detected by con-tinuously operating ground-basedcosmic ray detectors. The neutralradiations with an unambiguouslink to interactions of acceleratedparticles in the solar atmosphereare radio- and microwaves, soft

and hard X-rays, gamma rays andneutrons. On a long-term aver-age, cosmic ray ground level in-tensity enhancements due to rela-tivistic solar particles occur aboutonce per year, while events withlower-energy particles are muchmore frequent. There is a distinctdependence on solar sunspot ac-tivity, with particle events beingmore frequent and having a largerf luence from around two years be-fore to about four years after asolar maximum. However, there isevidence that the most energeticevents do not occur right in themaximum phase of solar activity.

In the current paradigm, solar en-ergetic particle events are general-ly classif ied as “impulsive events"or “gradual events". Impulsiveevents are characterized by thepresence of type III radio bursts,that are radio emissions generatedby streams of energetic electronsinjected from the Sun into inter-planetary space. Furthermore highenergy protons, ions, and electronsare emitted by the Sun and acceler-ated in the deep corona in regionswith temperatures �107 K. Ingradual events, the solar cosmicrays are produced by shock acceler-ation in the high corona and in in-terplanetary space near the Sun.An expanded classif ication ofsolar cosmic ray events includesalso the high-energy solar parti-cles, which cause nuclear reactionsin the solar atmosphere leading tothe emission of gamma rays andneutrons.

The Sun is the most powerful par-ticle accelerator in our neighbour-hood and, therefore of fundamen-

8SPAT IUM 11

Figure 4This artist’s illustration shows a supernova explosion (at left) and a conical sec-tion of the expanding cloud of ejected material. Atoms are torn from the brownishbands of “dust" material by shock waves (represented by orange rings). The shocks inthe expanding blast wave then accelerate the atoms to near light speeds f iring theminto interstellar space like cosmic bullets. (Credit: M. DeBord, R. Ramaty and B. Ko-zlovsky [GSFC],R.Lingenfelter [UCSD] , NASA)

tal physical and astrophysical in-terest. Ongoing research concen-trates on the nature and the prop-erties of the accelerationprocesses. One of the key instru-ments supporting these efforts isthe Reuven Ramaty High EnergySolar Spectroscopic Imager(RHESSI), a NASA Small Ex-plorer mission launched on Feb-ruary 5, 2002 with contributionsfrom the ETH Zürich and the PaulScherrer Institute, Würenlingen.Its primary mission is to explorethe basic physics of particle accel-eration and explosive energy re-lease in solar f lares.

DetectingCosmic Rays

The earliest cosmic radiation dataacquired on a worldwide basiswere obtained from Wilson cham-bers on ships. Routine monitoringof the cosmic radiation was initiat-ed in January 1932 with the op-eration of ionising chamber atHafelekar, Austria. Wilson cham-bers respond to muons generatedby incident high-energy protons;however only nucleons greaterthan about 4 GeV have sufficientenergy to generate a muon cascadecapable of penetrating the atmos-phere and reaching the Earth’ssurface. Thus it was desirable todevelop a detector that would re-spond to lower energy nucleons aswell as being relatively easy tomaintain. In the 1950s John A.Simpson, at the University ofChicago, invented and developedthe neutron monitor and foundthat the Earth’s magnetic f ieldcould be used as a spectrometer toallow measurements of the cosmicray spectrum down to low pri-mary energies. The magnetic lat-itude of a particular neutron monitor determines the lowestmagnetic rigidity of a primary thatcan reach the monitor, the so-called “cut-off rigidity". The sta-tion’s altitude determines theamount of absorbing atmosphereabove the station and hence theamount of absorption of the sec-ondary cosmic rays (the higher thestation, the higher the countingrate). By using a combination oflead (to produce local interac-

9SPAT IUM 11

Figure 5Sketch of the concept for the production and acceleration of the anomalous cosmicrays.

