17 Septembre – Thesis defense - Ricardo Pérez Sánchez

10 h Seminar room / CENBG (Gradignan)

The surrogate reaction method applied to 240Pu.

This PhD thesis revolves around the study the gamma-decay and fission probabilities of the compound nucleus (CN) 240Pu. These probabilities are obtained by using the surrogate reaction method, which, through charged particle reactions, aims to produce the same compound nucleus as the one that would be formed through a neutron induced reaction, or desired reaction. The objective is to cover the shortage of nuclear data, in cases in which the targets are too radioactive to be measured directly, for astrophysics and applications. As a matter of fact, if the measurement of the desired reaction is not possible, the reaction models reliance is compromised as their parameters cannot be adjusted. In this cases the gamma-decay and fission probabilities of the CN formed through the surrogate reaction, can help to improve the models. To this end, it is crucial to understand the difference between the formation and decay processes in the compound nuclei formed through a surrogate reaction and a neutron induced one.
A collaboration between the nuclear physics laboratories, CENBG and CEA/DAM/DIF, is making the state of the art of surrogate reactions advance. In particular giving some insight about the spin distribution of the CN formed with these reactions, which they proved different to that of the nuclei formed through neutron induced reactions and that this played an important role in the competition between gamma-decay and neutron emission. Nevertheless, this does not seem to be the case for fission, whose data are in agreement with neutron induced ones. To better understand this, we have studied 240Pu, an even-even nucleus, using an experimental setup developed by this collaboration to simultaneously measure gamma-decay and fission.
With this set-up, we performed an experiment in 2017 at the tandem accelerator at the IPN of Orsay (France). There a 30 MeV alpha particles beam interacted with the 240Pu target. The inelastically scattered alpha particles, ejectiles, were detected by two telescopes, which allow to identify the decaying nucleus and determine its excitation energy. The decay paths of the formed CN were identified, in coincidence with the telescopes, by detecting the gamma-rays and the fission fragments. With this information, the gamma-decay and fission probabilities were obtained by doing the ratio between the number of detected ejectiles and the number of measured coincidences correct by the detection efficiency.
To interpret these unique data, we proceeded in three steps. Firstly, we adjusted the reaction model parameters (nuclear level densities, fission barriers, etc.) of the compound nucleus 240Pu with the existing data of the n+239Pu reactions. Then we calculated the branching ratios G of the decaying nucleus, which represent the probability of the nucleus to decay through a certain channel, for a certain excitation energy, spin and parity. Finally, with M. Dupuis (CEA/DAM/DIF), a calculation to predict the spin distribution of the 240Pu formed through the inelastic scattering of alpha particles was done for the first time. The calculation combined a JLM optical potential with the states of the nucleus generated with a QRPA approach.
The spin distribution obtained with this calculation was combined with the calculated branching ratios G to calculate the decay probabilities. The comparison of this calculation to our measured probabilities shows a good agreement, which indicates a good a understanding of the reaction mechanism alpha,alpha'. Using this type of inelastic reaction in the future, could provide additional information about the radiative capture and fission cross sections of more exotic nuclei.

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