Speakers
Description
More accurate neutron capture cross-section data for $^{239}$Pu are required for the design of new critical nuclear systems such as Gen IV reactors and Accelerator Driven Systems (ADS) and for the operation of thermal reactors, especially when loaded with MOX fuels. Therefore, the capture cross-section of $^{239}$Pu is included in the NEA/OCDE High Priority Request List [1].
At present there are significant differences between the capture cross-section data for $^{239}$Pu that are recommended in the main evaluated nuclear data libraries. Furthermore, there are only two capture measurements in the Resolved Resonance Region with enough resolution to perform a reasonable resonance analysis. The first measurement, done by Gwin et al. [2] in 1971, covers the neutron energy range from 0.02 eV to 30 keV. In 2014, a measurement covering the range between 10 eV and 1.3 MeV was performed at LANSCE (Los Alamos, USA) by Mosby et al. [3-5]. In the latter, only the shape of the cross-section was measured and the results were normalized to the ENDF/B-VII.1 [6] cross-section between 17 eV and 18 eV. The scarcity of experimental data is partially a consequence of the challenging nature of $^{239}$Pu(n,γ) measurements due to the competing fission gamma-ray background and the high α-activity (~2 MBq/mg) of the samples.
A new capture measurement, which is part of the scientific program approved by the European Commission H2020 Supplying Accurate Nuclear Data for energy and non-energy Applications (SANDA), will be performed in the neutron time-of-flight facility n_TOF [7] at CERN in 2022. This measurement will provide additional cross section data with a better energy resolution thanks to the 10 times longer flight path than in [2] and [3]. The expertise acquired during the 235U(n,γ) measurement at n_TOF and the analysis of these data [8] has been applied to the new experimental setup. The detector system consists of the n_TOF Total Absorption Calorimeter (TAC) [9] with 40 BaF$_2$ crystals and a new ionization chamber for the fission fragments, developed specifically for this measurement. The fission chamber operates in coincidence with the TAC and will be used as a fission tagging detector to strongly reduce the background from fission reactions. This improved fission chamber will be tested in a $^{239}$Pu(n,f) measurement at the GELINA facility of JRC-Geel (Belgium).
The measurement will be performed with two different configurations of highly enriched samples manufactured at JRC Geel: a low mass configuration, involving ten $^{239}$Pu samples of 1 mg each placed inside the fission chamber, and a high mass configuration, with a 100 mg $^{239}$Pu encapsulated sample, without using the fission detector. Using the combination of the two different experimental setups, an overall uncertainty of ~3-4% is expected in the energy range from thermal up to 10 keV.
In addition to the cross-section data, the measurement will also provide valuable information on the distribution of the γ-rays cascades emitted in $^{239}$Pu(n,γ) and $^{239}$Pu(n,f) reactions, as experienced in previous experiments performed with the TAC [10-12].
In this conference some general results will be presented, including the test of the fission chamber, the details on the preparation of the samples, a description of the experimental setup at n_TOF and also some preliminary results from the $^{239}$Pu(n,γ) measurement.
References
[1] E. Dupont et al., EPJ Web of Conferences 239, 15005 (2020); the HPRL database is available at https://www.oecd-nea.org/dbdata/hprl/
[2] Gwin et al., Nucl. Sci. Eng. 45, 25 (1971).
[3] S. Mosby et al., Phys. Rev. C 89, 034610 (2014).
[4] S. Mosby et al., Phys. Rev. C 97, 041601 (2018).
[5] S. Mosby et al., Nucl. Data Sheets 148, 312 (2018).
[6] M. B. Chadwick et al., Nucl. Data Sheets 112, 2887 (2011).
[7] C. Rubbia, et al., A high Resolution Spallation driven Facility at the CERN-PS to Measure Neutron Cross Sections in the Interval from 1 eV to 250 MeV, CERN/ LHC/98-002-EET, 1998.
[8] J. Balibrea-Correa et al., Phys. Rev. C 102, 044615 (2020).
[9] C. Guerrero et al., Nucl. Instr. Meth. A 608 (2009)
[10] C. Guerrero et al., Phys. Rev. C 85, 044616 (2012).
[11] E. Mendoza et al., Phys. Rev. C 90, 034608 (2014).
[12] E. Mendoza et al., Phys. Rev. C 97, 054616 (2018).
