Polarization asymmetries in the ${}^9\mathrm{Be}(\gamma ,n_0)$ reaction

Measurements of the ${}^9\mathrm{Be}\left(\gamma ,n_0\right)$ reaction were performed using nearly 100% linearly polarized, high-intensity, and nearly monoenergetic $\gamma$-ray beams having energies between 5.5 and 15.5 MeV at the High Intensity $\gamma$-ray Source located at Duke University and Triangle Universities Nuclear Laboratory. Eighteen liquid scintillator detectors were used to measure neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, which are the differences between yields observed in detectors located in-plane and out-of-plane divided by their sums, were measured for the neutrons which left the residual nucleus $\left({}^8\mathrm{Be}\right)$ in its ground state, termed the $n_0$ group. Asymmetries between 0.4 to 0.7 were discovered over this energy region in addition to a clear trend of increasing asymmetries with increasing beam energy. A prediction of the polarization asymmetry based on a pure $E1$ direct capture model shows good agreement with the experimental measurements. These data and the prediction could be of interest for methods that rely on neutron measurements following photofission to identify the presence of special nuclear material.

Figure 1

A schematic of the detector array reproduced from Ref. [1]. The detectors were located at scattering angles of $\theta ={55}^{\circ },{72}^{\circ },{90}^{\circ },{107}^{\circ },{125}^{\circ }$, and ${142}^{\circ }$ and azimuthal angles of $\varphi =0^{\circ },{90}^{\circ },{180}^{\circ },{270}^{\circ }$.

Figure 2

Typical two dimensional pulse height vs PSD spectra for AmBe source measurements are shown with the hardware PSD threshold in the MPD-4 module disabled (top) and enabled (bottom). The dashed line is the location of the hardware PSD threshold.

Figure 3

A typical two dimensional time-of-flight vs PSD spectrum is shown for a beam energy of 10 MeV and a scattering angle of ${90}^{\circ }$. Some $\gamma$-ray events are visible at early time-of-flights, and both the $n_0$ and $n_1$ groups are observed at later time-of-flights.

Figure 4

The neutron energy spectrum is shown for a linearly polarized beam at an energy of 10.0 MeV for two detectors: one in the plane of beam polarization (black, upper histogram) and the other perpendicular to the plane of beam polarization (red, lower histogram). Both detectors were located at a scattering angle of $\theta ={90}^{\circ }$. The dashed line indicates the placement of the energy cut to eliminate the $n_1$ group.

Figure 5

The measured polarization asymmetries $\mathrm{\Sigma }\left(\theta \right)$ at a beam energy of 10.0 MeV are fit using Eq. (2). The uncertainties are statistical and systematic combined. Uncertainties not shown are smaller than the size of the data points.

Figure 6

The measured $b$ values as a function of beam energy (black points) are compared to the results of the calculation (red line). The uncertainties shown are statistical and systematic combined.