Isomeric ratio measurements for the radiative neutron capture ${}^{176}\mathrm{Lu}(n,\gamma )$ at the LANL DANCE facility

The isomeric ratios for the neutron capture reaction ${}^{176}\mathrm{Lu}(n,\gamma )$ to the $J^{\pi }=5/2^{-}$, 761.7 keV, $T_{1/2}=32.8$ ns and the $J^{\pi }=15/2^{+}$, 1356.9 keV, $T_{1/2}=11.1$ ns levels of ${}^{177}\mathrm{Lu}$ have been measured for the first time. The experiment was carried out with the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos National Laboratory. Measured isomeric ratios are compared with talys calculations using different models for photon strength functions, level densities, and optical potentials. In order to reproduce the experimental $\gamma$-ray spectra, a low-energy resonance must be added in the photon strength function used in our Hauser-Feshbach calculations.

Figure 1

Delayed $\gamma$-ray spectra obtained with the DANCE array for $M=3$ to $M=6$ cluster multiplicities. Spectra with only one delayed $\gamma$ ray for the Lu and the blank targets and two delayed $\gamma$ rays for the Lu target are plotted.

Figure 2

Time spectra obtained by applying energy gates on energy spectra with one delayed $\gamma$ ray for the Lu and blank targets. The exponential fits used to obtain the isomer population and lifetime are also plotted.

Figure 3

Partial level scheme of ${}^{177}\mathrm{Lu}$ [26, 27]. The absolute $\gamma$ intensities are reported.

Figure 4

Total energy spectra for the ${}^{176}\mathrm{Lu}$ and the blank targets. The background-subtracted spectrum, used to determine the number of detected cascades is also plotted with the integral limits.

Figure 5

Cumulative discrete and continuous level density. The positions of the isomers of interest and of the last discrete level taken into account are also plotted.

Figure 6

Comparison between experimental and calculated $\gamma$-ray spectra.

Figure 7

Comparison between experimental and calculated $\gamma$-ray spectra with $M1,E1$, and $E2$ resonances. All the resonances have the same parameters.

Figure 8

Experimental data from the EXFOR database and talys calculations of the ${}^{176}\mathrm{Lu}(n,\gamma )$ cross section with and without the added $M1$ resonance in the PSF. The PSF in the talys calculation are not renormalized: $G_{\mathrm{norm}}=1$.

Figure 9

Evolution of the isomeric ratios as a function of the incident neutron energy. Calculations performed with the $M1$ resonance added in the PSF.

Figure 10

Isomeric ratio evolutions as a function of the last discrete level energy.