JAERI 1287 JNDC Nuclear Data Library of Fission Products Fir

JAERI1287 2. Nuclear Data Library of Fission Products 15
well-known relation as
t/(A,Z) = l. (2.4.2)
Wahl et al. 43 > had proposed the Gaussian distribution of the independent yield in their phenomenological
model. After that several investigations 45>46) presented evidence for the fact
that the even-odd effect in proton- and neutron-pairing distorts the Gaussian distribution of
the yield. So we modify independent yield distribution as follows. Let FX(A, Z) be a correction
factor for the even-odd effect and letY(A)be the mass yield before delayed-neutron
emission. Then the yield Y(A, Z) is given by
Y(A, Z)=N(A>FX{A, Z)-f(A, Z), (2.4.3a)
, Z) = NVD-I, FXU, Z)f{A, Z), (2.4.3b)
z z
where N(A) is a normalization constant such that the sum of yields is 200% per fission. The
F(A, Z) 's are expressed in the following form 41 ' 47 *.
Fx(A,Z) = l + {p+n) foreven/1 - even Z
= l + {p-n) for odd A - even Z
= l-O-«) forodd A -odd Z
= 1 —(/?+«) for even A -odd Z
where p is proton-pairing term and n is neutron-pairing term.
Experimental data are not enough, but indicate that the magnitudes of p and n depend
on fissile and incident energy of neutron. These values were estimated with the simple empirical
equation proposed by Madland and England 47 ). The results are tabulated in Table 2.4.2.
The Gaussian width parameter a is taken to be 0.56 ± 0.06 44 ) irrespective of mass number,
fissile nuclide and neutron energy.
The most probable charge ZP(A) has been estimated by several investigators. Rider and
Meek adopted the values by Wahl et al. 44 ^ and Nethaway 48 ) and added another correction term
for ternary fission, which was estimated by Madland and Stewart 49 *. TheZ^C/Uis expressed
as the sum of standard value for thermal-neutron fission of 235 U and the small correction terms
Table 2.4.2 Parameters of pairing effects on the distribution
of independent yield in neutron-induced
fission (taken from the report by Rider and
Meek 41 ))
Fissile
23su(T)a
235 U (F) b
235 U (H) c
238 U(F)
238U(H)
239
Pu(T)
239
Pu (F)
241
Pu(T)
233
U (T)
232
Th (F)
Proton effects (p)
0.228
0.151
0.015
0.329
0.018
0.171
0.124
0.206
0.210
0.327
a thermal neutrons.
b fission spectrum neutrons.
c 14-MeV neutrons.
Neutron effects (n)
0.044
0.029
0.003
0.063
0.003
0.033
0.024
0.040
0.041
0.063