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

Prompt fission neutron emission: Problems and challenges

This paper presents some of the challenges ahead of us even after 75 years of the discovery of the fission process and large progress made since then. The focus is on application orientation, which requires improved measurements on fission cross-sections and neutron and γ-ray multiplicities. Experimental possibilities have vastly improved the past decade leading to developments of highly sophisticated detector systems and the use of digital data acquisition and signal processing. The development of innovative fast nuclear reactor technology needs improved respective nuclear data. Advancements in theoretical modelling also require better experimental data. Theory has made progress in calculating fission fragment distributions (i.e. GEF code) as well as prompt neutron and γ-ray emission to catch up with the improved experiments.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

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 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

Artificial diamonds as radiation-hard detectors for ultra-fast fission-fragment timing

In the framework of the construction of the double time-of-flight spectrometer VERDI, where we aim at measuring pre- and post-neutron masses directly and simultaneously, ultra-fast time pick-up detectors based on artificial diamond material were investigated for the first time with low-energy heavy ions produced in fission(0.5 MeV/u < E/A < 2 MeV/u). Signal stability under a high radiation dose was determined up to at least 10^9 fission fragments together with more than 3.5 x 10^9 neutrons and 3 x 10^10 alpha-particles. These doses are characteristic for fission experiments. The intrinsic timing resolution of a 100 micron thick polycrystalline CVD diamond detector with a size of 1 x 1 cm^2 was determined to sigma_{int} = (283 +- 41) ps by comparison with Monte-Carlo simulations. Using broadband pre-amplifiers, 4-fold segmented detectors of same total size and with a thickness of 180 micron show an intrinsic timing resolution of sigma_{int} = (124 +- 32) ps. This is highly competitive with the best micro-channel plate detectors. Due to the limited and batch-dependent charge collection efficiency of polycrystalline diamond material, the detection efficiency for fission fragments may be smaller than 100%. This restriction will no longer be an issue when detectors made from single-crystal diamonds are used.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

First use of single-crystal diamonds as fission-fragment detector

Single-crystal chemical vapor-deposited diamond (sCVD) was investigated for its ability to act as fission-fragment detector. In particular we investigated timing and energy resolution for application in a simultaneous time-of-flight and energy measurement to determe the mass of the detected fission fragment. Previous tests have shown that poly-crystalline chemical vapor deposited (pCVD) diamonds provide sufficient timing resolution, but their poor energy resolution did not allow complete separation between very low-energy fission fragments, -particles and noise. Our present investigations proof artificial sCVD diamonds to show similar timing resolution as pCVD diamonds close to 100 ps. Improved pulse-height resolution allows the unequivocal separation of fission fragments, and the detection efficiency reaches 100%, but remains with about a few per cent behind requirements for fragment-mass identification. With high-speed digital electronics a timing resolution well below 100 ps is possible. However, the strongly varying quality of the presently available diamond material does not allow application on a sufficiently large scale within reasonable investments.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

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 α-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, T1/2,SF = 1.165 × 1011 yr (1.1%), and for 242Pu, T1/2,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%).JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Prompt fission gamma-rays from the reactions 252Cf(SF) and 235U(nth, f) - new data

We present new spectral data of prompt γ-ray emission from the spontaneous fission of 252Cf. This work was performed in direct response to an OECD/NEA high priority data request. We discuss the impact of our new data on evaluated nuclear data tables not only for this nuclide, but also for 238U and 241Pu, which are always produced in a reactor. Furthermore, we will show results from our investigation of prompt γ-ray emission from the reaction 235 U(nth, f), measured in at the Centre for Energy Research of the Hungarian Academy of Sciences in Budapest, Hungary. Spectral data obtained with three different detectors are consistent and led to an uncertainty on total energy and multiplicity considerably smaller than requested by the OECD/NEA.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Fission fragment yield, cross section and prompt neutron and gamma emission data from actinide isotopes

Recent experimental investigations on major and minor actinides at the JRC-IRMM are presented. Fission-fragment distributions of isotopes with vibrational resonances in the sub-threshold fission cross section, i. e. 234,238U, have been measured. For 234U, the impact of an increased neutron multiplicity for the heavy fragments with higher incident neutron energies has been studied as observed in experiment and also recently theoretically predicted. The impact is found to be noticeable on post-neutron mass yields, which are the relevant quantities for a-priori waste assessments. The fission cross sections for 240,242Pu at threshold and in the plateau region are being investigated within the ANDES project. The results show some discrepancies to the ENDF/B-VII.1 evaluation mainly for 242Pu around 1 MeV, where the evaluation exhibits a resonance-like structure not observed so clearly in the present work. The requested target accuracy in design studies of innovative reactor concepts like Gen-IV is in the range of a few percent. In order to be able to respond to requests for measurements of prompt neutron and γ-ray emission in fission JRC-IRMM has also invested in setting up a neutron and γ-ray detector array. The neutron array is called SCINTIA and has so far been tested with 252Cf(SF). For γ-ray multiplicity and spectrum measurements of 252Cf(SF) and 235U(nth, f) lanthanum- and cerium-halide detectors were successfully used.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

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

240Pu has recently been pointed out by a sensitivity study of the OECD-Nuclear Energy Agency (NEA) to be one of the isotopes whose ssion 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 240Pu(n,f) cross section should be improved to an accuracy within 1-3%, compared to the present 5%. A measurement of the 240Pu cross section has been performed at the Van de Graa accelerator of the JRC-IRMM using quasimonoenergetic 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 conguration as ssion fragment detector. The 240Pu(n,f) cross section has been normalized to three dierent isotopes: 237Np(n,f), 235U(n,f) and 238U(n,f). Additionally, the secondary standard reactions were benchmarked through measurements against the primary standard reaction 235U(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).JRC.D.4-Standards for Nuclear Safety, Security and Safeguard