Spectroscopy of narrow, high-lying, low-spin states in ${}^{20}\mathrm{Ne}$

The excited states of the ${}^{20}\mathrm{Ne}$ nucleus from the ground state to $E_x$ = 25 MeV were investigated with the ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ reaction at a beam energy of $E_p$ = 60 MeV using the QDD magnetic spectrometer at iThemba LABS. Narrow, low-spin $J^{\pi }=0^{+},1^{-}$, and $2^{+}$ states have been observed for the first time at high-excitation energies in ${}^{20}\mathrm{Ne}$, namely at $E_x$ = 17.67, 18.84, 20.59, 21.16, 21.80, and 22.5 MeV. Shell-model and coupled reaction channel calculations were performed to determine the nature of the newly observed states. Candidates for $T$ = 2 states are proposed at 3.86(5), 4.43(5) MeV and 5.07(5) MeV above the lowest $0^{+},T$ = 2 state at $E_x=16.73$ MeV. Two of these $T$ = 2 candidates appear to result from core excitations in the $0p$ shell. The state at $E_x$ = 22.5 MeV, which could not be interpreted by the shell-model calculations, is a tentative candidate for the 5-$\alpha$ cluster state.

Figure 1

A particle identification spectrum, measured with a magnetic field setting for $E_x=17$ to 23 MeV for the ${}^{22}\mathrm{Ne}$($p,t$) reaction and a spectrometer angle of ${\theta }_{\mathrm{lab}}=-1^{\circ }$.

Figure 2

Excitation energy spectra in the excitation energy region $E_x$ = 17 to 23 MeV for the ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ reaction at ${\theta }_{\mathrm{lab}}=-1^{\circ }$ for the natural oxygen target as well as ${}^{12}\mathrm{C}$ and aramid foils.

Figure 3

Excitation energy spectrum for the ${}^{22}\mathrm{Ne}$ gas target in the excitation energy region 17–23 MeV at ${\theta }_{\mathrm{lab}}=-1^{\circ }$ (black spectrum), compared to charge normalized background data measured with a single aramid foil (red and purple spectra) in (a). The background subtracted ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ spectrum is shown in (b).

Figure 4

The excitation energy spectrum for the ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ reaction at 60 MeV measured at angles close to ${\theta }_{\mathrm{lab}}$ = $0^{\circ }$. New states are indicated by a black asterisk and colored in blue. The data were obtained at five different magnetic field settings, each normalized in total charge to the field setting for the $E_x=17–23$ MeV range, which is where the most statistics were accumulated.

Figure 5

Comparison of experimental and theoretical ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ cross sections for new states shown in Table 2. Values and angular distributions of measured states are indicated in black, while the angular distribution calculated with FRESCO is shown, for each state, in the same color as is used for the value calculated with OXBASH, which is indicated in each panel above the measured value.

Figure 6

Comparison of experimental and theoretical ${}^{22}\mathrm{Ne}$($p,t$)${}^{20}\mathrm{Ne}$ cross sections for $0^{+}$ states. Details are discussed in the text.

Figure 7

$T$ = 1 and $T$ = 2 states at excitation energies above 10 MeV in ${}^{20}\mathrm{Ne}$ and their known isobaric analog states in ${}^{20}\mathrm{Ne},{}^{20}\mathrm{F}$ and ${}^{20}\mathrm{O}$, incorporating the newly measured states, indicated in red as are their proposed partners, from the present measurement.