Spatial anisotropy of neutrons emitted from the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction with a linearly polarized $\gamma$-ray beam

We have measured the azimuthal anisotropy of neutrons emitted from the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction with a linearly polarized $\gamma$-ray beam generated by laser Compton scattering at NewSUBARU. Neutron yields at the polar angle of ${90}^{\circ }$ have been measured as a function of the azimuthal angle $\varphi$ between the detector and the linear polarization plane of the $\gamma$-ray beam. The azimuthal anisotropy of neutrons measured at $\varphi =0^{\circ }$, ${10}^{\circ }$, ${25}^{\circ }$, ${45}^{\circ }$, ${60}^{\circ }$, ${70}^{\circ }$, and ${90}^{\circ }$ has been well reproduced using a theoretically predicted function of $a+bcos\left(2\varphi \right)$.

Figure 1

Schematic view of the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction using a laser Compton scattering $\gamma$-ray beam. Excited states with energies of 14–16.7 MeV, which locate above the neutron separation energy of $\mathrm{Sn}=11.197$ MeV, are populated by the absorption of $\gamma$ rays. The individual excited states subsequently decay to states in a residual nucleus ${}^{55}\mathrm{Fe}$ by neutron emission.

Figure 2

Time-of-flight spectrum for the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction with linearly polarized photons. Neutrons and $\gamma$ rays from the ${}^{56}\mathrm{Fe}$ target are clearly separated. The natural backgrounds originating from radioactivities and cosmic rays are lower than the neutron yields.

Figure 3

Neutron anisotropy from the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction with the linearly polarized $\gamma$ beam. The solid line is the function of $a+bcos\left(2\varphi \right)$ obtained by ${\chi }^2$ fitting. The measured data are reproduced well.