A Comparison of ENDF/B-VI and UKFY3 Mass Chain Yields

Fission yield data are essential to many applications in nuclear technology, entering into all calculations of integral effects of the fission products. The fission yield is that quantity which characterizes the probability of a particular nuclide or mass to be formed after fission. The aim of this paper is to present a comparison of the mass chain yields contained in version VI of the U.S. Evaluated Nuclear Data File (ENDF/B-VI) and the U.K. evaluation, UKFY3, developed for the European Joint Evaluated File (JEF). In binary nuclear fission a heavy nucleus is split into two fragments (ternary fission is extremely rare, occurring in less than 0.2% of fission events 1). This process is non-deterministic with a large number of final states. In many fissioning systems there is a bimodal distribution of fission fragment masses with a predominance of fission fragments in the ~2/5 and ~3/5 mass range of the original nucleus. The fission fragments are formed in highly-excited, neutron-rich states resulting in the immediate release of prompt neutrons to relieve the neutron excess and prompt gammas to lower the excitation energy. These prompt neutrons account for about 99% of the total number of neutrons emitted from fission or an average of 2-3 neutrons. A nucleus so formed may be neutron-rich and unstable against the weak interaction, undergoing beta decay to further reduce the neutron number (alpha decay occurs in a few fission products; positron or elec- tron capture also occur in some low-yield fission products near stability). Each decay forms a "daughter" nuclide of the same mass (A) with an atomic number (Z) one greater than the parent. Fission products may undergo several beta decays along this mass chain before a beta-stable spe- cies is formed. A large number of fission products, then, can be formed either directly from fis- sion or through successive beta decays of parent nuclides. Beta decay often results in the formation of an excited state of the daughter nucleus. This nucleus can de-excite by emitting a gamma ray or a neutron (these delayed neutrons comprise the other ~1% of neutrons released in fission). These beta-delayed gamma rays and neutrons appear with the characteristic half-life of the parent, which can range from about 10 -3s for nuclides early in the decay chain to many years for nuclides at later stages in the chain. Due to the importance of fission nuclide yield data in applied fission calculations, evalua- tions of yield data have been undertaken in several countries. The most complete and widely- used evaluations are those compiled by England and Rider, 2,3 which is part of the U.S. Evaluated Nuclear Data File (ENDF/B-VI) and by Mills, James, and Weaver,4,5 developed for the European Joint Evaluated File (JEF). These files contain independent yield, cumulative yield, and mass chain yield information. A nuclide's independent or direct yield indicates the number of atoms of that nuclide produced directly from the fission process after emission of prompt neutrons but