Selective Population and Neutron Decay of an Excited State of ${}^{23}\mathrm{O}$

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of ${}^{23}\mathrm{O}$ at 2.8(1) MeV excitation energy which decays into the ground state of ${}^{22}\mathrm{O}$. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the $5/2^{+}$ hole state. This assignment and the recently observed $3/2^{+}$ particle state advance the understanding of ${}^{23}\mathrm{O}$.

Figure 1

(a) Energy losses in thin scintillator and ion chamber provide $Z$ identification. (b) TKE in thick scintillator vs corrected TOF for oxygen fragments provides isotope identification. (c) Neutron TOF for decay of ${}^{23}\mathrm{O}^{*}$, i.e., in coincidence with ${}^{22}\mathrm{O}$ fragments, and using a thick target in singles mode which also produces prompt $\gamma$ radiation (inset). The 1.4 ns FWHM of the $\gamma$ peak translates into a neutron-energy resolution of 1.4 MeV. (d) A one-dimensional mass-indicator spectrum for oxygen fragments is obtained by projecting the data from panel (b) onto the diagonal line there.

Figure 2

Decay-energy spectra of ${}^{23}\mathrm{O}^{*}$ (data points). The inset shows a close-up of the first 360 keV. The thick solid curve corresponds to the sum of the dashed (simulated resonant contribution) and dash-dotted (simulated thermal model) curves and agrees well with the data. Adding a strongly populated second resonance at a higher energy as observed in Ref. 5 (dotted line) is not consistent with our data. The nonobservation of the second resonance corresponds to an upper limit of its population of $\sim 20\%$ relative to the first resonance. The thin solid curve indicates the simulated effect of the geometric acceptance on the reconstructed decay-energy spectrum, by assuming an energy-independent decay energy.

Figure 3

Comparison of ${}^{23}\mathrm{O}$ excitation spectrum (this work and 5) to theory [15] (dotted lines, calculation relative to the ${}^{22}\mathrm{O}$ ground state, using the HBUSD interaction, a version of the old USD interaction which is slightly modified for heavier isotopes). The negative-parity state in ${}^{23}\mathrm{O}$ is outside of the USD model space. The agreement is of a similarly good quality as for the ${}^{22}\mathrm{O}$ excitation spectrum [3], and hence supports the assumed presence of large gaps in the single-particle spectrum at the Fermi energy for ${}^{22,24}\mathrm{O}$.