Photodisintegration of ${}^6\mathrm{Li}$

The cross sections for the photodisintegration of ${}^6\mathrm{Li}$ have been measured for all the available reaction channels that have neutrons in the final state, except the ${}^6\mathrm{Li}(\gamma ,p_0){}^5\mathrm{He}\text{(g.s.)}$ channel. The cross sections were measured at the photon energies, 8, 9, 10, 11, 12, 13, 15, and 15.6 MeV using linearly polarized photons, and 20, 25, 30, and 35 MeV using circularly polarized photons. Associated Legendre function coefficients are extracted for the ${}^6\mathrm{Li}(\gamma ,n_0){}^5\mathrm{Li}\text{(g.s.)}$ and ${}^6\mathrm{Li}(\gamma ,n_1){}^5\mathrm{Li}\mathrm{(1.49)}$ channels. For all observed reaction channels Legendre function parametrizations are used to determine total cross sections. Comparisons with recent theoretical calculations of the total photodisintegration cross section using various nucleon-nucleon interactions are inconclusive because of the large uncertainty in the cross sections for reactions channels that are not measured. We hope that the precision of our results for specific reaction channels will prompt calculations for those reaction channels.

Figure 1

The arrangement of neutron detectors in the Blowfish Neutron Detector Array. (a) Beam left to right. (b) Beam into page.

Figure 2

An example TDC spectrum with a log scale on the vertical axis. One TDC unit on the horizontal axes corresponds to a nominal 0.1 ns. The peak is due to $\gamma$ rays Compton scattering from the target. There is a small neutron contribution to the right of the peak. The drops to zero on either side are due to hardware windowing.

Figure 3

Example PSD spectrum. All $\gamma$ rays have negative PSD parameter and all events with positive PSD parameter are neutron events. Some neutron events have negative PSD parameter at low light outputs. Light-output cuts are chosen to ensure that the separation of $\gamma$ rays and neutrons by PSD will introduce uncertainties in the measured neutron yields of only a few percent at most.

Figure 4

Experimental neutron kinetic energy spectra reported for two neutron detectors at a polar angle ${\theta }_{\mathrm{lab}}={90}^{\circ }$. One detector is at an angle-to-polarization of $\varphi =0^{\circ }$ (red online) while the other is at $\varphi ={90}^{\circ }$ (blue online) with a linearly polarized photon beam of energy 13 MeV. Neutron kinetic energy spectra are computed from time-of-flight spectra. The drop off at approximately 2 MeV is due to the detector efficiency dropping to zero. The apparent lower energy neutrons are due to neutron scattering as explained in the text.

Figure 5

Neutron yields for the reaction channel $(\gamma ,n_0)$ for the detectors of Blowfish used in the analysis at a photon energy of 13 MeV. The data points are the measured yields and the histogram (red online) represents our fit to the data. The uncertainties in the measured yields have been rescaled in order to account for the uncertainty introduced in separating the reaction channels. There are eight detectors at each polar angle and the angle is shown on the plot with vertical lines dividing detectors at different angles. Detectors at specific polar angles are either in the plane of polarization, perpendicular to it, or at ${45}^{\circ }$ to it. Note that detectors numbered 14, 15, 25, and 26 are omitted due to technical issues with those detectors and the detectors numbered 81–88 at angle ${22}^{\circ }$ are also omitted as they did not function reliably due to the large number of gamma-rays scattered into them at this forward angle.

Figure 6

Comparison between a measured time-of-flight spectrum (data points) and a time-of-flight spectrum generated by a simulation using the relative rates and angular distributions of the $(\gamma ,n_0),(\gamma ,n_1)$ and $(\gamma ,p_1)$ reaction channels (histogram, red online). The uncertainties in the measured data are statistical only. This comparison indicates that our method of separating reaction channels is working as we intend it.

Figure 7

The light-output spectrum for one cell (cell 42) for one data run at a photon energy of 9 MeV. The light output is in equivalent electron energy units $\left({\text{keV}}_{\text{ee}}\right)$. (a) The calibrated light-output spectrum with no cuts applied. (b) The light-output spectrum with a PSD cut applied and a light-output cut of 500 ${\text{keV}}_{\text{ee}}$. With this light-output cut the PSD is essentially perfect so all the counts in this spectrum are neutrons.

Figure 8

The time-of-flight spectra from the simulation of the $(\gamma ,n_0)$ reaction channel (histogram, purple online) compared with the measured spectrum (data points) taken with a photon energy of 9 MeV. The simulated spectrum was normalized to the measured spectrum. The uncertainties in the measured data are statistical only.

Figure 9

The time-of-flight spectra for the beam energy of 11 MeV. Shown are the simulation of the $(\gamma ,n_0)$ (purple online), $(\gamma ,n_1)$ (blue online), and $(\gamma ,p_1)$ (green online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The uncertainties in the measured data are statistical only.

