fllJl'PLE~IEJ'(TO AI. VOLli~lE VIII, flEHIE IX

Dl<~L XliOVO cnlEX1'O

K, 2. 19582° 'I'rimest.re

General Observationson the Composition of the Primary Radiation.

B. PETERS

Toia J nstiiute for Fundamental Research - BOil/1m!!

As a result of papers presented at this conference some of the informationrelating to the energy spectrum and composition of the primary radiation eanbe stated with greater precision than was possible heretofore.

1. - The Energy Spectrum.

In the non-relativistic region the work of the Minnesota and Iowa groupshas confirmed earlier findings that iX-particles (and presumably therefore othercomponents) can arrive, at geomagnetic latitudes above 54°, with energieslower than those expected on the basis of existing information on the earth'sfield. They also find that there is at these latitudes no sharp energy cut-offand that the intensity of the low energy component can undergo appreciablevariations. These phenomena reflect presumably not properties of the primarycosmic radiation but properties of the earth's field and of interplanetary space.

In the latitude sensitive region there appears to be general agreement thatall heavy components follow approximately the same power law; if E desig­nates energy per nucleon {including rest mass) the integral spectra can herepresented by NA = IlA!E1 5

• The proton component in the latitude sensitivepart until now was believed to follow a flatter spectrum. However this con­clusion was based on large and uncertain estimates about the contributionof albedo particles to the counting rate at the equator. The experiment ofMcDonald reported here, is to my knowledge, the only one in which simul­taneously the charge, the velocity and the direction of motion of singly chargedparticles has been measured near the top of the atmosphere. It indicates avery much larger albedo than was believed so far, and if it is accepted the

GE!'iERAL OBSERVATIO:" O~ TH}~ COMPOSITION OF 1'111; PItI\I AR Y UAlJIAT[():" 557

proton spectrum has within errors of measurement the same slope as theheavy components.

In the high energy region we have the earlier measurements of the protonand <x-particle flux at 1.6.10:1 GeV by LAL and of 4.5 .103 GeV by KAPLaN

and RITSON. For these two components the power law exponent of 1.5 remainsvalid. At approximately the same energy measurements by PARKER andmyself made in 1950 and measurements by FOWLER reported in 1956 indicatethat, also the ratio of heavy primaries to protons and «-paxticles is approxi­mately the same as in the latitude sensitive region. This implies that thepower spectrum with exponent 1.5 is valid at least in the region from 1 to5000 GeV per nucleon and is accurately the same for all primary components.

2. - The charge spectrum.

From the point of view of astrophysics one is trying to answer three prin­cipal questions and they must be answered in the order in which they are stated:

(i) What happens to the primary particles after they have been acce­lerated'? Do they escape from the galaxy or do they break up and finallydissipate their energy in collisions with interstellar hydrogen?

If their energy is dissipated in the interstellar gas an equilibrium distri­bution will be established between the injected nuclei and their break upproducts and one can show that under these conditions the ratio of N L (thelight nuclei Li, Be, R) to N,[ (the medium nuclei C, N, 0) must exceed unity.All results on the ratio B = NrjN,[ which were reported here and most ofthe results published earlier agree that for particles of energy 1.6 GeV pernucleon, R is significantly smaller than unity and that therefore the ultimatefate of particles accelerated in our stellar system is escape from the volumewhich includes the earth as well as the region where cosmic-ray particlesoriginate.

')'he second question is the following:

(ii) Does the relative flux of primary components incident on the earthrepresent approximately the composition of the beam at the time of acce­leration or has the composition been modified substantially by fragmentationin the transition from the source to the earth? This depends of course on theaverage amount of matter traversed and determines the storage factor ofradiation in the galaxy and the amount of energy which must be fed intocosmic radiation to keep it continuously at its present level.

The wide divergence in the values. of R reported by various investigatorsdoes not yet permit a reliable answer to this question. Most investigatorsfavor a value R = 0.35 for particles whose energies exceed 1.6 GeVjnucleonand this corresponds to the traversal of 4.5 gjcm2 of hydrogen. The greatest

558 R. PETERS

uncertainty in R comes from the correction for break up processes in theatmosphere above the point of observation. Higher flights make this correctionsmaller thereby reducing the uncertainty.