Solar SystemMotion

NeutralsHe, O, Ne from LIS

Solar Modulation

Single Ionization

InterplanetaryAcceleration

Earth Orbit

Sun

tions), paraffin or polyethylene (tomoderate or slow down the neu-tron component) and multipleslow-neutron counters, Simpsongreatly increased the counting ratein his monitor design.After the in-vention of the standard neutronmonitor this type of detector be-came widely used for cosmic raymonitoring all over the world.One of the longest uninterruptedobservations is made by the Cli-max Neutron Monitor of the Uni-versity of Chicago, see Figure 6.

As the counting rate of high ener-gy particle counters increases withincreasing altitude high elevationresearch stations became attrac-tive locations for neutron moni-tors. In 1958 the f irst neutronmonitor was installed on the Jung-fraujoch where it is operated by the

International Foundation HighAltitude Research Stations Jung-fraujoch and Gornergrat (HFSJG)with headquarters in Bern, Figure 7.

A second detector was installed in1986. The effective vertical cut-off rigidity at Jungfraujoch is 4.5GeV. Hermann Debrunner, theformer President of the Pro ISSIassociation, has started his careerthere as a student and served lateras the foundation’s director.

10SPAT IUM 11

Figure 7Two neutron monitors are installed at the Sphinx laboratory of the Jungfraujoch atan altitude of 3570 m. The 18-IGY-Detector is in continuous operation since 1958,while the Standard 3-NM64-neutron monitor operates since 1986.

Mo

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ean

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/Ho

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75

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t N

umb

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Climax Corrected Neutron Monitor ValuesSmoothed Sunspot Numbers 1950–2002

1950 1960 1970 1980 1990 2000Monitor Data Smoothed SSN

Figure 6This plot shows data from the Climax Neutron Monitor operated by the Universityof Chicago. The cosmic rays show an inverse relationship to the sunspot cycle becauseSun’s magnetic f ield is stronger during sunspot maximum and shields the Earth fromcosmic rays.

Propagation of Cosmic Raysin the Earth’sEnvironment

The magnetic f ield and the atmos-phere form two powerful protec-tive layers against the cosmic radi-ation on the Earth’s surface. Themagnetic f ield acts both as a shieldand as a giant natural spectrometerfor cosmic ray particles. If the par-ticles possess energy, which isgreater than the magnetic cut-offenergy, they will cross through themagnetosphere and reach theupper layers of the atmosphere.But if their energy is insufficient,they will have a tendency to followthe magnetic lines of force, withwhich they move “easily", due totheir lack of energy, and succeed inreaching the poles. It is the reason

why the areas located near thepoles receive radiation in higherquantities than near the equator,which is better protected by theEarth’s magnetic f ield, see Figure 8.The second protective layer is theEarth’s atmosphere. Upon arrivingin the upper parts of the atmos-phere, the cosmic ray particles in-teract with the atoms, which theyencounter. As shown in Figure 9these collisions create new cas-cades of particles that producefurther successively lower energynuclear disintegrations. This nu-cleonic cascade process caused byprimary cosmic particles can bedetected at the surface of theEarth by means of fast neutrons

11SPAT IUM 11

Co

smic

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Par

ticle

Flu

x in

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nits 1.9

1.8

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1.3

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1.1

1.0

0 10 20 30 40 50 60 70 80

Magnetic Latitude

Solar Minimum

Solar Maximum

Figure 8Illustration of the cosmic ray latitude curve. The minimum value occur at theequator and the maximum values at polar latitudes. The values are relative since thenumbers vary with altitude and solar activity.At high latitudes the cosmic ray f lux lev-els off, since the shielding effect of the Earth’s atmosphere becomes larger than thecosmic ray cut-off by the magnetic f ield.

