Experiment giving approximate energy spectrum of electron component in primary cosmic radiatio

Lheureux, J.

Meyer, P.

English

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EF I NS-65-56 
THE FLUX AND ENERGY SPECTRUM OF PRIMARY COSMIC RAY ELECTRONS* 
Jacques L'Heureux and Peter Meyer 
Enrico Fermi Institute for Nuclear Studies 
and Department of Physics 
University of Chicago 
*?his research was supported by the National Aeronautics and Space Administration under 
Grant No. NASA-NsG-144-61 Res. 
- .  
THE FLUX AND ENERGY SPECTRUM OF PRIMARY COSMIC RAY ELECTRONS* 
Jacques L'Heureux and Peter Meyer 
and Department of Physics 
University of Chicago 
Enrico Fermi Institute for Nuclear Studies 
I f 2  Since the discovery of primary electrons in the cosmic radiation 
3f4f5f 6 
attempts have been made to measure the flux of these particles at  various energies. 
The low intensity of the electron component causes great difficulties in obtaining 
enough events for good statistics and energy resolution and has so far prevented the 
determination of a reliable energy spectrum. Yet, the knowledge of the electron 
spectrum in  the vicinity of the earth i s  important for several reasons. A t  the present 
time, i t  appears likely that the primary electrons observed near the earth are of 
galactic origin. A determination of their flux and spectrum makes i t  possible to 
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investigate in detail their relation to the nonthemal galactic radio emission. Further- 
more, the modulation of their intensity and spectrum due to solar controlled mechanisms 
may be different from the modulation of heavier primary particles. Parker has dis- 
7 
cussed the possibility of velocity dependent modulation, which can be tested by in- 
vestigating the primary cosmic my electrons. - 
I The experiment which we shall discuss here gives an approximate energy 
/- 
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spectrum of the primary electron component. I t  was carried out in the summer of -. 
1964 at a period near solar activity minimum. This period i s  most advantageous for 
*Th is  research was supported by the National Aeronautics and Space Administration 
under Grant No. NASA-NsG-144-61Res. 
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investigations of low energy galactic particles since the effects of solar modulation 
are approaching their minimum. Two balloon flights were made at Ft. Churchill on 
July 22 and 29, 1964, both of them floating under abcut 4g/cm of residual atmos- 
phere, in order to measure the flux and energy spectrum of the electron component. 
A schematic cross section of the instrument which was used i s  shown in Figure 1. Ver- 
tically incident particles trigger a counter telescope consisting of the scintillation 
counten T and I and a gas-Cerenkov counter C. The geometry factor of the tele- 
scope is  1.08 cm sr. The gas counter i s  f i l led with 2.4 atm. of SF and has a threshold 
of about 8 MeV for electrow andl5 BeV for protons. The energy loss of the particles 
in Counter I i s  measured in  order to discriminate between singly and multiply 
charged particles. After passing through Counter I, the particles penetrate a layer of 
10.8 g/cm of lead, a plastic scintillation counter S, and enter a lead glass Cerenkov 
Counter II (Schott glass No. SF6-FA) with a total depth of 13 radiation lengths. The 
amount of Cerenkov light produced in this counter i s  measured by a pulse height an- 
alyzer. Provided the incident particle is  an electron and the photon-electron 
shower produced by this electron i s  absorbed in the lead glass, then the light output 
i s  a measure of the energy of the incident electron. The lead glass Cerenkov counter 
i s  completely surrounded by anticoincidence counters in order to discriminate against 
events in  which one or several particles emerge from the lead glass. A l l  events in 
which the anticoincidence counter A3 i s  fired are also recorded and high energy 
protons which penetrate the entire telescope are used for in-flight calibration of 
the Counters I and II. In order to further discriminate against high energy protons and 
interactions produced by protons in the lead glass, the scintillation counter S i s  
2 
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connected to two independent triggers which respond to singly charged minimum 
ionizing particles and to particles with 1.6 times minimum ionization or more, respec- 
tively. The equipment was calibrated in a monoenergetic beam of electrons* a t  en- 
ergies ranging from 700 MeV to 4 BeV. These calibration runs served to determine: 
a) the pulse height in Counter II as a function of the energy of the incident elec- 
trons; b) the resolution of the lead glass Cerenkov counter at various energies; 
c) the resolution of Counter I; d) the fraction of events in which any of the anti- 
coincidence counten was triggered as a function of the energy of the incident 
electrons; e) the probability that an incident electron produces more than one 
particle in Counter S as a function of energy; and 9 the efficiency of the gas 
Cerenkov counter which was measured to be 95 percent. The resolution of Counter I 
i s  37 percent for relativistic electrons (full width at  half maximum) and the lead 
glass Cerenkov counter has a resolution which varies slightly with energy. We ob- 
tained 32 percent a t  700 MeV to 21 percent a t  1.3 BeV. Using the calibration data 
