Neutron-induced Fission Cross Section of240,242Pu

A sensitivity analysis for the new generation of fast reactors [Salvatores (2008)] has shown the importance of improved cross section data for several actinides. Among them, the240,242Pu(n,f) cross sections require an accuracy improvement to 1-3% and 3-5%, respectively, from the current level of 6% and 20%. At the Van de Graaff facility of the Institute for Reference Materials and Measurements (JRC-IRMM) the fission cross section of the two isotopes was measured relative to two secondary standard reactions,237Np(n,f) and238U(n,f), using a twin Frisch-grid ionization chamber. The secondary standard reactions were benchmarked through measurements against the primary standard reaction235U(n,f) in the same geometry. Sample masses were determined by means of low-geometry alpha counting or/and a 2p Frisch-grid ionization chamber, with an uncertainty lower than 2%. The neutron flux and the impact of scattering from material between source and target was examined, the largest effect having been found in cross section ratio measurements between a fissile and a fertile isotope. Our240,242Pu(n,f) cross sections are in agreement with previous experimental results and slightly lower than present evaluations. In case of the242Pu(n,f) reaction no evidence for a resonance at En=1.1 MeV was found.Postprint (published version

Highly accurate measurements of the spontaneous fission half-life of 240,242Pu

Fast spectrum neutron-induced fission cross-section data for transuranic isotopes are of special demand from the nuclear data community. In particular highly accurate data are needed for the new generation IV nuclear applications. The aim is to obtain precise neutron-induced fission cross sections for 240Pu and 242Pu. To do so, accurate data on spontaneous fission half-lives must be available. Also, minimizing uncertainties in the detector efficiency is a key point. We studied both isotopes by means of a twin Frisch-grid ionization chamber with the goal of improving the present data on the neutron-induced fission cross section. For the two plutonium isotopes the high a-particle decay rates pose a particular problem to experiments due to piling-up events in the counting gas. Argon methane and methane were employed as counting gases, the latter showed considerable improvement in signal generation due to its higher drift velocity. The detection efficiency for both samples was determined, and improved spontaneous fission half-lives were obtained with very low statistical uncertainty (0.13% for 240Pu and 0.04% for 242Pu): for 240Pu, T 1/2,SF=1.165×1011 yr (1.1%), and for 242Pu, T 1/2,SF=6.74×1010 yr (1.3%). Systematic uncertainties are due to sample mass (0.4% for 240Pu and 0.9% for 242Pu) and efficiency (1%). © 2013 American Physical Society.Postprint (published version

Neutron-induced Fission Cross Section of240,242Pu

A sensitivity analysis for the new generation of fast reactors [Salvatores (2008)] has shown the importance of improved cross section data for several actinides. Among them, the240,242Pu(n,f) cross sections require an accuracy improvement to 1-3% and 3-5%, respectively, from the current level of 6% and 20%. At the Van de Graaff facility of the Institute for Reference Materials and Measurements (JRC-IRMM) the fission cross section of the two isotopes was measured relative to two secondary standard reactions,237Np(n,f) and238U(n,f), using a twin Frisch-grid ionization chamber. The secondary standard reactions were benchmarked through measurements against the primary standard reaction235U(n,f) in the same geometry. Sample masses were determined by means of low-geometry alpha counting or/and a 2p Frisch-grid ionization chamber, with an uncertainty lower than 2%. The neutron flux and the impact of scattering from material between source and target was examined, the largest effect having been found in cross section ratio measurements between a fissile and a fertile isotope. Our240,242Pu(n,f) cross sections are in agreement with previous experimental results and slightly lower than present evaluations. In case of the242Pu(n,f) reaction no evidence for a resonance at En=1.1 MeV was found

Highly accurate measurements of the spontaneous fission half-life of 240,242Pu

Fast spectrum neutron-induced fission cross-section data for transuranic isotopes are of special demand from the nuclear data community. In particular highly accurate data are needed for the new generation IV nuclear applications. The aim is to obtain precise neutron-induced fission cross sections for 240Pu and 242Pu. To do so, accurate data on spontaneous fission half-lives must be available. Also, minimizing uncertainties in the detector efficiency is a key point. We studied both isotopes by means of a twin Frisch-grid ionization chamber with the goal of improving the present data on the neutron-induced fission cross section. For the two plutonium isotopes the high a-particle decay rates pose a particular problem to experiments due to piling-up events in the counting gas. Argon methane and methane were employed as counting gases, the latter showed considerable improvement in signal generation due to its higher drift velocity. The detection efficiency for both samples was determined, and improved spontaneous fission half-lives were obtained with very low statistical uncertainty (0.13% for 240Pu and 0.04% for 242Pu): for 240Pu, T 1/2,SF=1.165×1011 yr (1.1%), and for 242Pu, T 1/2,SF=6.74×1010 yr (1.3%). Systematic uncertainties are due to sample mass (0.4% for 240Pu and 0.9% for 242Pu) and efficiency (1%). © 2013 American Physical Society

