Primary cosmic ray electron spectrum in energy range from 300 MeV to 4 Be

Lheureux, J.

Meyer, P.

English

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RANGE FROM 300 MeV TO 4 BeV FROM 1964 TO 1966* 
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J. L'Heureux and P. Meyer 
Enrico Fermi Institute for Nuclear Studies 
The University of Chicago 
Chicago, Illinois 60637 
Laboratory for Astrophysics and Space Research 
EFINS Preprint No. 67-52 
June 1967 
* Research supported by the National Aeronautics and Space Administration under 
Grant NASA-NsG-144-61 and the National Science Foundation under Grant GP-4709. 
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a Also Department of Physics. 
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https://ntrs.nasa.gov/search.jsp?R=19670021898 2020-03-12T11:02:17+00:00Z
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THE PRIMARY COSMIC RAY ELECTRON SPECTR.LJM !N THE ENERGY 
RANGE FROM 300 MeV TO 4 BeV FROM 1964 TO 1966* 
J. L'Heureux and P. Meyer' 
Enrico Fermi Institute for Nuclear Studies 
The University of  Chicago 
Chicago, Illinois 60637 
A BS TRA CT 
Measurements of the primary electron spectrum were made during the summets 
of 1964, 1965 and 1966 using a balloon-borne counter telescope which was flown from 
Fort Churchill, Manitoba. SevemI balloon flights were carried out in each year in order 
to eliminate short term intensity variations. This paper addresses itself to two questions: 
(1) The determination of the energy spectrum of primary electrons in the energy interval 
from 300 MeV to 4 BeV; and (2) the long term intensity variations of the primary electron 
flux from 1964 through 1966. The energy spectrum was determined with improved accu- 
racy in 1966 and agrees with our previous results. Comparison of the electron spectra 
obtained in 1964, 65 and 66 shows that, within the errors of our measurements, there 
existed no long term intensity variation. The upper l i m i t  for the change of flux with 
respect to 1965 amounts to 20% in a l l  energy intervals studied. 
* Research supported by the National Aeronautics and Space Administration under 
Grant NASA-NsG-144-61 and the National Science Foundation under Grant GP-4709. 
Also Department of Physics. 
1. Introduction 
An understanding of the origin and the energy loss processes in  the galaxy 
and of the modulation in  the solar system of the primary cosmic rcy electron component 
requires accurate measurements of the electron energy spectrum and a study of the changes 
of this spectrum as a function of time. This has motivated our attempts to measure the 
primary electron spectrum with improved accuracy and to extend such measurements over 
many years. In this paper we wish to report our results which were obtained from 1964 
through 1966 on the spectrum of primary electrons in  the energy interval from 300 MeV 
to 4 BeV and on the intensity variation with time over t h i s  period. 
II. The Experiment 
Our measurements of the primary electron spectrum 
an energy-loss vs. total energy detector. We shall not describe - 
have been made with 
the instrument here, 
since details of the experimental method have been published elsewhere (L'Heureux 
and Meyer, 1965; L'Heureux, 1967). Seveml balloon flights were made with the same 
instrument in  each year in  order to eliminate the possibility of influencing the results 
by the occurrence of short term intensity variations. The dates of each flight and 
the atmospheric depth a t  which each measurement was made are summarized in  Table 1. 
Also given in  Table I are the Mount Washington and Deep River neutron monitor daily 
averages for the flight dates. An important aspect of a l l  1966 flights i s  the smaller 
depth of residual atmosphere. This led to more reliable results, particularly i n  the low 
energy portion of the spectrum. While in 1964 and 1965 the correction for secondaries 
amounted to as much as 50% of the total electron flux measured in  the low energy 
intervals, i t  never exceeded 30% in 1966. Fig. 1 shows the differential energy spectrum 
of a l l  electrons measured in  1966 under 2.5 g/cm 2 of atmosphere, together with the 
spectrum of atmospheric secondary electrons. The secondary spectrum was obtained 
from the electron intensity vs. altitude curves (see L‘Heureux, 1967) and agrees reason- 
ably well with theoretical estimates (Vena, 1967). 
- 
