Neutron-induced fission cross section of ${}^{240}\mathrm{Pu}$ from 0.5 MeV to 3 MeV

${}^{240}\mathrm{Pu}$ 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 ${}^{240}\mathrm{Pu}(n,f)$ cross section should be improved to an accuracy within 1–3 %, compared to the present 5%. A measurement of the ${}^{240}\mathrm{Pu}$ 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 ${}^{240}\mathrm{Pu}(n,f)$ cross section has been normalized to three different isotopes: ${}^{237}\mathrm{Np}(n,f)$, ${}^{235}\mathrm{U}(n,f)$, and ${}^{238}\mathrm{U}(n,f)$. Additionally, the secondary standard reactions were benchmarked through measurements against the primary standard reaction ${}^{235}\mathrm{U}(n,f)$ 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).

Figure 1

Summary of the most relevant experiments performed of the neutron-induced fission cross section of ${}^{240}\mathrm{Pu}$ compared with current evaluations. The evaluations chosen are ENDF/B-VII.1 [12], JEFF 3.1 [11] and JENDL 4.0 [13]. The experimental data shown are Ruddick and White (1964) [5] (open diamonds), White (1967) [6] (open crosses), Meadows (1981) [7] (open triangle up), Budtz-Jørgensen and Knitter (1981) [8] (open circles), Staples and Morley (1998) [9] (open triangles down), Laptev et al. (2004) [10] (open squares), and Tovesson et al. (2009) [4] (full circles). Selected data are shown for visibility of the plot. Data found as a ratio of ${}^{235}\mathrm{U}$ were normalized to the ENDF/B-VII.1 evaluation of this isotope. Further explanation is given in the text.

Figure 2

Scheme of the twin Frisch-grid ionization chamber with two samples in a back-to-back configuration followed by the digital electronics used for this experiment.

Figure 3

Left: setup configuration without the shielding (courtesy of D. Vanleeuw) (setup #1). The arrow points to the cap of the neutron producing target. Right: setup configuration with the paraffin-${\mathrm{B}}_4\mathrm{C}$ shielding (setup #2).

Figure 4

Pulse height (PH) distribution for the ${}^{240}\mathrm{Pu}$ (orange), ${}^{237}\mathrm{Np}$ (violet), and ${}^{235}\mathrm{U}$ (green) samples used in this experiment taken with $E_n=1.8$ MeV and using ${\mathrm{CH}}_4$ as counting gas. Higher fission fragment energy loss is seen at low PH values for the ${}^{237}\mathrm{Np}$ sample. Additionally, a degradation of the PH distribution is evident for the more active sample ${}^{240}\mathrm{Pu}$, even though it is the thinnest one.

Figure 5

Geometry used in the simulations with the MCNP code. Left: setup #1; right: setup #2. Besides the shielding, the main difference between the two setups is the thickness of the water layer (2 mm for setup #1; 1 mm for setup #2).

Figure 6

Correction factors to account for the neutrons outside the region of interest produced by a thermalization in setup #1 between the neutron producing target and the fissile deposits. The results given are the ratio of the flux at the neutron energy of interest folded with the $(n,f)$ cross section of the sample isotope considered [$\mathrm{\Phi }\left(E_n\right)\sigma \left(E_n\right)]$ and the flux as a function of the neutron energy impinging the sample deposits folded with the neutron-induced fission cross section of the isotope of interest [ ${\sum }_i\mathrm{\Phi }\left(E_i\right)\sigma \left(E_i\right)]$. (a) Using a LiF neutron producing target; (b) using a TiT neutron producing target. For each case the initial neutron energy was a probability distribution as a function of the emission angle. The values were taken from Refs. [25, 26].

Figure 7

Influence of the shielding, the water layer and the ionization chamber to the correction factors for each of the two setups for (a) 0.8 MeV LiF and 1.5 MeV LiF and (b) 1.8 MeV TiT and 2.2 MeV TiT. Labels are s1 (setup #1), s1 nW (setup #1 without water layer), s1 nW nIC (setup #1 without water layer and ionization chamber structure), s2 (setup #2), s2 nSH (setup #2 without shielding), s2 nSH nW (setup #2 without shielding and water layer), and s2 nSH nW nIC (setup #2 without shielding, water layer, and ionization chamber structure). For a better visualization the ${}^{240}\mathrm{Pu}$ correction factors are not shown since its cross section is very similar to the one of ${}^{237}\mathrm{Np}$. In the case of LiF only the correction factors of ${}^{235}\mathrm{U}$ and ${}^{237}\mathrm{Np}$ are presented, since no measurements were performed with ${}^{238}\mathrm{U}$. Noticeably, for the two setups, the biggest influence comes from the water layer (2 mm for setup #1 and 1 mm for setup #2); on the other hand, the shielding structure of setup #2 has a negligible influence on the neutron background. Additionally, it is clear that isotopes with a fission threshold at higher energies are less affected by the neutron background than fissile isotopes.

Figure 8

Neutron-induced fission cross section of ${}^{240}\mathrm{Pu}$ using as reference ${}^{237}\mathrm{Np}(n,f)$ (blue triangles), ${}^{235}\mathrm{U}(n,f)$ (green stars), and ${}^{238}\mathrm{U}(n,f)$ (red dots). The agreement of the three data sets is striking, especially at 1.6 and 1.8 MeV incoming neutron energy. Yet, the ratio plot shows a deviation of 3–7 % in the neutron energy range from 0.7 MeV up to 2.5 MeV compared with the ENDF/B-VII.1 evaluation.

Figure 9

Summary of the results of this experiment (red stars) compared with the most relevant experiments performed on the neutron-induced fission cross section of ${}^{240}\mathrm{Pu}$ and with current evaluations. The evaluations chosen are: ENDF/B-VII.1 [12], JEFF 3.1 [11], and JENDL 4.0 [13]. The experimental data shown are Ruddick and White (1964) [5] (open diamonds), White (1967) [6] (open crosses), Meadows (1981) [7] (open triangle up), Budtz-Jørgensen and Knitter (1981) [8] (open circles), Staples and Morley (1998) [9] (open triangles down), Laptev et al. (2004) [10] (open squares), and Tovesson et al. (2009) [4] (full circles). Selected data are shown for legibility of the plot. Data found as a ratio to ${}^{235}\mathrm{U}(n,f)$ were normalized to the ENDF/B-VII.1 evaluation of this isotope. Further explanation is given in the text.