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The T([alpha,gamma]) Li⁷ reaction and a standard Geiger-Muller counter for gamma ray flux determinations Morrow, Richard Alexander

Abstract

A study of the T(α,ɤ)Li⁷ reaction has been made at mean alpha particle energies of 1320, 560 and 350 Kev in a solid tritium-zirconium target. At the 1320 Kev energy the absolute 90° differential cross-section for transitions to the ground state has been found to be 1.89x10⁻³¹ cm² per steradian from analyses of spectra obtained in a 2¾"x4½” NaI crystal. At this energy the ratio of the differential cross-section for transitions to the first excited state to that for transitions to the ground state was found to be 0.446. This ratio was observed to remain the same at all detector angles. The excitation function roughly determined by additional runs at 560 and 350 Kev mean energy followed the form obtained by Riley of this laboratory in 1958. Angular distribution measurements at 1320 Kev gave the result that both the above mentioned gamma rays have the same angular dependence, this being 1-0.11 cos² θ + 0.39 cos⁴ θ. At 560 Kev mean energy their anisotropy was measured to be about 5%. A background from the target has been found to have an angular distribution at 1320 Kev mean alpha particle energy of 1+0.35 cos θ in laboratory co-ordinates. Both its excitation function, determined roughly from runs at 560 and 350 Kev, and its anisotropy followed a form in agreement with neutron counter measurements. It is suggested that this background is caused by neutrons from some still unknown reaction in the target. An investigation was also made of a standard thick-walled brass Geiger-Muller counter to determine its efficiency for photon energies from 0.5 to 20 Mev. A smooth curve was obtained showing efficiency increasing with gamma ray energy thus agreeing in form with previous experimental and theoretical results.

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