Measurement of the ${}^{238}\mathrm{U}(n,\gamma )$ cross section up to 80 keV with the Total Absorption Calorimeter at the CERN n_TOF facility

The radiative capture cross section of a highly pure (99.999%), 6.125(2) grams and 9.56(5)$\times {10}^{-4}$ atoms/barn areal density ${}^{238}\mathrm{U}$ sample has been measured with the Total Absorption Calorimeter (TAC) in the 185 m flight path at the CERN neutron time-of-flight facility n_TOF. This measurement is in response to the NEA High Priority Request list, which demands an accuracy in this cross section of less than 3% below 25 keV. These data have undergone careful background subtraction, with special care being given to the background originating from neutrons scattered by the ${}^{238}\mathrm{U}$ sample. Pileup and dead-time effects have been corrected for. The measured cross section covers an energy range between 0.2 eV and 80 keV, with an accuracy that varies with neutron energy, being better than 4% below 25 keV and reaching at most 6% at higher energies.

Figure 1

Raw counts per histogram bin with and without the ${}^{238}\mathrm{U}$ sample in place per pulse as a function of neutron energy (left) and deposited energy (in the neutron energy range 1–10 keV) (right) compared to counts with analysis conditions of $m_{\mathrm{cr}}>1$ and $2.5<E_{\mathrm{sum}}(\mathrm{MeV})<5.75$. The dip at 30 keV neutron energy is due to neutrons interacting with aluminium in the beam line and the structures in the deposited energy spectra correspond to neutron capture reactions with the sample plus surrounding materials.

Figure 2

Carbon and ${}^{238}\mathrm{U}$ deposited energy spectra, per histogram bin, for $m_{\mathrm{cr}}>1$ and with the sample-out and beam-off backgrounds subtracted, where the carbon spectrum has been scaled above 7 MeV to match the number of ${}^{238}\mathrm{U}$ counts to allow an estimation of the scattering contribution to the ${}^{238}\mathrm{U}$ data.

Figure 3

Neutron scattering contribution to the ${}^{238}\mathrm{U}$ data with 20 bins per decade.

Figure 4

The highest count rate in the data is found in the resonance at 66 eV. The asymmetrical resonance shape in the $1.0\times {10}^{12}\mathrm{ppp}$ (HIGH) data is removed after correcting for pileup and dead-time.

Figure 5

The saturated peak of the first 6.7 eV ${}^{238}\mathrm{U}$ resonance and residuals when compared to the sammy calculation.

Figure 6

The ${}^{238}\mathrm{U}$ experimental yield after initial background subtraction compared to the calculated yield from sammy using resonance parameters from JEFF 3.1.2. A small background still remains within the data, which can be quantified by finding the required normalization to match the data to what is expected as a function of neutron energy.

Figure 7

Measured residual background points as a function of energy and the function found to best fit these data points.

Figure 8

Background subtracted ${}^{238}\mathrm{U}$ counts are compared to the individual and total background contributions.

Figure 9

Experimental capture yield (black) and the sammy fit (red) in the neutron energy interval 4.5–5 keV, which represents the statistical upper limit of possible resonance analysis. The resonance parameters and resonance kernels can be found in the exfor entry [35]

Figure 10

Ratios of the capture resonance kernels determined from this work divided by those found by Mingrone et al. [10] and Kim et al. [11] and the JEFF 3.1.2 evaluation.

Figure 11

Experimental ${}^{238}\mathrm{U}\left(\mathrm{n},\gamma \right)$ cross section from this work compared to the other recent measurements and evaluations in the neutron energy range 5–80 keV.

Figure 12

Ratios of the measured ${}^{238}\mathrm{U}\left(\mathrm{n},\gamma \right)$ cross section from this work and other recent measurements and evaluations.