Fast neutron latitude variations in the atmosphere at solar minimum

Abstract

The fast-neutron flux in the atmosphere has been measured during solar minionurn. Data from ground level to about 4 g/cm 2 were obtained in a series of seven' high-altitude balloon flights. The flights were conducted between September 1964 and August 1965 at four locations between X _--8øN and X _--69øN, conventional geomagnetic latitude. The detector, a phoswichtype scintillator, was sensitive to neutrons in the range 1-10 Mev. A seven-channel pulseheight analyzer permitted the evaluation of the ,energy spectrum in this range. The neutron flux at the transition maximum increased from 0.17 ___ 0.02 cm -2 sec -• at X _--8øN to 1.9 ___ 0.1 cm -• see -• at X ----69øN. Neutron leakage from the top of the atmosphere was estimated by extrapolation. The magnitudes of the leakage fluxes, as well as the pole-to-equator ratio of 16, were in substantial agreement with the results of the diffusion calculation of Lingenfelter. The best fit to an inverse power law of the differential neutron energy spectrum was found to be independent of latitude within the limits o.f our experimental precision. Averaged over all flights, the best fit was for a special index of n ----1.05 ___ .15. This spectrum 'is harder than that calculated by Lingenfelter. Differences in the shapes of the neutron profiles of this experiment and of the calculation are consistent with the difference in spectrum.

Neutron measurements in space

“…For example, Haymes (1964) and Zych and Frye (1970) found good agreement with Newkirk's theoretical predictions for the counting rate versus atmospheric depth above 20 g cm-z. Furthermore, the measured neutron energy spectrum (Holt et al, 1966;Mendell, 1971 ;Jenkins et al, 1971) agree better with the Newkirk than with the Lingenfelter (1963) spectrum. The extrapolation factors used can easily be obtained from the figures in Newkirk's paper.…”

“…These measured values were then extrapolated, where necessary, to the top of the atmosphere, converted to a geomagnetic latitude of 42 ~ using the best known latitude dependence, normalized to sunspot minimum (the maximum in cosmic-ray intensity) and then converted to a neutron leakage flux F where the leakage flux is the number of neutrons moving upward per unit area per second I actual a neutron current density) (Appendix A-1). For compari- (1971) [5] Haymes (1964) [61 Zych andFrye (1970);Zych (1968) [7] Jenkins et al (1970,1971) [8] Intrilligator (1968) [9] Baird and Wilson (1966) [10] Klumpar and Lockwood (1972) [11] Bhatt and Parikh (1970) [12] Albernhe and Talon (1969) [13] Holt et al (1966);Merker et al (1972) a Assumed Lingenfelter (1963) spectral shape in measured energy interval. b Only the 42~ measurement extrapolated, see text.…”

“…Although in some measurements in the 1-10 MeV range it is difficult to determine the exact method for neutron spectrum unfolding, it must be remembered that the major contribution to the counting rate of the detector arises from neutrons below 3 MeV (Jenkins et al, 1971;Holt et aL, 1966;and Merker et al, 1972). Since only a few channels of pulse height data were available in these experiments the energy response functions from 1 to 10 MeV are very broad.…”

“…From 1 to 3 MeV the neutron energy spectrum and flux is not so well known. The measurements of Holt et al (1966) andJenkins et al (1970) yield 0.082_+0.003 neutrons cm-2s -1 for 1 <E<3 MeV. If the spectral index is y ~-1 for E< 3 MeV, then the spectrum must break rather sharply near 3 MeV.…”

Calculations of neutron flux spectra induced in the Earth's atmosphere by galactic cosmic rays

“…by Lingenfelter [1963]. * The Measurements of Holt et aL [1966] were made at X = 42 0N during the latter part of 1964 using a detector sensitive to neutrons in the 1-to 10-M6V range.…”