Angular distribution of $\gamma$ rays from the $p$-wave resonance of ${}^{118}\mathrm{Sn}$

The neutron energy-dependent angular distribution of $\gamma$ rays from the ${}^{117}\mathrm{Sn}(n,\gamma )$ reaction was measured with germanium detectors and a pulsed neutron beam. The angular distribution was clearly observed in $\gamma$-ray emissions with an energy of 9327 keV which corresponds to the transition from a neutron resonance of ${}^{117}\mathrm{Sn}+n$ to the ground state of ${}^{118}\mathrm{Sn}$. The angular distribution causes an angular-dependent asymmetric resonance shape. An asymmetry $A_{\mathrm{LH}}$ was defined as $(N_{\mathrm{L}}-N_{\mathrm{H}})/(N_{\mathrm{L}}+N_{\mathrm{H}})$, where $N_{\mathrm{L}}$ and $N_{\mathrm{H}}$ are integrated values for lower- and higher-energy regions of a neutron resonance, respectively. We found that the $A_{\mathrm{LH}}$ has the angular dependence of $(Acos{\theta }_{\gamma }+B)$, where ${\theta }_{\gamma }$ is the $\gamma$-ray emission angle with respect to the incident neutron momentum, with $A=0.394\pm 0.073$ and $B=0.118\pm 0.029$ in the 1.33 eV $p$-wave resonance.

Figure 1

Histogram of $\partial I_{\gamma }/\partial E_{\gamma }^{\mathrm{m}}$ in the range of 8–10 MeV.

Figure 2

Histogram of $\partial I_{\gamma }/\partial E_n^{\mathrm{m}}$ in the range of 0–10 eV. In this histogram, $\gamma$ rays whose energies were below about 200 keV were eliminated. The peak of 1.33 eV is a $p$-wave resonance of ${}^{118}\mathrm{Sn}$.

Figure 3

Histograms of $\partial I_{\gamma }/\partial E_{\gamma }^{\mathrm{m}}$ for each background event. Blue-open histogram is the measured spectrum. Red-hatched one is a simulated spectrum for monochromatic energy of 9563 keV. This was scaled so that the intensity of the full-absorption peak matched that of the measured one. Black-striped histogram is the spectrum of pileup events which were obtained by subtracting the red-hatched spectrum and a simulated spectrum of monochromatic energy of 9327 keV.

Figure 4

Histograms of $\partial I_{\gamma }/\partial E_n^{\mathrm{m}}$ around the 1.33 eV $p$-wave resonance in ${}^{118}\mathrm{Sn}$ for various angles accessible at ANNRI. The central figure shows the placement and shape of each germanium crystal. The detectors marked by red crosses were not used in this measurement.

Figure 5

Fitting result for $A_{\mathrm{LH}}\left({\theta }_{\gamma }\right)$ using Eq. (3). The angular-dependent parameter $A$ were obtained to be a significant value.