111. Results and Discussion 
a) The primary electron energy spectrum. 
Primary electron spectra are obtained by subtracting the atmospheric 
secondary electrons from the spectra measured a t  balloon altitude and correcting for 
energy loss by ionization and bremsstrahlung in the overlying atmosphere. In Fig. 2 we 
show the resulting primary spectra that were obtained from our measurements i n  1964, 
65 and 66. The 1966 spectrum i s  the most reliable in  the low energy range since 
secondary corrections were reduced by almost a factor of two as compared to our 
earlier measurements (L’Heureux, 1967). No difference appears between the spectra 
obtained in  the three years within the experimental errors. The power laws which 
give the best fit to a combination c f  these three spectra have the following forms: 
=31 electrons rn-2sec -1 ster-1 B ~ v - ~  dJ 
dE 
- 
250 MeV< E < 1.5 BeV 
dJ 
dE 
- = 54 electrons mm2sec -’ ster-’ BeV’l 
1.5BeV<E<4BeV 
b) Limits on the intensity variation of the primary electron flux from 
1964 to 1966. 
In order to investigate the time variation of the electron intensity i t  is  
of no advantage to compare the primary spectra of Fig. 2 since these contain uncer- 
tainties due to corrections for secondary electrons as well as due to extrapolations 
to the top of the atmosphere. Particularly, since the floating altitudes of the 1964 
-3- 
and 1965 flights were signficantly lower than in 1966, one has to be careful not to 
introduce (or remove) a possible time variation when one corrects for secondaries. 
We have found i t  advantageous to compare the intensities for the different years by 
grouping the data in three energy intewals and to compare the fluxes measured a t  
floating altitudes in each year w i th  a typical growth curve. In Fig. 3 we have super- 
posed the 1964 and 1965 fluxes measured a t  floating altitudes onto a growth curve 
constructed using a l l  the data from the 1966 flights. The solid line represents the 
contributions of secondaries in each energy interval. This flux of secondaries has 
two components which are respectively proportional to the depth and to the square 
of the atmospheric depth. To a l l  the data measured from 1964 to 1966 at  floating 
altitude we have fitted an intensity vs. depth curve which has the form - 
2 
flux = (Ax + Bx ) + Pexp(x/L) 
where the constants A and B are obtained from the calculation of Verma (1967). L i s  
the effective absorption length of primaries and depends on the shape of the primary 
2 spectrum. In the energy intervals considered here, L i s  of the order of 37 g/cm . 
Adjusting the parameter P, the best fits of t h i s  expression to the total electron fluxes 
are shown by dashed lines in Fig. 3. 
Examination of these altitude curves shows that a time variation, i f  i t  
exists, i s  smaller than the statistical fluctuations. We can put an upper l im i t  of 20% 
on this variation in a l l  energy intervals. These data alone do not yet provide a basis 
on which to critically evaluate modulation models. . *  
We are greatly indebted to H. Boersma for his invaluable help in carrying 
out th is  experiment and to R. Ekstrom and P. Mclntyre for their assistance in  the data 
ana I ys is. 
L'Heureux, J. 967, Ap. J. - 140, 399. 
L'Heureux, J. and P. Meyer 1965, Phys. Rev. Letters - 15, 93. 
Venna, S. D. 1967, Proc. Ind. Acad. Sci. (in press). 
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FIGURE CAPTIONS 
Fig. 1 The total differential electron energy spectrum (primary plus secondary) 
as measured a t  an average atmospheric depth of 2.4 s/cm in the 
summer of 1966. The contribution of atmospheric secondaries i s  also shown. 
The differential energy spectra of primary electrons measured in 1964, 
1 965, and 1 966. 
The vertical electron flux in three energy intervals, measured in 1964 
and 1965 a t  floating altitudes superposed on the 1966 growth curves. 
The solid line represents the calculated flux of atmospheric secondary 
electrons versus atmospheric depth. The dashed curve represents the 
sum of primaries and secondaries. It i s  normalized to the fluxes measured 
a t  floating altitudes. 
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Fig. 2 
Fig. 3 
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The primary cosmic ray electron spectrum in the energy range from 300 MeV to 4 BeV from 1964 to 1966

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