Evidence for a 3.8 MeV state in ${}^9\mathrm{Be}$

The breakup reaction ${}^9\mathrm{Be}({}^4\mathrm{He},3\alpha )\mathit{n}$ was measured using an array of four double-sided silicon strip detectors at beam energies of 22 and 26 MeV. Excited states in ${}^9\mathrm{Be}$ up to 8 MeV were populated and reconstructed through measurements of the charged reaction products. Evidence is given for a state in ${}^9\mathrm{Be}$ at $3.{82}_{\text{-0.09}}^{\text{+0.08}}$ MeV with $\mathrm{\Gamma }={1240}_{\text{-90}}^{\text{+270}}$ keV. This is consistent with two recent measurements of a state with similar properties in the mirror nucleus ${}^9\mathrm{B}$. An analysis of the reduced widths (${}^8\mathrm{Be}_{\mathrm{g}.\mathrm{s}.}$ channel) of this state along with the proposed mirror state has led to a firm limit of $J\le 7/2$ and a tentative assignment of $J^{\pi }=1/2^{-}$ or $3/2^{-}$.

Figure 1

Schematic drawing of the reaction chamber and detector positions along with an illustration of the inelastic scattering and breakup process.

Figure 2

Sum energy spectra for all four final-state particles, assumed to be $3\alpha +n$, corrected for the ${}^9\mathrm{Be}({}^4\mathrm{He},3\alpha )n$ reaction $Q$ value. The black line and shaded histogram give the spectra before and after target energy loss corrections were applied, respectively. The shaded histogram has been vertically scaled by a factor of 0.5 for plotting. Events within $2\sigma$ of the corrected peak are identified as ${}^9\mathrm{Be}$ breakups. The inset depicts a histogram of the relative energies between pairs of $\alpha$ particles in the final state. Events residing within the sharp peak at 92 keV correspond to breakup through the ${}^8\mathrm{Be}_{\mathrm{g}.\mathrm{s}.}$ channel.

Figure 3

Dalitz plot for the 26 MeV beam data. Natural parity states associated with ${}^{12}\mathrm{C}$ breakup appear as vertical bands and ${}^9\mathrm{Be}$ states form broad, horizontal bands. Neutron transfer events and subsequent breakups of the ${}^5\mathrm{He}_{\mathrm{g}.\mathrm{s}.}$ form the diagonal locus which is surrounded by the dot-dashed line. Only data to the right of the vertical dashed line were considered for further analysis in order to minimize contributions from ${}^{12}\mathrm{C}$ breakups.

Figure 4

The scaled Monte Carlo–derived efficiency profile for a 26 MeV beam energy, assuming isotropic particle emission, is given by the dashed line. The experimental data are marked by the points with error bars and the solid (red) line denotes the best fit to the spectrum (see Sec. 4).

Figure 5

Peak fits to the ${}^9\mathrm{Be}$ excitation spectra. Fits of the known levels are shown in the left panels and the fits inclusive of the proposed additional state are shown in the right panels. Plots corresponding to $E_{\mathrm{beam}}=22$ and 26 MeV occupy the upper and lower panels respectively. The solid red line on each plot represents the overall fit, individual states are marked by broken dot-dashed lines, and the fitted background profile is marked as a dashed line. In the right panels, the additional level is marked by the solid, black line. The insets of the left panels give the fit residuals, which highlight a discrepancy near to 4 MeV excitation.

Figure 6

The sensitivity of the ${\chi }^2$ value to the excitation energy (top panels), width (center panels), and amplitude (bottom panels) for the 3.8 MeV state. Fits to the 22 and 26 MeV data are shown in the left and right panels respectively and have ${\chi }^2$ per degree of freedom values of 1.17 and 0.95. The horizontal axis of the bottom panel is scaled by a factor of ${10}^{-5}$.