$N=14$ and 16 shell gaps in neutron-rich oxygen isotopes

In-beam $\gamma$-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from $A=20$ to 24. For the produced fragments, $\gamma$-ray energies, intensities, and $\gamma \text{−}\gamma$ coincidences have been measured. Based on this information new level schemes are proposed for ${}^{21,22}\mathrm{O}$ up to the neutron separation energy. The nonobservation of any $\gamma$-decay branch from ${}^{23}\mathrm{O}$ and ${}^{24}\mathrm{O}$ suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for ${}^{21}\mathrm{O}$ and ${}^{22}\mathrm{O}$, the size of the $N=14$ and 16 shell gaps in oxygen isotopes is discussed in the light of shell-model calculations.

Figure 1

(Color online) (a) Secondary beam composition given by the time of flight and energy loss information from the “active” target. (b) Energy loss vs mass over charge identification spectrum of the fragments produced in the active target and selected through the SPEG spectrometer.

Figure 2

$\gamma$-ray spectrum of ${}^{20}\mathrm{O}$ obtained from Ge detectors in the SSF experiment. The doublet of sharp lines at about $730\text{keV}$ and marked as Annih represents the $511\text{keV}$ annihilation $\gamma$-ray radiation after Doppler correction to account for the velocity of the emitting fragments and for the two different angles of the Ge detectors.

Figure 3

The ${}^{20}\mathrm{O}$ level scheme obtained from the present work, the USD shell-model calculations and from transfer reactions. Tentative associations between observed and calculated levels are indicated by dashed lines. Transition energies and relative intensities are indicated together with their uncertainties for the experimental work.

Figure 4

$\gamma$-ray spectrum of ${}^{21}\mathrm{O}$ obtained from the Ge detectors in the SSF experiment “Annih.” indicates the annihilation radiation line after Doppler correction.

Figure 5

Energy diagram for ${}^{21}\mathrm{O}$ from the present work with USD shell-model calculations and transfer reactions. Tentative associations between observed and calculated levels are indicated by dashed lines. Transition energies and relative intensities are indicated together with their uncertainties for the experimental work.

Figure 6

$\gamma \text{−}\gamma$ coincidences observed in ${}^{21}\mathrm{O}$ using the ${\text{BaF}}_2$ array in the SSF experiment.

Figure 7

$\gamma$-ray spectrum of ${}^{22}\mathrm{O}$. (a) Obtained from the ${\text{BaF}}_2$ detectors in the SSF experiment. (b) The same spectrum obtained during the DSF measurement. In both ${\text{BaF}}_2$ spectra of (a) and (b), the single escape line of the $3.15\text{MeV}$ $\gamma$ transition is indicated by “escape.” Note the lower energy threshold used for the ${\text{BaF}}_2$ detectors in the DSF experiment

Figure 8

High resolution $\gamma$-ray spectrum of ${}^{22}\mathrm{O}$ obtained from the Ge detectors in the experiment using the SSF method and the same spectrum obtained by requiring a least one ${\text{BaF}}_2$ detector in coincidence (insert).

Figure 9

${\text{BaF}}_2$ measured $\gamma$ rays observed in coincidence with the $3199\text{keV}$ transition in ${}^{22}\mathrm{O}$ during the SSF experiment.

Figure 10

The measured level scheme of ${}^{22}\mathrm{O}$ together with the $sd$-shell-model calculations. Transition energies and relative intensities are indicated together with their uncertainties for $\gamma$ lines.

Figure 11

(a) ${\text{BaF}}_2$ ${}^{23}\mathrm{O}$ $\gamma$-ray spectrum without Doppler correction. The origins of the quasitarget $\gamma$ emission are indicated. (b) The Doppler corrected $\gamma$-ray spectrum obtained for ${}^{23}\mathrm{O}$ and the corresponding Monte Carlo generated spectrum (dashed line) where a $2.7\text{MeV}$ simulated level deexcites (see text for details). The quality of the Monte Carlo simulation is illustrated in the inset of the figure where a comparison between the measured and simulated spectra in ${}^{22}\mathrm{O}$ is shown. Note that the low-energy part of the simulated spectrum comes mainly from the $511\text{keV}$ annihilation radiation, whereas in the measured spectrum the low-energy part is due to $\gamma$ rays from various targetlike fragments $\left({}^7\mathrm{Li},{}^{10}\mathrm{B},\dots \right)$. For that reason the agreement between the two spectra at low energy is purely coincidental.

Figure 12

(a) $\gamma$-ray spectrum of ${}^{24}\mathrm{O}$ from the ${\text{BaF}}_2$ detectors array with no Doppler correction. (b) The Doppler corrected $\gamma$-ray spectrum obtained for ${}^{24}\mathrm{O}$ and the corresponding Monte Carlo simulated spectrum (dashed line). In the simulation a $3.7\text{MeV}$ $\gamma$ ray is assumed to deexcite the first $2^{+}$ level in ${}^{24}\mathrm{O}$ (see text for details).