Search for excited states in ${}^{25}\mathrm{O}$

Background: Theoretical calculations suggest the presence of low-lying excited states in ${}^{25}\mathrm{O}$. Previous experimental searches by means of proton knockout on ${}^{26}\mathrm{F}$ produced no evidence for such excitations.

Purpose: We search for excited states in ${}^{25}\mathrm{O}$ using the ${}^{24}\mathrm{O}(d,p){}^{25}\mathrm{O}$ reaction. The theoretical analysis of excited states in unbound ${}^{25,27}\mathrm{O}$ is based on the configuration interaction approach that accounts for couplings to the scattering continuum.

Method: We use invariant-mass spectroscopy to measure neutron-unbound states in ${}^{25}\mathrm{O}$. For the theoretical approach, we use the complex-energy Gamow Shell Model and Density Matrix Renormalization Group method with a finite-range two-body interaction optimized to the bound states and resonances of ${}^{23–26}\mathrm{O}$, assuming a core of ${}^{22}\mathrm{O}$. We predict energies, decay widths, and asymptotic normalization coefficients.

Results: Our calculations in a large $spdf$ space predict several low-lying excited states in ${}^{25}\mathrm{O}$ of positive and negative parity, and we obtain an experimental limit on the relative cross section of a possible $J^{\pi }={1/2}^{+}$ state with respect to the ground state of ${}^{25}\mathrm{O}$ at ${\sigma }_{1/2+}/{\sigma }_{g.s.}=0.{25}_{-0.25}^{+1.0}$. We also discuss how the observation of negative parity states in ${}^{25}\mathrm{O}$ could guide the search for the low-lying negative parity states in ${}^{27}\mathrm{O}$.

Conclusion: Previous experiments based on the proton knockout of ${}^{26}\mathrm{F}$ suffered from the low cross sections for the population of excited states in ${}^{25}\mathrm{O}$ because of low spectroscopic factors. In this respect, neutron transfer reactions carry more promise.

Figure 1

Hartree-Fock-Bogoliubov (HFB) canonical energies as a function of the neutron number for oxygen isotopes. The calculations with the energy density functional UNEDF0 [10] were performed by means of the HFB solver [11] based on Pöschl-Teller-Ginocchio and Bessel-Coulomb wave functions, which properly takes the continuum couplings into account. The chemical potential (in MeV) is represented by a dashed line. While the ground state of ${}^{28}\mathrm{O}$ is bound in these calculations, it does not significantly affect the picture. The arrows show the decreasing energy gap between the last occupied level and the $fp$ levels in ${}^{24}\mathrm{O}$ and ${}^{26}\mathrm{O}$, respectively.

Figure 2

Corrected particle identification (PID) vs time-of-flight in the sweeper $\left(t_{\text{ToF}}\right)$ for the oxygen isotopes. The black dashed line denotes the selection of ${}^{24}\mathrm{O}$, and the red solid line the selection on time-of-flight. The $(d,p)$ reaction products are expected to be populated in the region of interest (denoted ROI, shaded gray).

Figure 3

Neutron kinetic energy for the full liquid deuterium target $({\mathrm{LD}}_2$, hatched blue) gated in the ROI of Fig. 2, and empty-target measurement (solid gray). The inset shows the background subtracted spectrum, with a Gaussian fit (solid red).

Figure 4

Two-body decay energy for ${}^{24}\mathrm{O}$ + $1n$. The best fit includes a 830 keV resonance (dashed red), a 3.3 MeV resonance from a possible $J=1/2^{+}$ state at ${\sigma }_{1/2+}/{\sigma }_{g.s.}=1.0$ (dot-dash, blue), and background contributions from the unreacted beam (dash-dot-dot green), in addition to deuteron breakup (shaded gray). The sum of all components is in solid black.

Figure 5

Energies of ${}^{23-28}\mathrm{O}$ obtained in the GSM and DMRG approaches compared to experiment. The widths are marked by shaded bands.

Figure 6

Overlap integrals for states in ${}^{24}\mathrm{O}$ and ${}^{25}\mathrm{O}$ (solid lines) from which the ANC values $C_{\ell j}$ (in ${\mathrm{fm}}^{-1/2})$ are extracted. The Whittaker functions used to fit the overlap integrals are marked by dashed lines. The ANCs for neutron transfer on ${}^{24}\mathrm{O}$ to negative parity states in ${}^{25}\mathrm{O}$ were too small $\left(\approx {10}^{-8}{\mathrm{fm}}^{-1/2}\right)$ to show.