An empirical formula for the average number of fission neutrons

Abstract

>The instantaneous'' fission energy E/sub F/, which is equal to the difference in energies between the fissioning nucleus and the fission fragments, is distributed between the kinetic energy of the fission fragments E/sub K/ and the excitation energy E/sub x/ of the fission fragnrents, which in turn is distributed between the prompt neutrons and prompt gamma quanta: E/sub K/ + E/ sub x/ E/sub K/ + nu E/sub nu / + E/sub gamma /. Here, nu is the number of neutrons; E/sub nu / is the averagc energy for the removal of a neutron from the fission fragment, and E/sub gamma / is the total energy of the prompt gamma quanta. Within a narrow region of nuclei with Z = 90 to 100 and A = 230 to 250, the average number of neutrons D can be considered as a linear function of Z and A: nu =C/sub 1/Z+C/sub 2/A+ C/sub 3/. Here, C/sub 1/, C/sub 2/and C/sub 3/ a re constants, and C/sub 3/ takes into account whether the nucleus is even or odd. Experimental data on the thermal neutron fission of the six odd nuclei Th/sup 229/ , U/sup 233/, U/sup 235/, Pu/sup 239/, Pu/sup 241/, and Am/supmore » 241/ were used to derive the following empirical relation: nu /sub T/ =0.1894 Z + 0.007 A- 16.60 + delta /sub nu /. Here delta /sub nu (d nu /dE/sub n/) delta m -- 0.09 xi (where xi = plus or minus 1 for the odd-odd, and even-even nuclei, and zero for nuclei with odd A). Values of nu /sub T/ were computed for the isotopes of Th, Pa, U, Np, Pu, Am, and Cm, and agree within 3% when compared with known experimental values of nu /sub T/. (TTT)« less