1 - IAEA Nuclear Data Services

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approximated by linear interpolation.
The errors in Table I take into account the statistical errors
on the fission tracks measure and the errors due to all intro­
duced corrections.
The behaviour of the a and b coefficients with energy is that
provided by Bohr's hypothesis near photofission threshold [6}•
The coefficient c is small but different from zero; statistical
errors are too big to give its behaviour with energy.
We have made also a quantitative analysis of the angular distri­
bution coefficients introducing the following simplifying hypo­
thesis: The states which can contribute to the photofission in
our energy interval are 1 k=0, 1~k=1 dipole absorption, and
2 k=0, quadrupole absorption (k is the projection of the total
angular momentum of the nucleus on the symmetry axis), but the
state 2 k=0 gives a constant contribution, in our energy inter­
val, corresponding to 0.06 of cr ; indicating with c", cr~
respectively the cross-sections for the states l""k=0, 1~k=1;
the coefficients a and b of the angular distribution W( ©) can
be expressed by
a , 3 °'l
4 a
tot
a
; 1 3 O
2
b + - a = — 4 ^
tot
.
Using, besides, the fission cross-section formula available
for energies near fission threshold, see papers [7] and [Vj,
U f
E B_u
a = const I e T /(l+^-^)dUf (1)
' o p
where the first factor in the integral measures the chance of
concentrating an amount of energy Up on the fission oscillation
rather than on the other degrees of freedom, and the second
factor is the fission barrier penetration given by Hill and