Towards high accurate neutron-induced fission cross sections of 240,242Pu: Spontaneous fission half-lives

Fast spectrum neutron-induced fission cross sections of transuranic isotopes are being of special demand in order to provide accurate data for the new GEN-IV nuclear power plants. To minimize the uncertainties on these measurements accurate data on spontaneous fission half-lives and detector efficiencies are a key point. High -active actinides need special attention since the misinterpretation of detector signals can lead to low efficiency values or underestimation in fission fragment detection. In that context, 240,242Pu isotopes have been studied by means of a Twin Frisch-Grid Ionization Chamber (TFGIC) for measurements of their neutron-induced fission cross section. Gases with different drift velocities have been used, namely P10 and CH4. The detector efficiencies for both samples have been determined and improved spontaneous fission half-life values were obtained.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Neutron-induced fission cross section of 242

Accurate nuclear-data needs in the fast-neutron-energy region have been recently addressed for the development of next generation nuclear power plants (GEN-IV) by the OECD Nuclear Energy Agency (NEA). This sensitivity study has shown that of particular interest is the 242Pu(n,f) cross section for fast reactor systems. Measurements have been performed with quasi-monoenergetic neutrons in the energy range from 15 MeV to 20 MeV produced by the Van de Graaff accelerator of the JRC-Geel. A twin Frisch-grid ionization chamber has been used in a back-to-back configuration as fission fragment detector. The 242Pu(n,f) cross section has been normalized to 238U(n,f) cross section data. The results were compared with existing literature data and show acceptable agreement within 5%

Fission cross section measurements for 240Pu, 242Pu

This report comprises the deliverable 1.5 of the ANDES project (EURATOM contract FP7-249671) of Task 3 "High accuracy measurements for fission" of Work Package 1 entitled "Measurements for advanced reactor systems". This deliverables provide evidence of a successful completion of the objectives of Task 3.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

First evidence of multiple beta-delayed neutron emission for isotopes with A > 100

The beta-delayed neutron emission probability, P-n, of very neutron-rich nuclei allows us to achieve a better understanding of the nuclear structure above the neutron separation energy, S-n. The emission of neutrons can become the dominant decay process in neutron-rich astrophysical phenomena such as the rapid neutron capture process (r-process). There are around 600 accessible isotopes for which beta-delayed one-neutron emission (beta 1n) is energetically allowed, but the branching ratio has only been determined for about one third of them. beta 1n decays have been experimentally measured up to the mass A similar to 1 5 0, plus a single measurement of Tl-210. Concerning two-neutron emitters (beta 2n), similar to 3 0 0 isotopes are accessible and only 24 have been measured so far up to the mass A = 100. In this contribution, we report recent experiments which allowed the measurement of beta 1n emitters for masses beyond A > 200 and N > 1 2 6 and identified the heaviest beta 2n emitter measured so far, Sb-136.Peer reviewe

Measurement of the heaviest beta-delayed 2-neutron emitter : Sb-136

The beta-delayed neutron emission probability, P-n, of very exotic nuclei is crucial for the understanding of nuclear structure properties of many isotopes and astrophysical processes such as the rapid neutron capture process (r-process). In addition beta-delayed neutrons are important in a nuclear power reactor operated in a prompt sub-critical, delayed critical condition, as they contribute to the decay heat inducing fission reactions after a shut down. The study of neutron-rich isotopes and the measurement of beta-delayed one-neutron emitters (beta 1n) is possible thanks to the Rare Isotope Beam (RIB) facilities, where radioactive beams allow the production of exotic nuclei of interest, which can be studied and analyzed using specific detection systems. This contribution reports two recent measurements of beta-delayed neutron emitters which allowed the determination of half-lives and the neutron branching ratio of isotopes in the mass region above A = 200 and N > 126, and a second experiment which confirmed Sb-136 as the heaviest double neutron emitter (beta 2n) measured so far.The beta-delayed neutron emission probability, P-n, of very exotic nuclei is crucial for the understanding of nuclear structure properties of many isotopes and astrophysical processes such as the rapid neutron capture process (r-process). In addition beta-delayed neutrons are important in a nuclear power reactor operated in a prompt sub-critical, delayed critical condition, as they contribute to the decay heat inducing fission reactions after a shut down. The study of neutron-rich isotopes and the measurement of beta-delayed one-neutron emitters (beta 1n) is possible thanks to the Rare Isotope Beam (RIB) facilities, where radioactive beams allow the production of exotic nuclei of interest, which can be studied and analyzed using specific detection systems. This contribution reports two recent measurements of beta-delayed neutron emitters which allowed the determination of half-lives and the neutron branching ratio of isotopes in the mass region above A = 200 and N > 126, and a second experiment which confirmed Sb-136 as the heaviest double neutron emitter (beta 2n) measured so far.The beta-delayed neutron emission probability, P-n, of very exotic nuclei is crucial for the understanding of nuclear structure properties of many isotopes and astrophysical processes such as the rapid neutron capture process (r-process). In addition beta-delayed neutrons are important in a nuclear power reactor operated in a prompt sub-critical, delayed critical condition, as they contribute to the decay heat inducing fission reactions after a shut down. The study of neutron-rich isotopes and the measurement of beta-delayed one-neutron emitters (beta 1n) is possible thanks to the Rare Isotope Beam (RIB) facilities, where radioactive beams allow the production of exotic nuclei of interest, which can be studied and analyzed using specific detection systems. This contribution reports two recent measurements of beta-delayed neutron emitters which allowed the determination of half-lives and the neutron branching ratio of isotopes in the mass region above A = 200 and N > 126, and a second experiment which confirmed Sb-136 as the heaviest double neutron emitter (beta 2n) measured so far.Peer reviewe