i t  i s  possible to show that contributions by protons to the measured electron flux are 
less than 5 percent at any energy. In Fig. 2 we present the electron energy spectrum 
under 4.1 g/cm 2 of residual atmosphere which we obtained from 20 hours of exposure 
at ceiling (both flights combined) and which i s  based on 330 electrons with energies 
exceeding 180 MeV. 
In order to arrive at  the spectrum of the primary electrons, the contribu- 
tion of secondary electrons which are produced in the residual atmosphere above the 
equipment has to be estimated. We have, a t  the present time, based this estimate on 
~~ ~~ ~ ~ 
*We wish to express our gratitude to Dr. Livingston for making a beam of the Cambridge 
Electron Accelerator available to us and to Drs. Fotino, Hand and EngeIs for help in 
setting up our experiment. 
. -4- 
the following data: 1) The measured altitude dependence of the electron flux; 2) Sim- 
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plif ied calculations based on the cross section and multiplicity of r-meson production 
by cosmic ray protons, and 3) Measurements of the 
at aircraft altitude. 
2 
4.1 g/cm of atmosphere i s  shown as a dashed line in Figure 2. I t  should be noted that 
the estimate of the secondary electron flux i s  preliminary. We shall investigate it in  
greater detail in the future. 
,,-meson energy spectrum obtained 
8f9 
The resulting energy spectrum of secondary electrons under 
A t  energies exceeding abwt 500 MeV the secondary correction becomes 
small compared to the measured flux of electrons. In this region our results wi l l  be 
l i t t le modified by a more refined analysis. We, therefore, restrict this discussion to 
primary electrons with energies above 500 MeV. In Figure 3 the energy spectrum of 
primary electrons in the range frDm 500 MeV to 3 BeV i s  shown after applying correc- 
tions for secondariesand for energy loss (ionization and bremsstrahlung). We may 
represent this spectrum as a power law of the form 
-1 
dJ = 11 x x (m2 sec sr Bev) 
where E i s  measured in BeV. The exponent has an error of about - + 0.5. This spectrum falls 
1,2,5 
inside the limits set by earlier determinations and, if extrapolated beyond 4.5 BeV, 
3 
i s  compatible with the results of Agrenier et a1 . An extrapolation to lower energies 
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falls closely on the data which Cline e t  a1 obtained below 10 MeV. Although it i s  
interesting to note this point, i t  does not lead to any conclusions at  this time. In view 
of the large error limits in  the earlier data 
tion effects, even i f  present, 
this paper. 
1, 2 , it  i s  not surprising that solar modula- 
cannot be noticed in  the comparison with the results of 
. -5- 
v 
in the energy interval which we cover in this experiment, the exponent of 
the power law i s  appreciably smaller than would be expected on the basis of a col- 
10.) 1 
lision origin of the electron components in the galaxy . It i s  lower, although not 
incompatible, with the exponent deduced from observation of the frequency spectrum 
12 
of the nonthermal galactic radio emission. 
We are indebted to Dr. R. Vogt who participated in  the early phases of 
this experiment, to Mr. R. C. Hartman, who developed the gas Cerenkov counter, and 
to Messrs. T. Burdick, H. Boersma, W. Johnson, A. Hoteko and S. Avery for their 
participotion in  the development and construction of the equipment. We greatly 
profited from discussions with Dr. Y. A. Smorodin concerning the secondary electrons. 
Mr. R. Eckstrom provided the computer program for the analysis of the data. 
We gratefully acknowledge the support given us by the Skyhook program 
of the US. Office of Naval Research and the Royal Canadian Air Force at Ft. Chur- 
chill, Manitoba. 
Ref e re n ces 
1. J. A. Earl, P. R. Letters 6, 125 (1961) - 
2. P. Meyer and R. Vogt, P. R. Letters 6, 193 (1961) t - 
3. Agrenier, Koechlin, Parlier, Boella, Degliantoni, Dilworth, Scarsi, Sironi, 4 
P. R. Letters, 13, 377 (1964) -
4. J. W. Schmocker and J. A. Earl, P. R. 138, 300 (1965) 
5. P. S, Freier and C. J. Waddington (Private communication) 
- 
6. T. L. Cline, G. H. Ludwig and F. B. McDonald, P. R. Letters 13, 786 (1964) -
7. E. N. Parker, Interplanetary Dynamical Processes, lnterscience Publishers (1 963) 
8. M, Adams, C. D. Anderson, E. W. Cowan, Rev. Mod. Phys. 21, 72 (1949) 
9. L. T. Bamdzei, M. V. Solovlev, Z, I .  Tulinova, L. 1. Filatova, JETP 9, 1151 (1959) 
- 
- 
10. S, Hayakawa and H. Okuda, Progr. Theoret. Phys. (Kyoto) - 28, 517 (1962) 
11. V. L. Ginsburg and S, I. Syrovatskii, The Origin of Cosmic Rays, Pergamon Press 
(1 944) 
12. See fa. D. Walsh, F. T. Haddock and H. F. Schulte, Proceed. Fourth International 
Space Science Symposium, Warsaw 1963, p. 935, John WiIey and Sons, 
1964. 
Figure G p t i o n s  
Figure 1. 
Figure 2. 
Schemat ic  Cross-section of t h e  Counter  Telescope 
The ene rgy  spectrum of e lectrons as measured under  4.1 s/cm of 2 
residual a tmosphere 
Experimental Points 
Estimated f lux a n d  energy  spectrum of secondary  electrons.  
+ - - - - _ -  
Figure 3. The flux a n d  ene rgy  spectrum of primary e lec t rons  be tween 500 MeV a n d  
3.5 BeV. 
(corrected for secondary electrons a n d  energy  loss i n  the  atmosphere) 
, 
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The flux and energy spectrum of primary cosmic ray electrons

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