Structure of the isomeric states in ${}^{123,125}\mathrm{Sb}$

Excited states in ${}^{123,125}\mathrm{Sb}$ have been studied following the ${}^{122,124}\mathrm{Sn}$(${}^7\mathrm{Li}$,α2n)${}^{123,125}\mathrm{Sb}$ reactions at beam energies of 35 and 37 MeV, respectively. Conversion coefficients for transitions depopulating isomeric states in ${}^{125}\mathrm{Sb}$ have been measured using an electron spectrometer. This has allowed firm $J^{\pi }$ assignments to be made and enabled the structure of the isomeric states to be compared with those in ${}^{121,127-131}$Sb and in the neighboring even-$A$ tin isotopes. The half-lives of the $J^{\pi }={\frac{23}{2}}^{+}$ isomeric states in ${}^{123}\mathrm{Sb}$ and ${}^{125}\mathrm{Sb}$ have been measured to be 66(4) μs and 272(16) ns, respectively. The half-lives of the $J^{\pi }={\frac{19}{2}}^{+},{\frac{19}{2}}^{-}$, and ${\frac{15}{2}}^{-}$ states in ${}^{125}\mathrm{Sb}$ have been measured as 31(2) ns, 28.0(7) μs, and 4.1(2) μs, respectively. The transition probabilities for the transitions depopulating these states are compared with those in ${}^{121,127-131}$Sb and with the results of shell-model calculations using the Oxbash code and the SN100PN interaction in a restricted model space.

Figure 1

Comparison of the $J^{\pi }={\frac{23}{2}}^{+},{\frac{19}{2}}^{-}$, and ${\frac{15}{2}}^{-}$ states in the odd-$A$ antimony isotopes [1, 2, 3, 4, 5, 7, 8] with the energies of the $J^{\pi }={10}^{+},7^{-}$, and $5^{-}$ states in the even-$A$ tin isotopes [9]. The $J^{\pi }$ values for the levels in ${}^{125}\mathrm{Sb}$ are uncertain and therefore are shown as open symbols.

Figure 2

Level scheme for ${}^{125}\mathrm{Sb}$ from the present work. The γ-ray energies are given in keV and the thickness of the arrows represents the relative γ-ray intensity. The prompt 1419.9, 1087.7, 469.7, and 332.1 keV transitions are known from β-decay studies [13]; however, the intensities of these states were too weak to be measured in the current work.

Figure 3

(a) A single-component fit to the decay profile of the 146.3 keV transition de-exciting the 2472 keV state in ${}^{125}\mathrm{Sb}$. The straight line shows the fit to the data giving a half-life of 272(16) ns. (b) A two-component fit to the decay spectrum produced by gating on the 1067.8 keV transition in ${}^{125}\mathrm{Sb}$. Assuming a half-life of 28.0(7) μs for the 2113 keV level, the fit gives the half-life of the 1972 keV state as 4.1(2) μs.

Figure 4

Results of the unfolding of the 139.7/140.9 keV doublet for the electron and γ-ray spectra. Panels (a) and (b) are the raw electron and γ-ray spectra. Panels (c) and (d) show the unfolded 140.9 keV peaks from the transition in ${}^{125}\mathrm{Sb}$. Panels (e) and (f) show the unfolded 139.7 keV contaminant peak. Transition energies are calculated using the antimony $K$-shell binding energy of 30.5 keV.

Figure 5

Summed time spectrum obtained by gating on the 127.6, 442.0, 955.9, and 1088.7 keV transitions below the 2614 keV state in ${}^{123}\mathrm{Sb}$. The straight line shows the fit to the data, giving a half-life of 66(4) μs.

Figure 6

Calculated level scheme for ${}^{125}\mathrm{Sb}$ produced using the restricted SN100PN model space and showing only the levels that were experimentally observed in this work. Level energies were multiplied by a factor of 1.44 to account for the effect of the restricted model space. The width of the arrows does not represent the transition strengths.

Figure 7

A comparison of calculated and experimental level energies for ${}^{123}\mathrm{Sb}$ showing selected levels below the 2614 keV isomer. Calculated level energies were multiplied by a factor of 1.52 to account for the effect of the restricted model space.

Figure 8

Calculated and experimentally observed transition probabilities as a function of mass number. (a) The $B(E2:{\frac{23}{2}}^{+}\to {\frac{19}{2}}^{+})$ values in the odd-$A$ antimony isotopes plotted with the $B(E2:{10}^{+}\to 8^{+})$ in the even-$A$ tin isotopes. (b) The $B(E2:{\frac{19}{2}}^{-}\to {\frac{15}{2}}^{-})$ values in the odd-$A$ antimony isotopes plotted with the $B(E2:7^{-}\to 5^{-})$ in the even-$A$ tin isotopes. Experimental errors are smaller than the size of the data points unless shown.