Figure 10

The time-of-flight spectra for the beam energy of 13 MeV. Shown are the simulation of the $(\gamma ,n_0)$ (purple online), $(\gamma ,n_1)$ (blue online), and $(\gamma ,p_1)$ (green online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The uncertainties in the measured data are statistical only.

Figure 11

The time-of-flight spectra for the beam energy of 15 MeV. Shown are the simulation of the $(\gamma ,n_0)$ (purple online), $(\gamma ,n_1)$ (blue online), and $(\gamma ,p_1)$ (green online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The uncertainties in the measured data are statistical only.

Figure 12

The time-of-flight spectra for the beam energy of 20 MeV. Shown are the simulation of the $(\gamma ,n_0)$ (purple online), $(\gamma ,n_1)$ (blue online), and $(\gamma ,p_1)$ (green online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simul-ation histograms. The small background contribution from the photodisintegration of atmospheric nitrogen is represented by the black histogram. The uncertainties in the measured data are statistical only.

Figure 13

The time-of-flight spectra for the beam energy of 25 MeV. Shown are the simulation of the $(\gamma ,n_1)$ (blue online) and $(\gamma ,p_1)$ (green online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The small contribution from the photodisintegration of atmospheric nitrogen is represented by the black histogram. The uncertainties in the measured data are statistical only.

Figure 14

The time-of-flight spectra for the beam energy of 30 MeV. Shown are the simulation of the $(\gamma ,n_1)$ (blue online), $(\gamma ,p_1)$ (green online), $(\gamma ,n_2)$ (cyan online), and $(\gamma ,p_2)$ (orange online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The small contribution from the photodisintegration of atmospheric nitrogen is represented by the black histogram. The uncertainties in the measured data are statistical only.

Figure 15

The time-of-flight spectra for the beam energy of 35 MeV. Shown are the simulation of the $(\gamma ,n_1)$ (blue online), $(\gamma ,p_1)$ (green online) $(\gamma ,n_2)$ (cyan online), $(\gamma ,p_2)+(\gamma ,p_3)$ (orange online), and $(\gamma ,n_3)$ (purple online) reaction channels and the measured spectrum (black data points). The histogram through the data points (red online) is the sum of the simulation histograms. The small contribution from the photodisintegration of atmospheric nitrogen is represented by the black histogram. The uncertainties in the measured data are statistical only.

Figure 16

Associated Legendre function coefficients for the reaction channel $(\gamma ,n_0)$.

Figure 17

Cross sections for the reaction channels that could be extracted from our data at each energy. It was not possible to separate all reaction channels at all energies, therefore some reaction channels are combined at some energies. The points labeled ‘Observable’ represents the sum of the extracted reaction channel [i.e., all reaction channels that produce neutrons, excluding the $(\gamma ,p_0)$ channel]. Where data points are overlapping they have been displaced in energy for clarity (color online). Note that there is some model dependence in the extraction of reaction channels (see text for discussion).

Figure 18

The total photoneutron cross section for ${}^6\mathrm{Li}$ estimated from our data (closed circles) compared with that of the Livermore group [32] (open squares, blue online). The closed squares is our observable photoneutron cross section (red online). The total photoneutron cross section is estimated assuming that the unmeasured $(\gamma ,p_0)$ reaction channel is similar to the $(\gamma ,n_0)$ cross section. The cross-hatched area illustrates the systematic uncertainty inherent in this assumption; it shows an uncertainty equal to 50% of the $(\gamma ,n_0)$ cross section.

Figure 19

Absolute cross section for the reaction channel ${}^6\mathrm{Li}+\gamma {\to }^3\text{He}{+}^3\text{H}$ from Manuzio et al. (open circles) [44], Sherman et al. (open squares) [36, 45], Shin, Skopik, and Murphy (closed squares) [46], and Junghans et al. (open triangles) [29] with our composite values (closed circles).

Figure 20

Our constructed total ${}^6\mathrm{Li}$ photodisintegration cross section for all reaction channels (closed circles). Error bars are calculated using the assumed values for the $(\gamma ,p_0)$ cross section and the $\left(\gamma {,}^3\text{He}{+}^3\text{H}\right)$ as discussed in the text. The cross-hatched area illustrates the systematic uncertainty which is dominated below 20 MeV by the assumption that the $(\gamma ,p_0)$ cross section is similar to the $(\gamma ,n_0)$ cross section, and above 20 MeV by the uncertainty in the average measured $\left(\gamma {,}^3\text{He}{+}^3\text{H}\right)$ cross sections. These data are compared with the prediction of the Lorentz integral transform calculation [4] using the AV4' model (line, red online), the MN model (dashed line, green online), and the MTI-III model (dot-dashed line, blue online). Reaction threshold energies are shown by the arrows.