Electromagneticor “Soft”Component

Mesonor “Hard”Component Nucleonic Component

Energy Feeds across from Nuclearto Electromagnetic Interactions

Small Energy Feedback fromMeson to Nucleonic Component

Incident PrimaryParticle

Low EnergyNucleonicComponent(DisintegrationProductNeutrons Degenerate to “Slow” Neutrons)

N, P = High Energy Nucleons

n, p = Disintegration Product Nucleons

= Nuclear Disintegration

NP

N P

P

P

N

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n

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Figure 9Schematic Diagram of a Cosmic Ray Shower.An incident cosmic ray particle inter-acts with the atoms at the top of the atmosphere. Due to its high energy it disintegratesthe atoms producing a cascade of electromagnetic radiation, of muons and nucleons,of which the neutrons are detected by the neutron monitors.

produced in the atmosphere, whichare counted by neutron monitors.As shown in Figure 10 cosmic radia-tion represents at ground levelonly a small part (11%) of the ion-ising radiation to which an indi-vidual is commonly exposed. Nat-ural land-based sources exposeeach of us to an average total doseof 2.4 mSv per year (see box),though with significant variationsaccording to regions. The largerpart of the sources is a gaseous de-scendant of natural uranium, i.e.radon, which concentrates in en-closed areas such as houses. Thereis also soil-based radiation, comingfrom surface rocks, granite in par-ticular, which contain radioactiveelements such as uranium, datingfrom the formation of the planet.The water and foods,which we in-gest also contain radioactive ele-ments. Finally, there is also the in-ternal radiation, i.e. coming fromwithin our own bodies, namelyfrom the potassium 40 which isnaturally present in our tissues.

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Telluric Radiation

Cosmic Radiation

Industrial Activities

Mean Exposure of Medical Origin

Inhaled (Radon) or Ingested Radioactive Elements

1.5 mSv

1.3 mSv0.1 mSv0.4 mSv

0.5 mSv

Figure 10Sources of radioactivity as a percent-age of the dose received by an averageindividual. (UNSCEAR, United Na-tions Scientif ic Committee on the Ef-fects of Atomic Radiations)

The SievertAssessing the biological risk of ionising radiation

When it comes into contact with matter, ionising radiation collides withthe atoms comprising it. During these interactions, it releases a part or all of its energy. The absorbed dose (expressed in Gray) is defined bythe ratio of this released energy over the mass of the matter. A Graycorresponds to one Joule of energy released in one kilogram of matter.

In order to have a single unit which expresses the risk of the occurrenceof the stochastic effects associated with all possible exposure situations,physicists developed an indicator known as the “effective dose”, ameasurement using the Sievert (Sv), named after the Swedish physicistwho was one of the pioneers in protection against ionising radiation.

The effective dose is calculated from the dose (expressed in Gy) ab-sorbed by the various exposed tissues and organs, by applying weightingfactors which take into account the radiation types (alpha, beta, gamma,X, neutrons), the means of exposure (external or internal) and the specificsensitivity of the organs or tissues.

Cosmic Raysand Climate

Changes in the Earth’s climate aregenerally attributed to internalcauses, e.g. volcanic dust in the at-mosphere, atmospheric/ocean os-cillations like the El Niño South-ern Oscillation, and to externalcauses like variations in the Earth’sorbital parameters and in the Sun’sluminosity. Recently, it was sug-gested that the Earth’s cloud coveris correlated with the intensity ofgalactic cosmic rays. The idea isthat droplet formation is inf lu-enced by ionisation of atmospher-ic molecules by cosmic rays. Theionisation could potentially inf lu-ence optical transparency of theatmosphere, by either a change inaerosol formation or an inf luenceon the transition between differ-ent phases of water. As thesehypotheses are still controversialand highly speculative they arecurrently the objects of intenseworld-wide research activities.

Cosmic Raysand Life

Since the cosmic radiation increas-es with increasing altitude, it couldbe expected that people living athigh altitudes suffer more fromcosmic rays than those at sea level.For example at Denver, Coloradoat an altitude of 1’600 m the expo-sure to cosmic radiation is twicethan for people living near sealevel. Medical records for thesepopulations exist for a significantnumber of years thus making itpossible to search for health prob-lems that might be correlated to differences in the exposure tobackground cosmic radiation.When these studies are done, asurprising paradox is found: thepopulation living at mountain alti-tudes is generally healthier and haslonger life span than the popula-tions living at sea level. The con-clusion is that other factors mustbe dominant and exposure to cos-mic radiation at altitudes wherepeople live does not appear topresent a health hazard.