Neutron-induced fission cross sections of Pu-242 from 0.3 MeV to 3 MeV

The majority of the next generation of nuclear power plants (GEN-IV) will work in the fast-neutron-energy region, as opposed to present day thermal reactors. This leads to new and more accurate nuclear-data needs for some minor actinides and structural materials. Following those upcoming demands, the Organisation for Economic Cooperation and Development Nuclear Energy Agency performed a sensitivity study. Based on the latter, an improvement in accuracy from the present 20% to 5% is required for the Pu-242(n, f) cross section. Within the same project both the Pu-240(n, f) cross section and the Pu-242(n, f) cross section were measured at the Van de Graaff accelerator of the Joint Research Centre at the Institute for Reference Materials and Measurements, where quasimonoenergetic neutrons were produced in an energy range from 0.3 MeV up to 3 MeV. A twin Frisch-grid ionization chamber has been used in a back-to-back configuration as fission-fragment detector. The Pu-242(n, f) cross section has been normalized to three different isotopes: Np-237(n, f), U-235(n, f), and U-238(n, f). A comprehensive study of the corrections applied to the data and the uncertainties associated is given. The results obtained are in agreement with previous experimental data at the threshold region up to 0.8 MeV. The resonance-like structure at 0.8 to 1.1 MeV, visible in the evaluations and in most previous experimental values, was not reproduced with the same intensity in this experiment. For neutron energies higher than 1.1 MeV, the results of this experiment are slightly lower than the Evaluated Nuclear Data File/B-VII.1 evaluation but in agreement with the experiment of Tovesson et al. (2009) as well as Staples and Morley (1998). Finally, for energies above 1.5 MeV, the results show consistency with the present evaluations

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Neutron-induced fission cross sections of Pu-242 from 0.3 MeV to 3 MeV

The majority of the next generation of nuclear power plants (GEN-IV) will work in the fast-neutron-energy region, as opposed to present day thermal reactors. This leads to new and more accurate nuclear-data needs for some minor actinides and structural materials. Following those upcoming demands, the Organisation for Economic Cooperation and Development Nuclear Energy Agency performed a sensitivity study. Based on the latter, an improvement in accuracy from the present 20% to 5% is required for the Pu-242(n, f) cross section. Within the same project both the Pu-240(n, f) cross section and the Pu-242(n, f) cross section were measured at the Van de Graaff accelerator of the Joint Research Centre at the Institute for Reference Materials and Measurements, where quasimonoenergetic neutrons were produced in an energy range from 0.3 MeV up to 3 MeV. A twin Frisch-grid ionization chamber has been used in a back-to-back configuration as fission-fragment detector. The Pu-242(n, f) cross section has been normalized to three different isotopes: Np-237(n, f), U-235(n, f), and U-238(n, f). A comprehensive study of the corrections applied to the data and the uncertainties associated is given. The results obtained are in agreement with previous experimental data at the threshold region up to 0.8 MeV. The resonance-like structure at 0.8 to 1.1 MeV, visible in the evaluations and in most previous experimental values, was not reproduced with the same intensity in this experiment. For neutron energies higher than 1.1 MeV, the results of this experiment are slightly lower than the Evaluated Nuclear Data File/B-VII.1 evaluation but in agreement with the experiment of Tovesson et al. (2009) as well as Staples and Morley (1998). Finally, for energies above 1.5 MeV, the results show consistency with the present evaluations

Neutron-induced fission cross section of Pu-240 from 0.5 MeV to 3 MeV

Pu-240 has recently been pointed out by a sensitivity study of the Organization for Economic Cooperation and Development (OECD) Nuclear Energy Agency (NEA) to be one of the isotopes whose fission cross section lacks accuracy to meet the upcoming needs for the future generation of nuclear power plants (GEN-IV). In the High Priority Request List (HPRL) of the OECD, it is suggested that the knowledge of the Pu-240(n, f) cross section should be improved to an accuracy within 1-3 %, compared to the present 5%. A measurement of the Pu-240 cross section has been performed at the Van de Graaff accelerator of the Joint Research Center (JRC) Institute for Reference Materials and Measurements (IRMM) using quasi-monoenergetic neutrons in the energy range from 0.5 MeV to 3 MeV. A twin Frisch-grid ionization chamber (TFGIC) has been used in a back-to-back configuration as fission fragment detector. The Pu-240(n, f) cross section has been normalized to three different isotopes: Np-237(n, f), U-235(n, f), and U-238(n, f). Additionally, the secondary standard reactions were benchmarked through measurements against the primary standard reaction U-235(n, f) in the same geometry. A comprehensive study of the corrections applied to the data and the associated uncertainties is given. The results obtained are in agreement with previous experimental data at the threshold region. For neutron energies higher than 1 MeV, the results of this experiment are slightly lower than the ENDF/B-VII.1 evaluation, but in agreement with the experiments of Laptev et al. (2004) as well as Staples and Morley (1998)