Observation of the competing fission modes in 178 Pt

Fragment mass distributions from fission of the excited compound nucleus 178 Pt have been deduced from the measured fragment velocities. The 178 Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction 36 Ar + 142 Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of ≈1/3. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, A L /A H ≈79/99, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation

Study of fission using multi-nucleon transfer reactions

Multi-nucleon transfer channels of the reactions of 18O+232Th, 18O+238U, 18O+248Cm were used to measure fission-fragment mass distribution for various nuclides and their excitation energy dependence. Predominantly asymmetric fission is observed at low excitation energies for all the studied cases, with an increase of the symmetric fission towards high excitation energies. Experimental data are compared with predictions of the fluctuation-dissipation model, where effects of multi-chance fission (neutron evaporation prior to fission) was introduced. It was shown that a reliable understanding of the observed fission fragment mass distributions can be obtained only invoking multi-chance fissions

Mass and charge distributions in the very asymmetric mass region of the neutron induced fission of Np-238

The mass-separator Lohengrin was used to measure the yields of the light fission products with A = 74-85 and their nuclear charge and kinetic energy distributions from the odd-Z compound nucleus Np-239* formed by double capture of thermal neutrons. The mass yield distribution reveals an influence of the fragment shell with N = 50 affecting also the nuclear charge and kinetic energy distributions. An odd-even effect for protons is found in the very asymmetric mass division, increasing from 4% to 35% with increasing fission asymmetry. This is in contrast to findings in normal asymmetric fission (region of high fission yields) where no odd-even effect for protons was observed. An odd-even effect for neutrons is also found comparable in size with that for protons. The latter effect exists also in normal asymmetric fission and is at least partly attributed to prompt neutron emission from the fragments. Some information on the number of prompt neutrons emitted is also obtained. From this information and from the energy dependence of the odd-even effect for protons it is concluded that the very light fragments originate from a cold and nearly undeformed light sphere of a dumbbell scission configuration characterised by a double shell closure with 28 protons and about 50 neutrons. In contrast to the double shell closure at mass 132 (Z = 50, N approximate to 82) the two shell closures in the light sphere do not coincide at one mass but are realised at A = 70 and 80, respectively. This leads to a layered structure of the light sphere of the dumbbell scission configuration. (C) 2001 Elsevier Science B.V, All rights reserved

Development of a gaseous proton-recoil telescope for neutron flux measurements between 0.2 and 2 MeV neutron energy

Absolute measurements of neutron fluence are an essential prerequisite of neutron-induced cross section measurements, neutron beam lines characterization and dosimetric investigations. The H(n,p) elastic scattering cross section is a very well known standard used to perform precise neutron flux measurements in high precision measurements. The use of this technique, with proton recoil detectors, is not straightforward below incident neutron energy of 1 MeV, due to a high background in the detected proton spectrum. Experiments have been carried out at the AIFIRA facility to investigate such background and to determine its origin and components. Based on these investigations, a gaseous proton-recoil telescope has been designed with a reduced low energy background. A first test of this detector has been carried out at the AIFIRA facility, and first results will be presented

Detection of Fission Coincidences With Plastic Scintillators for the Characterization of Radioactive Waste Drums