Biological effects of ionising radi-ation in living cells begin with theionisation of atoms. Ionising radia-tion absorbed by living cells hasenough energy to remove elec-trons from the atoms that make upmolecules of the cell. When theelectron that was shared by thetwo atoms to form a molecularbond is dislodged by ionising radi-ation, the bond is broken and thus,the molecule falls apart. This is a

basic model for understanding ra-diation damage.When ionising ra-diation interacts with cells, it mayor may not strike a critical part ofthe cell. We consider the chromo-somes to be the most critical partof the cell since they contain thegenetic information and instruc-tions required for the cell to per-form its function and to makecopies of itself for reproductionpurposes. Obviously, cosmic radia-tion has been an intrinsic bound-ary condition for the evolution oflife on Earth since its very begin-nings and nature, therefore, had todevelop eff icient repair mecha-nisms, which are able to repair cel-lular damage – including chromo-some damage.

Public concern and legal regula-tions that became effective in Eu-rope in May 2000 address the radi-ation risk and the individual doseassessment of airline crewmem-bers. On the average, at f light alti-tude radiation dosage is of theorder of 10 microsievert / hour. Ofcourse, this value varies with alti-tude, latitude, and solar activity,and it must be interpreted in com-parison with the average naturaldosage. Thus, approx. 400 hoursper year at f light altitude wouldlead to the equivalent of the natu-ral yearly radiation load of around4 mSv. Interested f light passengerscan evaluate the expected dosageusing publicly available programsas e. g. SIEVERT (http://www.sievert-system.org/).

Above the Earth’s f irst protectivelayer, the atmosphere, the radiationexposure increases strongly. Theassessment of the radiation risk of

13SPAT IUM 11

astronauts for example during theongoing construction of the Inter-national Space Station is essentialto safeguard their health. The as-tronauts’ suits provide some pro-tection against cosmic rays espe-cially against low energy particlesand in areas, where human organsmore susceptible to radiation haz-ards are, like for example the heador the heart. In any case, however,the layout of such a suit remains acompromise between its protec-tive eff iciency and the residualmobility required by the astronaut.The US Space Shuttle orbit theEarth with an inclination of28 ° and receive between 42 to 62 Gy per day, while in the 51°inclination orbit of the Inter-national Space Station the radia-tion dose is higher due to thecoverage of higher geomagneticlatitudes and amounts to 90 to 150 Gy per day.

Outside the Earth’s second protec-tive layer, the magnetosphere, theradiation loads become still moreimportant.When it comes to plan-ning a mission to the other planetslike for example to Mars the radia-tion protection of the astronautson their long-duration f lights is akey issue. In order to systematical-ly broaden the available radiationdatabase the European SpaceAgency regularly equips its space-craft with the Standard RadiationEnvironment Monitor (SR EM).The SR EM, a high-energy particlecounter for electrons with up to 6MeV and protons up to 300 MeVhas been jointly developed byContraves Space AG, of Zürichand the Paul Scherrer Institute ofWürenlingen.

Cosmic Raysand Matter

The disintegrating effect of cos-mic rays on molecular bonds notonly effects living cells but ofcourse also anorganic materials.Due to their large areas and theirlong exposure time the solar pan-els of spacecraft are especially en-dangered by cosmic rays.A contin-uous degradation of their powerproduction efficiency or even theircomplete breakdown may be theresult of cosmic rays. In addition,electronic micro-components areprone to radiation damages lead-ing to malfunctioning of equip-

ment or even to the complete lossof the spacecraft. The most widelyknown disturbances and failuresare those of the Canadian ANIKsatellite on January 20/21, 1994and of the Telstar satellite on Janu-ary 11, 1997, which both have beenattributed to cosmic ray effects.The commercial losses amountedto more than 135 millions US$.

