Measurement and resonance analysis of the ${}^{237}$Np neutron capture cross section

The neutron capture cross section of ${}^{237}$Np was measured between 0.7 and 500 eV at the CERN n_TOF facility using the 4$\pi$ BaF${}_2$ Total Absorption Calorimeter. The experimental capture yield was extracted minimizing all the systematic uncertainties and was analyzed together with the most reliable transmission data available using the sammy code. The result is a complete set of individual as well as average resonance parameters [$D_0=0.56\left(2\right)$ eV, $\langle {\Gamma }_{\gamma }\rangle =40.9\left(18\right)$ meV, ${10}^4{S}_0=0.98\left(6\right)$, $R^{\prime }=9.8\left(6\right)$ fm]. The capture cross section obtained in this work is in overall agreement with the evaluations and the data of Weston and Todd [Nucl. Sci. Eng. 79, 184 (1981)], thus showing sizable differences with respect to previous data from Scherbakov et al. [J. Nucl. Sci. Technol. 42, 135 (2005)] and large discrepancies with data Kobayashi et al. [J. Nucl. Sci. Technol. 39, 111 (2002)]. The results indicate that a new evaluation combining the present capture data with reliable transmission data would allow reaching an accuracy better than 4$\%$, in line with the uncertainty requirements of the nuclear data community for the design and operation of current and future nuclear devices.

Figure 1

View of the Total Absorption Calorimeter as it is implemented in the code geant4 [33] used for the Monte Carlo simulations.

Figure 2

Deposited energy distribution measured with the neptunium and the auxiliary samples. The data correspond to neutron energies between 1 and 10 eV.

Figure 3

Deposited energy distribution from ${}^{237}$Np($n$,$\gamma$) under different conditions in multiplicity. The data correspond to neutron energies between 1 and 10 eV.

Figure 4

Neutron energy distribution measured with the neptunium and auxiliary samples under the conditions $m_{\mathrm{cr}}$ > 2 and 2.5 < $E_{\mathrm{sum}}\left(\mathrm{MeV}\right)$ < 6.

Figure 5

Measured and simulated TAC response corresponding to the ${}^{237}$Np ($E_n=1.32$ eV, $J=3^{+}$). (a) Deposited energy distribution with and without conditions in $m_{\mathrm{cr}}$. (b) crystal multiplicity distribution ($E_{\mathrm{sum}}$ > 1 MeV).

Figure 6

sammy fits (solid lines) to the capture yield (red markers) and the transmission (green markers) data between 1 and 20 eV.

Figure 7

sammy fits (solid lines) to the capture yield (red markers) and the transmission (green markers) data between 20 and 43 eV.

Figure 8

Correlation matrix for the first 35 resonances ($E$${}_n$, ${\Gamma }_{\gamma }$, and ${\Gamma }_n$ for each resonance) including the negative resonance (first three parameters) and the normalization and constant background variables (parameters 106 and 107, respectively).

Figure 9

sammy fits (solid lines) to the capture yield (red markers) and the transmission (green markers) data between 43 and 80 eV.

Figure 10

sammy fits (solid lines) to the capture yield (red markers) and the transmission (green markers) data between 430 and 500 eV.

Figure 11

Variation around the mean value of the normalization found in the different energy intervals.

Figure 12

Cumulative number of levels as a function of neutron energy. Both $J=2$ and $J=-3$ resonances are included.

Figure 13

Cumulative sum of reduced neutron widths ${\Gamma }_n^0={\Gamma }_n{(1\mathrm{eV}/E_n)}^{1/2}$.

Figure 14

Level spacing distribution of the n_TOF resonance parameters below 90 eV compared to the expected Wigner distribution.

Figure 15

Ratios of the n_TOF average capture cross section of ${}^{237}$Np from 1 to 500 eV and those of previous measurements and evaluations (see Table 5).