Speaker: Olivier SEROT (CEA-Cadarache)

SYSTEMATIC INVESTIGATION OF THE EMISSION OF LIGHT TERNARY FISSION PARTICLES

During the last few years, three campaigns of measurements were performed in order to enlarge the database concerning the emission of light ternary fission particles and to improve our understanding of the ternary fission process.
a)The first one was dedicated to the determination of the energy distributions and yields of ternary alpha particles (also called Long Range Alpha (LRA) particles) emitted during spontaneous fission of 238,240,242,244Pu-isotopes. This study has permitted to demonstrate the strong influence of the fission modes on the LRA emission process.
b)The second campaign was devoted to the investigation of the influence of the excitation energy on the ternary alpha and triton emission probability (denoted LRA/B and t/B respectively). This was done by comparing LRA/B and t/B data measured from spontaneous fission decay and thermal neutron induced fission reaction of the same fissioning system such as 246Cm(sf) and 245Cm(nth,f). In this way, a same fissioning nucleus at 0 MeV and at 6-7 MeV excitation energy can be compared. Up to now, three fissioning systems were measured (246Cm, 248Cm and 252Cf). In each case, a decrease of the ternary alpha emission probability and an increase of the triton emission probability with the excitation energy of the compound nucleus could be observed. The (nth,f) measurements were performed at the cold neutron guide of the Institute Laue Langevin in Grenoble (France).
c)Lastly, in the third campaign, we have investigated the LRA emission probabilities for 239Pu(n,f) reaction in the thermal and in the resonance region. The experiment was carried out at an 9m flight path of the GELINA facility of the Institute for Reference Materials and Measurements in Geel (Belgium). The energy of the incoming neutrons was determined by their time of flight. Correlations were searched between these LRA emission probabilities and the parameters characterizing each resonance. Our results confirm the negligible role of the resonance spin on the LRA emission process but suggests that there exists an anti-correlation between LRA/B and the average prompt neutron multiplicity measured in binary fission.
Using the experimental data from the present work combined with other results, we have also observed that LRA emission process is governed by the pre-formation of an alpha-cluster which is not the case for the ternary triton emission. The different behavior of tritons and alpha's suggests a possible difference in their emission mechanism.