Our own work at Bombay was carried out with an exposure at 7 g/cm2,

a greater height than that obtained in earlier investigations. The investigationis not completed but indicates a value of R which may be substantially lowerand could be less than 10%.

One should expect that the lower the energy and the radius of curvatureof the primary particles, the larger the amount of matter traversed beforethey diffuse to the solar system. The corresponding decrease of R with energyis perhaps indicated in the results of McDoNALD who finds

R

0.320.180.17

Energy

> 400 MeV/nucleon> 600 » >}

> 1.2 GeV/nucleon

Our own observations tend to agree with McDoNALD which correspondsto less than 2.25 g/cm2 of hydrogen traversed by the average primary of energyabove 1.5 GeV/nucleon. But the question can not be considered settled untildifferent types of experiments and especially experiments carried out at greateraltitudes get into agreement.

An average amount of 4.5 g/cm2 of hydrogen corresponds to a lifetimeof t = (3 '10 6)/e years where e is the number of hydrogen atoms per cm3 alongthe trajectory.

The lower value favored by McDoNALD and by our group correspondsto half that time.

This leads to the third question:

(iii) Does the composition observed in cosmic radiation correspond tothe chemical composition of any stellar object which can be considered as apossible source for cosmic radiation particles?

It seems almost impossible to answer this question if we accept an averageamount of traversed matter of 4.5 g/cm2 which corresponds to two collisionmean free paths of iron in hydrogen. Even if the amount is half that muchthe finer features of the chemical composition particularly among the heavierelements should already be obliterated. The structural features which SCHEIN

found in the «harge speetrum of the heavy component, the apparent alternationin the abundance of odd and even nuclei and the correspondence with certainfeatures in the chemical composition of young stars leads one to suspect thatthe average amount of interstellar matter traversed and therefore the ratio Ris still smaller than the value one obtains if one extrapolates the measure­ments made under ] 5 g/cm2 of air to the top of the atmosphere.

(jENERAI. OBSERVATION OK THE COMPOSITION OF THE PRIMARY RADIATIOK 559

In summary, it seems safe to conclude that all primary components havethe same energy spectrum and that it can be expressed as a power law withexponent 1.5 ± 0.05. The proton spectrum below 10 GeV is perhaps somewhatflatter but this is not certain.

Cosmic ray particles produced in the galaxy escape after traversing onthe average not more than 4.5 gjcm2 of interstellar hydrogen. Certain expe­riments favor a considerably smaller value for the amount of matter traversed.Until this point is settled it is not clear whether detailed features of the com­position of the primary radiation can be related to relative abundances inthe chemical composition of various stellar objects.

INTERVENTI E DISCUSSIONI

~ H. MESSEU

I agree with PETERS that the light to medium ratio is not compatible with an equi­librium ratio of one; however, it is my belief that one of the first things on which wemust agree is that there is a substantial amount of Li, Be and B entering into theearth's atmosphere as primary cosmic radiation. As far as mechanisms of origin, storageand acceleration are concerned it is not necessary at the present stage to know whetherthe light to medium ratio is exactly t, l or the like. After all we do not know e,the density of matter in interstellar space to an accuracy of more than say a factorof five or ten.

L. BIERMANN:

Do I understand correctly, that the OpInIOn prevails, that the abundance of Li,Be, B is probably a fraction, but not a very small fraction of the abundance to beexpected in fragmentation equilibrium in interstellar space; that hence the storage ofcosmic radiation in the interstellar magnetic fields is somewhat less effective than onethought some years ago, such that for stationary conditions not 10-4 of the thermalradiation are required for the reproduction of cosmic radiation as in case of perfectstorage (which would be indicated by fragmentation equilibrium) but that rather afraction nearer to 10- 3 of the thermal radiation (averaging over the galaxy and longperiods of time) would maintain the present energy density of cosmical radiation inour galaxy.