Investigation of the ${}^{240}\mathrm{Pu}(n,f)$ reaction at the n_TOF/EAR2 facility in the 9 meV–6 MeV range

Background: Nuclear waste management is considered amongst the major challenges in the field of nuclear energy. A possible means of addressing this issue is waste transmutation in advanced nuclear systems, whose operation requires a fast neutron spectrum. In this regard, the accurate knowledge of neutron-induced reaction cross sections of several (minor) actinide isotopes is essential for design optimization and improvement of safety margins of such systems. One such case is ${}^{240}\mathrm{Pu}$, due to its accumulation in spent nuclear fuel of thermal reactors and its usage in fast reactor fuel. The measurement of the ${}^{240}\mathrm{Pu}(n,f)$ cross section was previously attempted at the CERN n_TOF facility EAR1 measuring station using the time-of-flight technique. Due to the low amount of available material and the given flux at EAR1, the measurement had to last several months to achieve a sufficient statistical accuracy. This long duration led to detector deterioration due to the prolonged exposure to the high $\alpha$ activity of the fission foils, therefore the measurement could not be successfully completed.

Purpose: It is aimed to determine whether it is feasible to study neutron-induced fission at n_TOF/EAR2 and provide data on the ${}^{240}\mathrm{Pu}(n,f)$ reaction in energy regions requested for applications.

Methods: The study of the ${}^{240}\mathrm{Pu}(n,f)$ reaction was made at a new experimental area (EAR2) with a shorter flight path which delivered on average 30 times higher flux at fast neutron energies. This enabled the measurement to be performed much faster, thus limiting the exposure of the detectors to the intrinsic activity of the fission foils. The experimental setup was based on microbulk Micromegas detectors and the time-of-flight data were analyzed with an optimized pulse-shape analysis algorithm. Special attention was dedicated to the estimation of the non-negligible counting loss corrections with the development of a new methodology, and other corrections were estimated via Monte Carlo simulations of the experimental setup.

Results: This new measurement of the ${}^{240}\mathrm{Pu}(n,f)$ cross section yielded data from $9\mathrm{meV}$ up to $6\mathrm{MeV}$ incident neutron energy and fission resonance kernels were extracted up to $10\mathrm{keV}$.

Conclusions: Neutron-induced fission of high activity samples can be successfully studied at the n_TOF/EAR2 facility at CERN covering a wide range of neutron energies, from thermal to a few MeV.

Figure 1

Amplitude spectra recorded in EAR1 and EAR2 for a ${}^{240}\mathrm{Pu}$ sample. The $\alpha$-particle background in EAR2 is appreciably suppressed while the fission rate is significantly higher.

Figure 2

Schematic view of the fission foil stack, with respect to the neutron beam direction. Apart from the fission samples, an empty cathode was placed to monitor possible proton and $\alpha$ recoils from the detector itself.

Figure 3

Stacked recorded waveforms in the $\gamma$-flash region for a ${}^{240}\mathrm{Pu}$ sample. The solid line corresponds to the calculated average. The signals shown correspond to $1\%$ of the statistics. A few indicative neutron energies are also shown.

Figure 4

Stacked residuals between the average $\gamma$-flash and the recorded waveforms in the $\gamma$-flash region for a ${}^{240}\mathrm{Pu}$ sample. The inset contains the projection of the residuals to the $y$ axis, up to $10\mathrm{MeV}$ neutron energy. The signals shown correspond to $1\%$ of the statistics.

Figure 5

Typical 2D distribution of the reconstructed time-of-flight and amplitude signals for a ${}^{240}\mathrm{Pu}$ sample. Residuals from the $\gamma$-flash subtraction and signals from the $\alpha$ activity are illustrated in the bottom left and right parts of the figure, respectively. Resonances are also visible. A few indicative neutron energies are shown.

Figure 6

Statistical uncertainties, after applying the corrections, in the $100\mathrm{keV}$–$6\mathrm{MeV}$ high-energy region concerning the lightest ${}^{240}\mathrm{Pu}$ sample. Up to $1\mathrm{MeV}$ an isolethargic binning of 100 bins per decade was used whereas in the MeV region a custom binning that is shown in Appendix pp2 was adopted.

Figure 7

Comparison between the experimental and simulated amplitude spectra from a ${}^{240}\mathrm{Pu}$ sample. For the low amplitude region, a beam-off spectrum was added to the simulated one. The reproduction of the experimental points is quite satisfactory. The shaded area represents the fraction of the rejected FF for an amplitude threshold equal to 30 channels.

Figure 8

The $f_{\text{imp}}$ correction factor (top panel) applied to ${}^{240}\mathrm{Pu}$ with respect to the neutron energy. The bottom panel shows the total estimated uncertainty, which was obtained from the diagonal elements of the covariance matrix.

Figure 9

Exponential fits in waiting time distributions are a useful experimental tool in estimating counting losses by calculating the integral below the extrapolated fitting function [46].

Figure 10

Estimated correction factors for counting losses. Below $1\mathrm{MeV}$ the methodology proposed by Coates [43] and Moore [44] was applied, while above $1\mathrm{MeV}$ the correction was based on Ref. [45]. Average correction factors are shown per $0.5\mathrm{MeV}$, above $1\mathrm{MeV}$.

Figure 11

The neutron self-shielding correction was based on the Beer-Lambert law and ENDF/B-VIII.0 ($n$,tot) cross sections for the materials seen in the figure.

Figure 12

Correction factors for neutron beam attenuation that were applied to ${}^{240}\mathrm{Pu}$ and ${}^{238}\mathrm{U}$.

Figure 13

Comparison between beam-on and -off spectra recorded from the most massive ${}^{240}\mathrm{Pu}$ sample. The contribution of spontaneous fission was considered negligible. Spectra are normalized to the number of triggers for a direct comparison.

Figure 14

The neutron flux calculated from ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and SiMon2 was found in satisfactory agreement with the evaluated and the simulated ones.

Figure 15

The ${}^{238}\mathrm{U}(n,f)$ cross section that was calculated with reference to the ${}^{235}\mathrm{U}(n,f)$ one was in satisfactory agreement with the ENDF/B-VIII.0 evaluation.

Figure 16

The ${}^{240}\mathrm{Pu}(n,f)$ cross section that was derived in the present work spanned across a wide range in neutron energy, from $9\mathrm{meV}$ up to $6\mathrm{MeV}$.

Figure 17

The ${}^{240}\mathrm{Pu}(n,f)$ cross section between 9 and $100\mathrm{meV}$ in comparison with the experimental data of Eastwood et al. [14] and the evaluation by Bouland et al. [19] as well as the most common evaluation libraries [52, 53, 54, 55].

Figure 18

The high resolution ${}^{240}\mathrm{Pu}(n,f)$ cross section in the 1.05 eV region demonstrates the impressive capabilities of EAR2 in low energy fission measurements.

Figure 19

The cross section in the $10–21\mathrm{keV}$ energy region. It is evident that, despite the availability of high resolution data, the observed structures are only considered in the ENDF/B-VIII.0 evaluation [54].

Figure 20

The cross section in the $100\mathrm{keV}$–$1\mathrm{MeV}$ region. An overall agreement with reported data sets was observed apart from the one reported by Tovesson et al. [22].

Figure 21

Comparison of the cross section in the $1–6\mathrm{MeV}$ region with the respective statistical uncertainties.

Figure 22

The correlations of the ${}^{240}\mathrm{Pu}(n,f)$ cross section, which were calculated by means of covariance propagation.

Figure 23

Resonance at 1.05 eV where a fission width with a $5\%$ uncertainty was derived.

Figure 24

A few resonances that were resolved in the $19–400$ eV region. (a) Resonance at 20.4 eV. (b) Resonance at 152 eV. (c) Isolated class-I resonances at 260.2 and 286.9 eV. (d) Resonance at 405 eV. An overall agreement within uncertainties was observed with the evaluation by Bouland et al. [19], except for the resonance at 20.4 eV. See text for further details.

Figure 25

Cross sections in regions where resonances with high fission widths were observed. (a) The cross section close to the 782 eV resonance. (b) The cross section close to the 1402 eV resonance.

Figure 26

Prominent resonance structures that were observed between 6.2 and $10.2\mathrm{keV}$. A parametrization of the cross section is provided in Appendix pp1 using Reich-Moore resonance parameters.