PHOTONEUTRON CROSS SECTIONS IN SILICON AND CALCIUM

Abstract

Using the dropping apparatus and dose monitoring method previously reported, the cross sections for the reactions Si28(γ, n)Si27 and Ca40(γ, n)Ca39 have been measured by detecting positron activities in the residual nuclei. The Si28(γ, n)Si27 cross section has a peak value of 21 mbarns at 20.9 Mev. and an integrated cross section to 24 Mev. of 0.070 Mev-barns. The threshold energy is 16.9 ± 0.1 Mev. The Ca40(γ, n)Ca39 cross section has a peak value of 15 mbarns at 19.3 Mev. and an integrated cross section to 24 Mev. of 0.065 Mev-barns. The threshold is 15.8 ± 0.1 Mev.A comparison of our results with the neutron yield measurement of Price and Kerst at 18 and 22 Mev. gives good agreement. A further comparison with the neutron yield work of Baldwin and Elder is made.The sharp discontinuity in (γ, n) yields using lithium γ rays observed by Wäffler and Hirzel is explained as a threshold effect. Integrated cross sections increase more or less smoothly with Z.In addition, improved values for the half-lives of the residual nuclei Si27 and Ca39 have been measured. These are 4.45 ± 0.05 and 1.00 ± 0.03 sec. respectively.