atmospheric gamma ray angle and energy distributions from 2 to 25 MeV

Abstract

Angle and energy distributions are reported for gamma rays near the top of the atmosphere for energies of 2–25 MeV and for zenith angles of 0°–50° (downward moving) and 130°–180° (upward moving). These distributions were obtained with the University of California, Riverside (UCR) double Compton scatter gamma ray telescope flown on a balloon to 3.0 g/cm² residual atmosphere from Palestine, Texas, on May 13, 1975 (geomagnetic cutoff: 4.5 GV). Growth curves from 3.4 to 100 g/cm² of residual atmosphere are used to determine the downward‐moving atmospheric fluxes. Results are given for six energy intervals from 2–25 MeV and for five angle intervals from 0°–50°. The downward fluxes are in reasonable agreement with those of Schönfelder and Lichti (1975, 1976) at the lower energies and with those of Kinzer et al. (1974) at the higher energies, where our respective data overlap. The shape of the angular distribution is consistent with that of Lichti et al. (1975) and Thompson (1974) at higher energies. The absolute fluxes of upward moving gamma rays from 130°–170° at 4.2 g/cm² are in agreement with the semiempirical values of Ling (1975) but increase away from the vertical contrary to his predictions. Integrating over zenith angles of 10°–40° and 140°–170°, our ratios of upward moving to downward moving gamma ray fluxes are 12 ± 1 and 9 ± 2 for 3–10 and 10–25 MeV, respectively, in agreement with the theoretical model of Graser and Schönfelder (1977).

Fermi Large Area Telescope Observations of the Cosmic-Ray Induced

“…This instrument detected an enhancement of γ-rays when pointing towards the Earth, consistent with balloon flight measurements [4,5,6,7,8] and followed up by other satellite missions [9,10,11,12]. A more detailed study of the γ-ray emission from interactions of cosmic rays with the Earth's atmosphere was done by Thompson et al [13] using approximately 6700 γ-ray events from the Earth, recorded with the second Small Astronomy Satellite, SAS-2 .…”

“…The measurements from different instruments are not readily comparable since they have been taken in different points in the solar cycle and are also integrated over different areas of the Earth. Data points are taken from Ryan et al [7], Share and Murphy [12], Ajello et al [25], Frontera et al [26], Schwartz and Peterson [27], Churazov et al [28], Gehrels [29].…”

Gamma-ray detectors (>10MeV)

“…A time of fright requirement between the planes further reduces background by rejecting neutrons and upward moving y-rays. The telescope has been used to investigate both atmospheric background (Ryan et al, 1977) and cosmic sources (White et aL, 1980). A second-generation UCR Compton telescope, designed to operate over an energy range of 0.1 to 35 MeV, has been described by Zych etal.…”

Atmospheric gamma rays at geomagnetic latitudes of −29° and +43°

“…In his calculation the maximum near 120 ø is caused by gamma rays which originated mainly in the horizontal direction, but which were Compton scattered on the way from their source to the detector. comparison with FTS89 we quote the measurements of Sch6nfelder et al[1977], which give a ratio of about 9 for energies of 1.5-10 MeV, and ofRyan et al [1977], which obtained 12 _+ 1 in the energy interval of 3-10 MeV.Energy DistributionsFigure 6shows the differential energy spectra for vertical downward moving atmospheric gamma rays at float for zenith angles from 0 ø to 40 ø. These measurements were extrapolated to 1 g cm -2 for comparison with other data at 4.5 GV.…”

“…The zenith angle dependence of atmospheric gamma rays has been studied from balloons at energies above 35 MeV by Fichtel et al [1969] and Thompson [1974] and at energies below 25 MeV by Ling [1975], Sch6nfelder et al [1977,1980], Ryan et al [1977Ryan et al [ , 1979 Thompson et al [1981], and Dean et al [1989]. In general, the flux values at the top of the atmosphere are minimum at the zenith direction, gradually increasing toward the horizon, then peaking near 120 ø before decreasing again.…”