The production of the fissile nucleus U is controlled by its predecessor Pa nucleus. The latter plays a major role in this chain due to its long half life (27 days). It has time to capture neutron, then to fission or emit gamma ray, many processes that can modify the quantity of U present in the fuel and thus affect the neutron equilibrium in the reactor environment. Thus, the knowledge of fission and radiative capture reaction cross-sections is of primary importance. It is because of high radio-toxicity of Pa and of the difficulties arising in its chemical separation that such measurements were never implemented.
The originality of our approach is in the use of transfer reaction to measure the fission probability for the nucleus (A) as a function of its excitation energy.
Concerning the U nucleus which is the fissile nucleus in the chain, we re-measured the fission cross-section U(n,f) in the range of fast neutrons and the experiment planned for 2007 will be concerned with simultaneous measurements of fission and radiative capture cross-sections for U in the range of epithermal neutrons.
In order to determine the cross-sections for reactions Pa(n,f) and Pa(n,), the fission and radiative capture probabilities must be multiplied by the cross section σ for the formation of the compound nucleus (A) formed through the reaction n+(A-1).
We have validated this procedure, in the case of fission, by comparing with measurements induced with neutrons.
In the first case, we were interested in the fission of the excited nucleus Pa* formed in the reaction Pa+n (En = 0.5-10 MeV). We formed the nucleus Pa* in the transfer reaction Th(He,p).
Then, we studied the radiative capture (n,γ) (En = 0-1 MeV) through the same transfer reaction.
The fission cross-section U(n,f) was measured directly with the help of fast neutrons obtained from electrostatic accelerator of CENBG in the energy range of neutrons from 1 to 6 MeV. ( see fission U(n,f)).
In the second case, we proposed to simultaneously measure radiative capture and fission cross-sections for U nucleus in the region of epithermal neutrons.
The yield ratio of these cross-sections σ(capture)/ σ(fission) is an essential parameter in the calculation of regeneration power in the chain Th- U. Very recently, analyses of sensitivity and uncertainty showed evidence of a lack of reliability in the evaluations of this ratio which are based on old measurements attaining 25% dispersion.
(see alpha yield ratio)
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The main difficulty in the measurement of the $^233$U $\alpha$ yield ratio ( $\alpha$ = σ(capture)/ σ(fission) is the separation of γ rays associated with the reaction $^233$U(n,γ) from the ones that come from fission fragments and which represent parasitic radiation. The prototype of an ionization chamber that would allow to identify fission fragments was constructed and tested on AIFIRA accelerator of CENBG. Detection efficiency for fission exceeds 90% and (...)