$\gamma$-ray strength function for ${}^{116,117}\mathrm{Sn}$ with the pygmy dipole resonance balanced in the photoneutron and neutron capture channels

Photoneutron cross sections were measured for ${}^{117}\mathrm{Sn}$ and ${}^{116}\mathrm{Sn}$ near the neutron thresholds at $6.94$ and $9.56$ MeV, respectively, with quasi-monochromatic laser-Compton scattering $\gamma$ rays. The ${}^{117}\mathrm{Sn}$ cross section, which is strongly enhanced near the low threshold, provides evidence for the presence of extra $\gamma$ strength in the low-energy tail of the giant dipole resonance. A coherent analysis of the photoneutron data for ${}^{117}\mathrm{Sn}$ together with the neutron capture on ${}^{116}\mathrm{Sn}$ shows that the $\gamma$-ray strength function is balanced in the photoneutron and neutron capture channels in terms of the microscopic Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation model of $E1$ strength combined with a pygmy $E1$ resonance at $8.5$ MeV. The high-energy part of the pygmy resonance is also suggested in the photoneutron cross section for ${}^{116}\mathrm{Sn}$.

Figure 1

Photoneutron cross section for ${}^{117}\mathrm{Sn}$. The open and solid circles show the results deduced with the ${\chi }^2$-fit and the Taylor expansion methods, respectively. The data of [29, 30] are also shown for comparison. The experimental cross section is compared with the $E1$ threshold behavior $\sigma (E_{\gamma })\propto (E_{\gamma }-S_n){}^{\ell +1/2}$ with $\ell =1$ (dashed line).

Figure 2

Same as Fig. 1 for ${}^{116}\mathrm{Sn}$. The data of [29], [30], [35] are also shown.

Figure 3

${}^{117}\mathrm{Sn}$($\gamma ,n$)${}^{116}\mathrm{Sn}$ photoneutron cross section. The full dots correspond to our new measurements and the other symbols to the former photoabsorption data [29, 30]. Note that the two sets of photoneutron cross sections shown in Fig. 1 are averaged. The theoretical cross sections considered in this study (see text) are compared with the $E1$ threshold behavior $\sigma (E_{\gamma })\propto (E_{\gamma }-S_n){}^{\ell +1/2}$ with $\ell =1$ (dot-dash line).

Figure 4

${}^{116}\mathrm{Sn}$($n,\gamma$)${}^{117}\mathrm{Sn}$ cross section with different $E1$ strengths (see Fig. 3) and compared with experimental data [39, 40, 41, 42].

Figure 5

Same as Fig. 3 for ${}^{116}\mathrm{Sn}$($\gamma ,n$)${}^{115}\mathrm{Sn}$. Note that the two sets of photoneutron cross sections shown in Fig. 2 are averaged. The data of [29, 30, 35] are also shown.