1 - IAEA Nuclear Data Services

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Wheeler [9], we can express a and b through the T, B and E
parameters which characterize the nucleus. Fitting the fun­
ctions a, b+^-a and — separately, for the parameters T, B and
a
E we have obtained the values reported in Table II.
The values reported in Table II are calculated using the values
of a and b of Table I.
In fig. 2 are shown the experimental and calculated behaviour
of the fitted functions whit energy and the agreement of the
experimental U photofission cross-section with that calcu­
lated with formula 1) and the parameters of Table II as far
as about 6.5 MeV. Hereafter the competition of the (y,n)
process does not render our formulas available.
2.1. Cross-section of ¿5¿ i Th.
Thorium loaded nuclear emulsions have been exposed to mono­
chromatic Y-rays obtained from (n,y) reactions on the follow­
ing elements: S, Dy, Y, Ca, Ti, Be, Mn, Pb, Fe, Al, Cu, Ni.
As far as details are concerned, we refer to our previous
papers whether for obtaining monochromatic Y- ravs I. 1 -*» 1 ^J »
or for loading nuclear emulsions with fissile materials £10 ,
15 J, or finally for experimental data handling [11, 12~J.
Here we report the experimental results that are summarized
in Table III: column 1 lists the reactor targets; column 2 the
main line energies; column 3 the corresponding Y- ra y s fluxes;
column 4 the ratio N./® Th, that is the fission tracks density
f m J
by unity of Y-ray flux and thorium nuclei; and column 5 the
cross-section values obtained after correction for neutron
background and for the secondary lines contribution.
These last values are also shown in fig. 3 together with the
corresponding U values. The behaviour of the two cross-sections
is roughly the same and is explained with the growth of the
(Y,n) reaction which starts at about 6 MeV and at about 7 MeV
becomes competitive with fission process. That the minimum