14SPAT IUM 11

Reference List

1) Excerpt from Frank B. Mc. Donald and Vladimir S. Ptuskin, GalacticCosmic Rays,The Century of Space Science, Kluwer Academic Publish-ers 2001; p. 677–697

2) Excerpt from John A. Simpson,The Cosmic Radiation,The Century ofSpace Science, Kluwer Academic Publishers 2001; p.117–151

3) Extracts from M. A. Shea and D. F. Smart; Fifty Years of Cosmic RadiationData; Cosmic Rays and Earth; Space Science Series of ISSI; KluwerAcademic Publishers 2000; p. 229–262

4) NOAA Satellite and Information Services,http://www.ngdc.noaa.gov/stp/SOLAR/COSMIC_RAYS/cosmic.html

5) The International Foundation High Altitude Research Stations Jungfrau-joch and Gornergrat (HFSJG); http: //www.ifjungo.ch

6) Extracts from M. A. Shea and D. F. Smart; Cosmic Ray Implications forHuman Health; Cosmic Rays and Earth; Space Science Series of ISSI;Kluwer Academic Publishers 2000; p. 187–205

7) The Sievert System; http: //www.sievert-system.org

15SPAT IUM 11

Conclusions

Cosmic rays are the messengersfrom distant regions in our galaxyand beyond. Solar cosmic rays re-leased occasionally in associationwith energetic processes at theSun, provide first-hand informa-tion on astrophysical processes re-sponsible for particle acceleration.Anomalous cosmic rays reveal newinsight in the dynamic processes inthe heliosphere and its interactionwith the local interstellar medium.Apart from the electromagneticspectrum, where the classical as-tronomical observations take place,cosmic rays open a second windowto the universe providing an attrac-tive complementary diagnostic toolfor our understanding of the pro-cesses in the universe.Although theresearch of cosmic rays began near-ly 100 years ago much is still un-known especially with regard totheir origins and the mechanismsproviding the particles nearly thespeed of light. That is why the cos-mic ray research continues to beone of the most fascinating adven-tures of modern space science.Thisarticle is based on publications ofthe International Space ScienceInstitute on cosmic rays,which arestrongly recommended for furtherreading. In addition I am verythankful to Erwin Flückiger, In-stitute for Physics, University ofBern and Rudolf von Steiger, In-ternational Space Science Institutefor their his most valuable contri-butions.

Hansjörg Schlaepfer

Figure 1115,000 years ago a star in the constellation of Cygnus exploded – the shockwavefrom this supernova explosion is still expanding into interstellar space! Credit: J.Hester (ASU), NASA

The Publications of ISSI as of October 2003

All books are published by Kluwer Academic Publishers, Dordrecht, Boston, London

Reports from Workshops

ISSI Scientific Reports

Volume 1 Analysis Methods for Multi-Spacecraft Data, 1998

Volume 2 The Radiometric Calibration of SOHO, 2002

Space Science Series of ISSI

Volume 1 The Heliosphere in the Local Interstellar Medium, 1996

Volume 2 Transport Across the Boundaries of the Magnetosphere, 1997

Volume 3 Cosmic Rays in the Heliosphere, 1998

Volume 4 Primordial Nuclei and Their Galactic Evolution, 1998

Volume 5 Solar Composition and its Evolution – from Core to Corona, 1998

Volume 6 Magnetospheric Plasma Sources and Losses, 1999

Volume 7 Corotating Interaction Regions, 2000

Volume 8 Composition and Origin of Cometary Materials, 1999

Volume 9 From Dust to Terrestrial Planets, 2000

Volume 10 Cosmic Rays and Earth, 2000

Volume 11 Solar Variability and Climate, 2000

Volume 12 Chronology and Evolution of Mars, 2001

Volume 13 The Astrophysics of Galactic Cosmic Rays, 2002

Volume 14 Matter in the Universe, 2002

Volume 15 Auroral Plasma Physics, 2003

Volume 16 Solar System History from Isotopic Signatures of Volatile Elements, 2003

Review of the Space Science Achievements in the 20th Century

The Century of Space Science

Two volumes, 1846 pages, 2002