Identification and measurement of helium formed in beryllium by γ -rays

Abstract

For reasons explained elsewhere (Paneth 1937) helium is at present the only product of artificial disintegration which can be separated in bulk by chemical methods. Some time ago helium artificially produced by bombard­ment of boron with slow neutrons was spectroscopically identified and measured by a manometric device (Paneth, Glückauf and Loleit 1936). This reaction 10 5 B + 1 0 n → 7 3 Li + 4 2 He (1) had been investigated before by physical methods (Chadwick and Goldhaber 1935 a , b ; Taylor and Goldhaber 1935; Amaldi and others 1935). The fact of the production of helium was already known and chemistry could merely contribute to its quantitative study. There are, however, other processes of artificial disintegration where at present only microchemical methods seem capable of deciding between different possibilities. Since the discovery of the “nuclear photo-effect” by Chadwick and Goldhaber, it is known that γ -rays produce neutrons from deuterium and beryllium (Chadwick and Goldhaber 1934, 1935 c ; Szilard and Chalmers 1934); in this case the formation either of 8 Be or of 4 He is to be expected according to the equation 9 4 Be + hv → 8 4 Be + 1 0 n (2) or 9 4 Be + hv → 2 4 2 He + 1 0 n (3) Each reaction is equally possible, although Chadwick and Goldhaber give reasons why reaction (2) is probably the main one. Now if reaction (3) occurs to any considerable extent, then it should be possible to detect the helium by gas-analytical methods, while in the case of formation of a stable, or slowly disintegrating, 8 Be, no helium should be found.