Neutron capture cross section of ${}^{232}\mathrm{Th}$ measured at the n_TOF facility at CERN in the unresolved resonance region up to 1 MeV

We have measured the neutron capture reaction yield of ${}^{232}\mathrm{Th}$ at the neutron time-of-flight facility n_TOF at CERN in the energy range from 1 eV to 1 MeV. The average capture cross section has been extracted in the energy range from 4 keV up to 1 MeV with an overall accuracy better than 4%. An independent IAEA evaluation shows good agreement with the data. The average cross section has been expressed in terms of average resonance parameters using the partial waves $\ell =0,1,$ and 2.

Figure 1

Schematic view of the horizontal plane of detectors and sample geometry in the ${}^{232}\mathrm{Th}$($n,\gamma$) measurement. Neutrons going through the 5.8 cm inner diameter beam tube are incident on the sample placed in the beam. Two ${\mathrm{C}}_6{\mathrm{D}}_6$ detectors, each consisting of a scintillator cell 12.7 cm in diameter and 7.62 cm high coupled to a photomultiplier, are placed 9.2 cm upstream of the sample center.

Figure 2

Energy density, multiplied by kinetic energy, of the total number of neutrons $n$ incident on the sample plane for a standard burst of $7\times {10}^{12}$ protons as a function of neutron energy. Energy density is given as $\mathit{dn}/d\ln (E)=E\mathit{dn}/\mathit{dE}$.

Figure 3

Part of the counting spectrum of the Th sample with the Al filter in the neutron beam together with the fit of the 34.8 keV black resonance of Al. Energy density of the number of counts is given as $1/d\ln (E)=E/\mathit{dE}$.

Figure 4

Measured count rate of ${}^{232}\mathrm{Th}$($n,\gamma$) reaction per bunch of $7\times {10}^{12}$ protons and derived background applying γ-ray thresholds of 160 and 1000 keV.

Figure 5

Derived capture yields using spectra with γ-ray thresholds of 160 and 1000 keV. Yields are identical within their uncertainties up to a neutron energy of about 700 keV. Beyond that energy, γ rays from inelastic scattering become visible as a background that is eliminated only with the high threshold.

Figure 6

Correction factor $f(E_n)$ due to self-absorption and multiple scattering calculated with the code SESH [44].

Figure 7

Neutron capture cross section ${}^{232}\mathrm{Th}$($n,\gamma$) multiplied by the square root of the neutron energy, showing the measured data and an average parameter fit, compared to several other experimental data sets. To keep the figure readable, data covering energy bins are represented by points and uncertainties for some data sets are omitted.

Figure 8

Measured cross section and a description with average resonance parameters using the code FITACS. Partial wave contributions for $\ell =0,1$, and 2 are also shown.