Nuclear levels in proton-unbound ${}^{109}\mathrm{I}$: Relative single-particle energies beyond the proton drip line

A level scheme has been constructed for the proton-unbound, $T_z=3/2$ nuclide ${}_{53}^{109}{\mathrm{I}}_{56}^{}$ following a recoil-decay-tagging experiment using the ${}^{58}\mathrm{Ni}$(${}^{54}\mathrm{Fe}$, $p2n\gamma$) reaction at a beam energy of 195 MeV. The experiment was performed using the highly efficient JUROGAM γ-ray spectrometer in conjunction with the RITU gas-filled recoil separator and the GREAT focal-plane spectrometer. Cranking calculations are used to interpret band structures built on $\pi g_{7/2}$ and $\pi h_{11/2}$ states in a weakly deformed, triaxial nucleus.

Figure 1

(a) Prompt γ-ray spectrum (1 keV/channel) correlated with any recoil detected at the focal plane of RITU. Peaks marked with filled circles are strong transitions in the dominant exit channel, ${}^{109}\mathrm{Sb}$. (b) Prompt γ-ray spectrum (1 keV/channel) correlated with the ground-state proton decay of ${}^{109}\mathrm{I}$. The inset of (b) shows the decay-energy spectrum recorded by the DSSD for a 300 μs correlation time between a recoil implantation and its subsequent decay. The strong peak in the decay spectrum corresponds to the 813 keV ground-state proton decay of ${}^{109}\mathrm{I}$, while the tail on the left consists of escaped protons and β particles. The dotted lines define the energy gate in the time-correlated decay spectrum for which the prompt γ rays in JUROGAM have been assigned as transitions in ${}^{109}\mathrm{I}$. (c) Same as (b), but with an expanded energy axis and with the assigned ${}^{109}\mathrm{I}$ transitions labeled by their energies.

Figure 2

Level scheme deduced for ${}^{109}\mathrm{I}$. Level and transition energies are labeled in keV. The widths of the arrows are proportional to the relative $\gamma \text{−}\gamma$ coincident transition intensities, as derived from a global fit to the proton-correlated $\gamma \text{−}\gamma$ matrix, and hence different from those given in Table . The level energies are given relative to the lowest $(7/2^{+})$ state, which is probably not the ground state. The tentative spin-parity assignments are discussed in the text.

Figure 3

$\gamma$-ray spectra (1 keV/channel) without background subtraction showing transitions in coincidence with (a) the 594 keV transition, (b) the 548 keV transition, and (c) the 1056 keV transition of ${}^{109}\mathrm{I}$. The transitions labeled with their energies are shown in the level scheme.

Figure 4

Systematics of bands built on the $\pi h_{11/2}$ orbital in odd-$A$ iodine isotopes. The energies of low-lying $5^{/}$ (thin blue line) and $7^{/}$ (thick red line) states relative to the ${11}^{/}$ states are also shown. The $5/2^{+}$ state forms the ground state for odd-$A$ iodine isotopes with $111⩽A⩽127$, while the $7/2^{+}$ state becomes the ground state for $A⩾129$.

Figure 5

Energy-ratio systematics of the first three states in the ground-state bands of even-$A$ Te and Xe isotopes, and the $\pi h_{11/2}$ bands of odd-$A$ I and Cs isotopes.

Figure 6

TRS surfaces calculated for various one- and three-quasiparticle configurations in ${}^{109}\mathrm{I}$ at a rotational frequency of 0.325 MeV/ħ. The energy contours are separated by 250 keV.

Figure 7

Representative single-quasiparticle positive-parity (red) and negative-parity (blue) proton (a) and neutron (b) levels calculated with a cranked Woods-Saxon potential with deformation parameters ${\beta }_2=0.143,{\beta }_4=0.040$, and $\gamma ={10}^{°}$, and pairing parameters ${\Delta }_{\pi }=0.68$ MeV and ${\Delta }_{\nu }=1.10$ MeV. The parity and signature ($\pi ,\alpha$) of the levels are: (+, +1/2), solid lines; (+, –1/2), dotted lines; (–, –1/2), dashed lines; and (–, +1/2), dot-dashed lines.

Figure 8

Experimental alignments (a) and Routhians (b) of the bands in ${}^{109}\mathrm{I}$.