Halogen Delayed-Neutron Activities

A gas-flow technique has been developed to make rapid chemical separations of bromine and of iodine fission products from a solution of ${\mathrm{U}}^{235}$ irradiated with thermal neutrons. The active bromine or iodine is observed by its delayed neutron emission. Analysis of the decay curves so obtained has been made by graphical and by computer methods. Four delayed-neutron periods have been found in the bromine fraction and three in the iodine. Including several already well known, the half-lives and probable mass assignments are as follows: 54.5-sec ${\mathrm{Br}}^{87}$, 16.3-sec ${\mathrm{Br}}^{88}$, 4.4-sec ${\mathrm{Br}}^{89}$, and 1.6-sec ${\mathrm{Br}}^{90}$; 24.4-sec ${\mathrm{I}}^{137}$, 6.3-sec ${\mathrm{I}}^{138}$, and 2.0-sec ${\mathrm{I}}^{139}$. Shorter-lived halogens would not have been detected. The relative yields of each neutron activity in the order of decreasing half-life are, for bromine, 0.37:1.0:1.9:1.5; while for iodine they are 1.0:0.47:0.38 in the same order. If we assume that there are no other important contributors of delayed neutrons in this half-life range, the numerical values of yields for bromine may be compared directly with those for iodine with an uncertainty of a factor of 2.