Physical vapour deposition of metallic lithium

The preparation of thin films of LiF restarted a few years ago at the Institute for Reference Materials and Measurements (IRMM). Deposits of 6LiF and 7LiF with an areal density up to approximately 500 µg·cm-2 are prepared by physical vapour deposition for neutron production (Li(p,n)) in the Van de Graaff accelerator at IRMM. In order to reach higher neutron yields, a larger number of Lithium nuclei per unit of surface is requested. In a proton beam however, high-energetic gamma rays are produced by interaction with Fluorine and disturb experiments making use of gamma spectrometry. In this regard, the deposition of metallic Lithium by means of physical vapour deposition up to an areal density of 300 µg·cm-2 was examined. Yet, metallic Li is known to diffuse into certain materials and react with atmospheric air by forming several reaction products. In this regard, the effect of protective covers of Au or LiF (sandwiches), deposited by means of a multi-crucible evaporation system, was investigated and the stability of the layers in the proton beam was measured.JRC.D.2-Standards for Innovation and sustainable Developmen

The fast neutron induced fission cross section of 240,242Pu

Based on a sensitivity study performed by the Nuclear Energy Agency (NEA) for the next generation of nuclear power plants (GEN-IV), a list of high priorities of the most important isotopes and their relevant quantities was prepared. Within the high priority list it is requested to improve the uncertainty of the neutron-induced fission cross section of 242Pu from the current 20% to 5%, and that of 240Pu to an uncertainty within 1-3%, compared to the present 5%. In this paper the results from recent 240,242Pu(n,f) cross-section measurements will be presented. The major difference to other literature studies of these fission cross sections are the fact that three different reference targets have been used, namely 235,238U and 237Np. In addition the spontaneous fission half-lives of 240,242Pu have been re-measured with unprecedented statistical accuracy. For 240Pu(n,f) our results reproduce the fission threshold very well but in the plateau region the difference to the evaluations is about 5 %. For 242Pu(n,f) our results show a deviation already in the threshold region where the present data tend to a less steep rise of the fission cross section. Furthermore the resonance like structure around 1.1 MeV seen in the evaluations is not present in our data. In addition, in the plateau region up to about 2.5 MeV our data are 7-9% lower than the evaluations.JRC.G.2-Standards for Nuclear Safety, Security and Safeguar

Neutron-induced fission cross section of 240,242Pu up to En = 3 MeV

The neutron-induced fission cross sections of 240,242Pu have been measured at JRC-IRMM with incident neutron energy from 0.2 MeV up to 3 MeV. A Twin-Frisch Grid Ionization Chamber (TFGIC) has been used in a back-to-back geometry. The measurements have been performed using the secondary standards 237Np and 238U as a reference. The purity of the plutonium samples was 99.89% for 240Pu and 99.97% for 242Pu. The results obtained follow the ENDF/B-VII.1 evaluation for 240Pu, but some discrepancies are visible around En = 1 MeV for 242Pu. In addition, the spontaneous fission half-life has been measured for both isotopes.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Neutron-induced fission cross section of 242Pu from 15 MeV to 20 MeV

Accurate nuclear-data needs in the fast-neutron-energy region have been recently addressed for the development of next generation nuclear power plants (GEN-IV) by the OECD Nuclear Energy Agency (NEA). This sensitivity study has shown that of particular interest is the 242Pu(n,f) cross section for fast reactor systems. Measurements have been performed with quasi-monoenergetic neutrons in the energy range from 15 MeV to 20 MeV produced by the Van de Graaff accelerator of the JRC-Geel. A twin Frischgrid ionization chamber has been used in a back-to-back configuration as fission fragment detector. The 242Pu(n,f) cross section has been normalized to 238U(n,f) cross section data. The results were compared with existing literature data and show acceptable agreement within 5%.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Absolute and relative cross section measurements of 237Np(n,f) and 238U(n,f) at the National Physical Laboratory

Cross section measurements in the fast energy region are being demanded as one of the key ingredients for modelling Generation-IV nuclear power plants. However, in facilities where there are no time-of-flight possibilities or it is not convenient to use them, using the 235U(n,f) cross section as a benchmark would require a careful knowledge of the room scatter in the experimental area. In this paper we present measurements of two threshold reactions, 238U and 237Np, that could become a standard between their fission threshold and 2.5 MeV, if the discrepancies shown in the evaluations and in some experimental data can be solved. The preliminary results are in agreement with the present ENDF/B-VII.1 evaluation.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard