Measurement of the neutron-induced fission cross section of ${}^{230}\mathrm{Th}$ at the CERN n_TOF facility

The neutron-induced fission cross section of ${}^{230}\mathrm{Th}$ has been measured at the neutron time-of-flight facility n_TOF located at CERN. The experiment was performed at the experimental area EAR-1 with a neutron flight path of 185 m, using Micromegas detectors for the detection of the fission fragments. The ${}^{230}\mathrm{Th}(n,f)$ cross section was determined relative to the ${}^{235}\mathrm{U}(n,f)$ one, covering the energy range from the fission threshold up to 400 MeV. The results from the present work are compared with existing cross-section datasets and the observed discrepancies are discussed and analyzed. Finally, using the code empire 3.2.3 a theoretical study, based on the statistical model, was performed leading to a satisfactory reproduction of the experimental results with the proper tuning of the respective parameters, while for incident neutron energy beyond 200 MeV the fission of ${}^{230}\mathrm{Th}$ was described by Monte Carlo simulations.

Figure 1

Experimental data for the neutron-induced fission cross section of ${}^{230}\mathrm{Th}$ available in the literature. The data present discrepancies and cover the energy range from the fission threshold up to 25 MeV. The measurements of the present work can assist in resolving the discrepancies and provide data at higher energies.

Figure 2

Evaluated cross-section data for the ${}^{230}\mathrm{Th}(n,f)$ reaction from ENDF/B-VIII.0 [19] which adopts the JENDL-5.0 library [20] (red), JEFF-3.3 [21] (blue), and TENDL-2021 [22] (green). Large differences are observed between the evaluation libraries. The data of this work can help in improving the evaluations.

Figure 3

A photograph of the stack of the Micromegas detectors and samples assembly used in the experiment. The seven ${}^{230}\mathrm{Th}$ targets were placed between the reference targets.

Figure 4

$\gamma$-flash stack from various events. The average $\gamma$-flash shape and the subsequent baseline (red line) are estimated from the stack.

Figure 5

Distribution of the moderation length $\lambda$ as a function of the neutron energy.

Figure 6

Distribution of the amplitude of the signals for the ${}^{235}\mathrm{U}$ sample as a function of the neutron energy. The $\alpha$ particles from the natural radioactivity of the target are present in the low energy region and the $\gamma$-flash residuals (increasing with energy) are present in the high energy region.

Figure 7

Comparison between experimental (blue line) and simulated (red line) spectra (normalized in the counts of fission fragments) for one of the ${}^{230}\mathrm{Th}$ (#3) targets, after the calibration and application of a skewed Gaussian response function to the simulated one. The amplitude cut for this energy region is presented with the dashed green line.

Figure 8

Counting rate of the reference ${}^{235}\mathrm{U}$ target (red line) and for one of the ${}^{230}\mathrm{Th}$ targets (black line), corresponding to the dedicated pulses.

Figure 9

Expected counts per proton for ${}^{230}\mathrm{Th}$ (black line) and for all the Pu isotopes present in the ${}^{230}\mathrm{Th}$ target (red line) for the experimental area EAR-1 measurement. The yield of all Pu contaminants is negligible when compared to the yield of ${}^{230}\mathrm{Th}$ in this energy region.

Figure 10

Comparison between experimental counts (red line) and expected counts from the Pu contaminants present in the ${}^{230}\mathrm{Th}$ samples (blue line) of the experimental area EAR-2 measurement. The expected counts are normalized to the experimental ones in the energy region between 4 eV and 3 keV.

Figure 11

Cross section of the ${}^{238}\mathrm{U}(n,f)$ reaction (black circle), estimated using the ${}^{235}\mathrm{U}(n,f)$ reaction as reference, plotted along with the reference cross section of ${}^{238}\mathrm{U}(n,f)$ [19, 44].

Figure 12

Cross section from the seven ${}^{230}\mathrm{Th}$ samples for the ${}^{230}\mathrm{Th}(n,f)$ reaction, estimated using the ${}^{235}\mathrm{U}(n,f)$ reaction as reference. Only the statistical errors are shown in the figure.

Figure 13

Cross section of the ${}^{230}\mathrm{Th}(n,f)$ reaction (black circle), estimated using the ${}^{235}\mathrm{U}(n,f)$ reaction as reference, plotted along with the previous datasets. The binning is isolethargic with 2000 bins per decade (bpd) up to 0.8 MeV, 100 bpd from 0.8 to 30 MeV, and 20 bpd from 30 to 400 MeV. Only the statistical uncertainties are shown in the figure.

Figure 14

Cross section of the ${}^{230}\mathrm{Th}(n,f)$ reaction (black circle), estimated using the ${}^{235}\mathrm{U}(n,f)$ reaction as reference, plotted along with the previous datasets in the region of the resonance near 700 keV. The binning used is isolethargic with 2000 bins per decade (bpd). Only the statistical uncertainties are shown in the figure.

Figure 15

Cross section of the ${}^{230}\mathrm{Th}(n,f)$ reaction (black circle), estimated using the ${}^{235}\mathrm{U}(n,f)$ reaction as reference, plotted along with the data of Blons et al. (orange points) and James et al. (cyan cross) normalized to the current data. Only the statistical uncertainties are shown in the figure.

Figure 16

Uncertainty due to counting statistics of the ${}^{230}\mathrm{Th}(n,f)$ cross section, for energies higher than 0.8 MeV.

Figure 17

empire calculation for the ${}^{230}\mathrm{Th}$ neutron induced fission cross section (up to 200 MeV) with the default parameters (blue line) and the modified parameters (red line) along with the experimental cross section of this work (black points).

Figure 18

Comparison of Monte Carlo calculated fission cross sections with experimental data.