JAERI 1287 JNDC Nuclear Data Library of Fission Products Fir

10 JNDC Nucleai Data Library of Fission Products JAERI 1287
values when evaluated for the whole nuclides but not the individual. The adopted data of Qp,
£>and Er are given in Ref. 39 for these 87 nuclides as well as those with unknown data.
2.4 Fission Yields
Data of independent and cumulative yields of fission products are very important in
calculation of the decay power. The experimental data are, however, far from complete,
because the measured yields are a very complicated function of independent yields of the
preceding nuclides in the decay chain including many short-lived nuclides. Thus it is usually
difficult to compare and evaluate many measured data on yields in a region of interest even
for the case of thermal-neutron fission of 235 U. Generally the individual yields have been
evaluated empirically by using an appropriate theoretical assumption.
The fission yield data included in the present library are based on those compiled by
Rider and Meek 41) . When a new isomer is found or some isomer adopted proves not to exist,
the independent yield given by Rider and Meek should be revised as will be described in this
section. The cumulative yields have been calculated from the independent yields using the
branching ratios adopted in our decay scheme evaluation. The fission yield data thus evaluated
are those for thermal neutron fissions of 235 U, 239 Pu, 241 Pu and 233 U, fast neutron fissions of
235U; 238U> 239Pu and 232Th) and high-energy neutron fissions of 235 U and 238 U.
In the compilation work in 1977, Rider and Meek 41) postulated that the independent
yields were expressed as the symmetric Gaussian charge distribution with even-odd effects
in proton and neutron pairing. They made further adjustment by taking account of available
experimental data. The results were adopted in ENDF/B-IV. In the present work most of the
yield data has been taken from the compilation by Rider and Meek. However it requested
further information about short-lived nuclides far from stability line than theirs and up to
date data on isomeric state. Thus we calculated the necessary yield values by following the
treatment described in Ref. 41. The main items of the revision are summarized as follows.
1) The distribution of independent yield strength between ground and isomeric states of
primary fission products was evaluated by Madland and England method with recent
experimental data of the spin-parity values 42 ^. The values of fraction at yields for a total
of 191 nuclides are tabulated in Table 2.4.1. We have used these values for all yields of
relevant nuclides in our file, while Rider and Meek used only a part of them.
2) The increase in number of nuclides in this library is about 140 and their yields have been
estimated.
3) Recent experiments have denied the existence of the isomeric state of 74 As, 85 Se, 86 Rb,
109 162
Rh and Tb adopted in Ref. 41. Thus these isomeric states were removed and their
yields were added to those of the ground states.
4) The increase in number of isomeric states is 68 and the nuclides are expressed with
asterisk at upper right of the names in Table 2.4.1.
The equations and the parameter values used in the present evaluation will be briefly
described in the following. In a given mass chain of mass number A the fractional independent
yield of an isobar of atomic number Z is approximated by the symmetric Gaussian
function 43 >44) as follows:
o.s r (Z-Zp(A)) 2
[ ^ ]dZ. (2.4.1)
5 e x p [ ^
where Zp (A) is the most probable charge and a the Gaussian width. The sum is given by the