International audienceThis work addresses the use of plastic scintillators as an alternative to 3He neutron detectors for radioactive waste drum characterization. The time response of scintillators is three orders of magnitude faster than that of gas proportional counters, and they offer similar neutron detection efficiency at lower cost. However, they are sensitive to gamma rays and the neutron-gamma pulse shape discrimination (PSD) is not possible with standard PVT scintillators. The proposed approach uses the neutron and gamma times of flight in triple coincidences recorded with 252Cf, AmBe, and 60Co sources. A 2-D histogram of time delays between the second and first detected pulses, on the x -axis, and between the third and second pulses, on the y -axis, evidences a specific region of interest for spontaneous fission coincidences. MCNPX-PoliMi simulations are performed, which are in good agreement with previous experiments and allow for investigating the types of coincidences ( γγγ , nnn, γ nn, γγn ) and optimizing the rejection of neutron and gamma scattering crosstalk. The method was also experimentally tested with a 118-L mock-up drum filled with a metallic or an organic matrix, showing a correct estimation of the net fission signal up to an “alpha ratio” of 12 between fission and ( α ,n) neutron emissions. Matrix and localization effects were also measured, showing a sensitivity (useful signal per gram of 240Pueq) for a homogeneous distribution of plutonium in the iron matrix about three times larger than in the wood matrix, due to neutron slowing-down in the latter. This difference can be taken into account by the prior knowledge of the matrix characteristics or by neutron and gamma attenuation measurements. In contrast, in case of a point source with an unknown position in the matrix, the relative localization uncertainty is 26% for the metallic drum, and 41% for the organic one

Experimental fission study using multi-nucleon transfer reactions

It is shown that the multi-nucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multi-nucleon transfer channels of the reactions of ¹⁸O+²³²Th, ¹⁸O+²³⁸U and ¹⁸O+²⁴⁸Cm are used to study fission for various nuclei from many excited states. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel. Predominantly asymmetric fission is observed at low excitation energies for all studied cases, with a gradual increase of the symmetric mode towards higher excitation energy. The experimental distributions are found to be in general agreement with predictions of the fluctuation-dissipation model. Role of multi-chance fission in fission fragment mass distributions is discussed, where it is shown that mass-asymmetric structure remaining at high excitation energies originates from low-excited nuclei by evaporation of neutrons

Measurement of 242^{242}Pu(n,f) in the 1-2 MeV energy range

International audienceThe design of new generation fast nuclear reactors requires highly accurate cross-section measurements in the MeV energy region. The 242Pu fission cross section is of particular interest for Pu incineration and nuclear waste production. There are discrepancies around 1 MeV incident neutron energy between libraries and among experimental data. Some data suggest the presence of a strong structure between 1 and 1.2 MeV whereas it is barely visible on some other data and its shape is very different among evaluations. The large majority of the 242Pu(n,f) measurements have been carried out with respect to the 235U(n,f) secondary-standard cross section. This introduces a strong correlation between measurements from different research teams. Moreover, this reference cross section exhibits structures, in particular a steep increase of +10% at 1 MeV. Therefore, we aim to re-measure the 242Pu(n,f) cross section relative to the primary-standard 1H(n,n)p cross section, by using a proton recoil detector. This standard has a very high accuracy (0.4%), is not used for other 242Pu measurements, and is structureless. An experiment has been carried out in October 2022 at the MONNET facility in JRC Geel, with incident neutron energies from 0.9 MeV to 2.0 MeV. The experimental setup will be presented, and the analysis procedure will be detailed

Measurement of 242^{242}Pu(n,f) in the [1;2MeV] energy range

International audienceThe design of new generation fast nuclear reactors requires highly accurate cross-section measurements in the MeV energy region. The 242 Pu fission cross section is of particular interest for Pu incineration and nuclear waste production. There are discrepancies around 1 MeV incident neutron energy between libraries and among experimental data. Some data suggest the presence of a strong structure between 1 and 1.2 MeV whereas it is barely visible on some other data and its shape is very different among evaluations. The large majority of the 242 Pu(n,f) measurements have been carried out with respect to the 235 U(n,f) secondary-standard cross section. This introduces a strong correlation between independent measurements and this cross section exhibits structures, in particular a steep increase of +10% at 1 MeV. Therefore, we aim to re-measure the 242 Pu(n,f) cross section relative to the primary-standard 1 H(n,n)p cross section, by using a proton recoil detector. This standard has a very high accuracy (0.4%), is not used for of other 242 Pu measurements, and is structureless. An experiment has been carried out in October 2022 at the MONNET facility in JRC Geel, with incident neutron energies from 0.9 MeV to 2.0 MeV. The experimental setup will be presented, and the analysis procedure will be detailed