Constraining the calculation of ${}^{234,236,238}\mathrm{U}(n,\gamma )$ cross sections with measurements of the $\gamma$-ray spectra at the DANCE facility

The cross section for neutron capture in the continuum region has been difficult to calculate accurately. Previous results for ${}^{238}\mathrm{U}$ show that including an $M1$ scissors-mode contribution to the photon strength function resulted in very good agreement between calculation and measurement. This paper extends that analysis to ${}^{234,236}\mathrm{U}$ by using $\gamma$-ray spectra measured with the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center to constrain the photon strength function used to calculate the capture cross section. Calculations using a strong scissors-mode contribution reproduced the measured $\gamma$-ray spectra and were in excellent agreement with the reported cross sections for all three isotopes.

Figure 1

Nuclear level density for ${}^{237,239}\mathrm{U}$ obtained by the Oslo method [13] compared to estimates from global fits for ${}^{237}\mathrm{U}$. “Oslo Renorm” designates the Oslo NLD renormalized as described in the text. The CT predictions are from Refs. [17] and [18] and the ${\mathrm{CoH}}_3$ prediction used an updated parametrization of the model in Ref. [19].

Figure 2

Comparison of PSF models used in simulations and the renormalized experimental data from Oslo [8]. “Renorm Oslo” designates the Oslo PSFs renormalized as described in the text. See text for a description of the other models. The total photoneutron cross-section data of Caldwell [15] are also shown.

Figure 3

Measured $\gamma$-ray spectra for several $1/2^{+}$ resonances in ${}^{234,236,238}\mathrm{U}(n,\gamma )$ compared to calculations made with photon strength-function and nuclear level-density parameters obtained from the Oslo method and by renormalizing the Oslo results as described in the text. The resonance energies are indicated in each panel. The $y$-axis counts are arbitrarily normalized.

Figure 4

Measured $\gamma$-ray spectra for several $1/2^{+}$ resonances in ${}^{234,236,238}\mathrm{U}(n,\gamma )$ compared to calculations made with photon strength-function and nuclear level-density parameters obtained from systematics used in the ${\mathrm{CoH}}_3$ code and using the MGLO and ${\mathrm{CoH}}_3$ models for the $E1$ strength. The resonance energies are indicated in each panel. The $y$-axis counts are arbitrarily normalized.

Figure 5

Summed-energy spectra for two resonances in ${}^{236}\mathrm{U}(n,\gamma )$ compared to calculations using the PSF and NLD from the Oslo method [8] and using the parameters in the ${\mathrm{CoH}}_3$ code. The $y$-axis counts are arbitrarily normalized.

Figure 6

Calculated cross sections for ${}^{234,236,238}\mathrm{U}(n,\gamma )$ compared to the ENDFB/VII-1 evaluation [3] and representative data. For ${}^{236}\mathrm{U}$, measurements by Adamchuk [34], Buleeva [33], and Carlson [32] are shown. The tabulated data were obtained from the EXFOR data base [35]. The ${}^{238}\mathrm{U}(n,\gamma )$ results were published previously [2].