1 - IAEA Nuclear Data Services

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happens for Th and U at the same energy does not necessarily-
mean that the (y»n) thresholds for the two nuclei coincide.
The position of the minimum may depend in fact on the behaviour
and absolute value of the two cross-sections; with respect to
Uranium, Thorium has actually a much less photofission cross-
-section, while his ( y,n) cross-section grows very sharply [l6J.
The ratio a TV/ a u i- s reported in fig. 3 as a function of energy.
Disregarding the first point because of the large error it is
affected by, this ratio shows a broad maximum at about 7 MeV
which agrees with the Huizenga et al. results [_16J and which
is explained by these authors in terms of the competitivity of
(Y»n) reactions with fission, together with the different thesh-
olds of Uranium and Thorium (y,n) reactions.
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