Neutron knockout of ${}^{12}\mathrm{Be}$ populating neutron-unbound states in ${}^{11}\mathrm{Be}$

Neutron-unbound resonant states of ${}^{11}\mathrm{Be}$ were populated in neutron knockout reactions from ${}^{12}\mathrm{Be}$ and identified by ${}^{10}\mathrm{Be}$–$n$ coincidence measurements. A resonance in the decay-energy spectrum at 80(2) keV was attributed to a highly excited unbound state in ${}^{11}\mathrm{Be}$ at 3.949(2) MeV decaying to the $2^{+}$ excited state in ${}^{10}\mathrm{Be}$. A knockout cross section of 15(3) mb was inferred for this 3.949(2) MeV state, suggesting a spectroscopic factor near unity for this $0p3/2^{-}$ level, consistent with the detailed shell model calculations.

Figure 1

${}^{11}\mathrm{Be}$ level scheme up to 4 MeV including the first two states in ${}^{10}\mathrm{Be}$. Only the neutron decay energies observed in this experiment are shown. Energies are given in keV along with known spin and parity assignments. (${}^{\mathrm{a}}$ denotes values derived in the current work incorporating the recently remeasured separation energy from Ref. [11].)

Figure 2

Decay energy spectrum from ${}^{10}\mathrm{Be}$–neutron coincidence data. The simulation (solid line) is the sum of three resonances with decay energies of 80 keV (dotted line), 1.28 MeV (dot-dashed line), and 2.19 MeV (dashed line). In addition, a nonresonant background component (double-dot dashed line) was included. The insert shows a separate fit to the low energy range with higher fidelity, confirming only one low-energy peak at 80 keV.