CV. Internal pair creation in Na²⁴

Abstract

A measurement has been made of the internal pair-creation coefficient of the 2.76 MeV. gamma-ray line from Na 24. This is found to lie very close to the curve of electric dipole transitions predicted by Jaeger and Hulme for Z-=0 and suggests that measurement of this coefficient for high energy transitions in light nuclei may provide a useful method of determining multipole order. On the assumption that the 2.76 MeV. line is of dipole origin, an energy level diagram has been constructed for the Mg "4 nuc]eus. INTRODUCTION.THE internal pair-creation coefficient (I) of nuclei of charge Z=0 and Z-----84 for electric dipole and quadrupole transitions was calculated by Jaeger and Hulme in 1935. Recently in a letter to Nature, Wang (1948) presented a curve showing the variation with energy of the coefficient for Z= 84 and for magnetic dipole transitions. In each case the authors gave the energy spectrum of the positrons which should be produced. The energy spectra of the positrons from ThB and Radon in equilibrium with their decay products have been investigated by a number of authors (Alichanov et al. 1936, and later work reviewed by Latyshev 1947;Bradt et al. 1946) and have shown good general agreement with the theory. In particular the variation with energy of the internal paircreation coefficient for both electric dipole and quadrupole transitions has been verified accurately by Aliehanov for the above two sources, (Z~.~84), over the energy range 1 to 2.62 MeV. The only absolute determination of I published however, appears to be that of Aliehanov for the 2.62 MeV. Hne of ThC". This agrees with the curve of electric quadrupole transitions for Z= 84 given by Jaeger and Hnlme.No precise measurement of I seems to have been made for a light element, and so the variation of I with Z predicted by the theory has not been verified. Consequently it has been thought worth while to examine the positron spectrum of Na ~4 which has a simple beta-decay followed by two gammarays in cascade of energies 1.38 and 2.76 MeV., (Wiedenbeck 1947, and see also references quoted) and to obtain an absolute measurement of the internal pair-creation coefficient for the 2.76 MeV. line. As a further