Azimuthal angle distributions of neutrons emitted from the ${}^9\mathrm{Be}(\gamma ,n)$ reaction with linearly polarized $\gamma$ rays

The electromagnetic transitions of ${}^9\mathrm{Be}$ with linearly polarized $\gamma$ rays are calculated by using the $\alpha +\alpha +n$ three-body model and the complex-scaled solutions of the Lippmann-Schwinger equation; the azimuthal angle distributions of the emitted neutrons are investigated. We calculate the anisotropy parameter as a function of the photon incident energy $E_{\gamma }$ and discuss how sensitive the anisotropy parameter is to nuclear structure and transition modes. The result suggests that the azimuthal angle distribution of neutrons emitted from the ${}^9\mathrm{Be}(\gamma ,n)$ reaction with the linearly polarized $\gamma$ rays is useful to identify the resonances in the final states even if it is not clearly observed in the cross section.

Figure 1

Calculated cross sections of the ${}^9\mathrm{Be}(\gamma ,n)$ reaction. Panel (a) represents the cross sections of up to $E_{\gamma }=16\mathrm{MeV}$, and panel (b) is the enlarged figure of panel (a) in the range of $1.5\le E_{\gamma }\le 5.5\mathrm{MeV}$. The contributions of the $E1$ and $M1$ transitions are shown as the red (dashed) and blue (dotted) lines, respectively. The sum of the contributions is shown as the black (solid) line. The open squares and open circles represent experimental data taken from Refs. [3] and [6], respectively.

Figure 2

Anisotropy parameters in the ${}^9\mathrm{Be}(\vec{\gamma },n)$ reaction. Those for the $E1$ and $M1$ transitions are represented as the red (solid) and blue (dotted) lines, respectively. The vertical line indicates the breakup threshold of ${}^9\mathrm{Be}$. The inset shows the enlarged figure for the energy region corresponding to the $5/2^{-}$ resonance.

Figure 3

Anisotropy parameters for the $E1$ transitions with and without the effect of the ${}^8\mathrm{Be}$ excitation in comparison with the experimental data. The red (solid) line represents the result which includes the excitation of the ${}^8\mathrm{Be}$ subsystem and the blue (dotted) line is the result obtained by switching off the ${}^8\mathrm{Be}$ excitation. The experimental data are represented as open circles and are taken from Ref. [4].