$\gamma$ decay from the quasicontinuum of ${}^{197,198}\mathrm{Au}$

The average electromagnetic dipole response of levels in the quasicontinuum of ${}^{197,198}\mathrm{Au}$ has been measured using $({}^3\mathrm{He},{}^3{\mathrm{He}}^{\prime })$ and $\left(d,p\right)$ reactions. The extracted $\gamma$-ray strength functions have been normalized according to three model assumptions for the nuclear spin distribution. An enhancement in the energy region $E_{\gamma }=3.0\text{–}6.5$ MeV is observed for both isotopes. The $E1$ component of such excess of strength is studied in detail for ${}^{198}\mathrm{Au}$ and is interpreted as the pygmy dipole resonance with an energy centroid of $5.9\left(1\right)$ MeV and exhausts about $1\%$ of the total integrated strength. The pygmy dipole resonance is shown to have a significant impact on the calculated ${}^{197}\mathrm{Au}{(n,\gamma )}^{198}\mathrm{Au}$ cross section.

Figure 1

Excitation energy versus primary $\gamma$ ray energy of ${}^{198}\mathrm{Au}$. The black diagonal line $E=E_{\gamma }$ defines the $\gamma$ rays of highest energy within each excitation energy bin. The intensity of $\gamma$ transitions drops in correspondence with the neutron separation energy $S_n=6.512$ MeV.

Figure 2

$\gamma \mathrm{SFs}$ of ${}^{197}\mathrm{Au}$ (black filled diamonds) and ${}^{198}\mathrm{Au}$ (red filled circles). For the odd-mass isotope, in addition to the statistical error bars, a large systematic error (hatched area between black lines) is included, due to the uncertainties of the absolute normalization.

Figure 3

(a) Spin cutoff distributions as a function of the excitation energy chosen for the normalization of the $\gamma \mathrm{SF}$ of ${}^{198}\mathrm{Au}$: the rigid moment of inertia (RMI, EB2006) formula as provided by von Egidy and Bucurescu [38], the latest energy-dependent formula from the same authors (EB2009) [30], and the spin cutoff from a microscopic model for the level density (HFB+comb) [39]. (b) The spin distributions at $S_n=6.512$ MeV and (c) the normalized $\gamma \mathrm{SFs}$ corresponding to the three models adopted for $g(J,\sigma )$.

Figure 4

The $\gamma \mathrm{SF}$ of ${}^{198}\mathrm{Au}$ for two final excitation energies (black filled circles) $E_f=0.32$ MeV (a) and $E_f=1.1$ MeV (b). Comparison with the total $\gamma \mathrm{SF}$ (green dashed line) and the selected $\gamma \mathrm{SFs}$ (solid red and violet lines) extracted for a narrow window of final excitation energies centered at the chosen $E_f$.

Figure 5

(a) Comparison between the $\gamma \mathrm{SF}$ of ${}^{197,198}\mathrm{Au}$ from this work and from $\left(\gamma ,n\right)$ reactions [43, 44, 45, 46, 47]. The data are modeled with a combination of the EGLO model and four SLO components (model A). (b) Three possible normalizations for the $\gamma \mathrm{SF}$ of ${}^{198}\mathrm{Au}$ are compared (see text). Another model parametrization is proposed, based on the QRPA approach (red dashed line) for the $E1$ strength (model B).

Figure 6

(a) Calculated ${}^{197}\mathrm{Au}{(n,\gamma )}^{198}\mathrm{Au}$ cross section using three sets of level densities and $\gamma \mathrm{SFs}$ obtained from the normalization of the experimental data presented in this work assuming different spin distributions for ${}^{198}\mathrm{Au}$. (b) Calculated neutron capture cross section for ${}^{197}\mathrm{Au}$ using the EB2009 inputs, with (red line) and without (dashed black line) including the PDR contribution in the $\gamma \mathrm{SF}$. The predictions are compared with measured data from Refs. [65, 66, 67, 68, 69, 70, 71, 72].