Identification of isomeric states in the $N=73$ neutron-deficient nuclei ${}^{132}$Pr and ${}^{130}$La

Decays from isomeric states in the neutron-deficient $N=73$ nuclei ${}^{132}$Pr and ${}^{130}$La have been observed for the first time. Half-lives of 486(70) ns and 2.46(4) $\mu$s were measured for two isomeric states in ${}^{132}$Pr. The decay from the 486 ns $\left(8^{-}\right)$ isomer has been interpreted as a hindered $E1$ transition from the bandhead state of the excited $\pi h_{11/2}\otimes \nu g_{7/2}$ configuration. The decay from the 2.5 $\mu$s $\left(8^{+}\right)$ isomer is consistent with the Weisskopf estimate for a low-energy $E2$ transition. An analogous 0.74(3) $\mu$s decay from an $\left(8^{+}\right)$ isomer in the neighboring isotone ${}^{130}$La has also been observed which similarly can be explained if the transition has $E2$ character. The Weisskopf interpretation for the isomer hindrance is strengthened by the lack of evidence for shape or $K$ isomerism due to the $\gamma$-soft shapes predicted by configuration-constrained potential-energy-surface calculations.

Figure 1

Planar Ge detector spectra showing recoil-correlated delayed $\gamma$ rays. (a) $\gamma$ rays recorded within 40 $\mu$s of a recoil implantation. The majority of the transitions result from states populated by $\beta$ decay, the most intense of which are labeled with the parent nucleus. (b) $\gamma$ rays recorded within 11 $\mu$s of a recoil implantation. A random-correlation background spectrum of $\gamma$ rays, recorded up to 11 $\mu$s prior to the recoil signal, was subtracted.

Figure 2

Recoil-$\gamma$ time distribution for the delayed 176 keV decay in ${}^{133}$Nd. A least-squares fit yielded a half-life of 291(4) ns.

Figure 3

Recoil-$\gamma$ time distributions for the delayed (a) 118 keV, (b) 102 keV, and (c) 53 keV decays in ${}^{132}$Pr along with least-squares fit curves.

Figure 4

Spectra produced from a prompt-delayed $\gamma$-$\gamma$ coincidence matrix for ${}^{132}$Pr. The matrix had an optimum recoil-delayed $\gamma$ correlation time of 9 $\mu$s and was background subtracted. (a) Prompt $\gamma$ rays that correlate with delayed 118 keV decays. (b) Delayed $\gamma$ rays that correlate with prompt 283 keV decays.

Figure 5

Planar Ge spectra produced from a prompt-delayed $\gamma$-$\gamma$ coincidence matrix with a 9 $\mu$s recoil-delayed $\gamma$ correlation time (a) gated by the prompt 283, 130, and 266 keV transitions in ${}^{132}$Pr and (b) gated by the prompt 116, 178, 228, and 277 keV transitions in ${}^{132}$Pr.

Figure 6

Background-subtracted planar Ge detector spectrum showing delayed $\gamma$ rays that were in coincidence (within 400 ns) with ${}^{132}$Pr 118 keV decays recorded by the focal-plane clover detector. Both planar and clover signals correlated with a recoil implantation within 9 $\mu$s.

Figure 7

Partial ${}^{132}$Pr decay scheme produced in this work. The newly observed delayed $\gamma$ rays at 53, 102, and 118 keV are tentatively positioned below the proposed $\left(8^{-}\right)$ and $\left(8^{+}\right)$ isomeric states. The weighted mean of the half-lives measured from the 102 and 118 decay time distributions is adopted as the half-life for the $\left(8^{+}\right)$ state. Bracketed decays were reported in a previous study [14] but not observed in this work.

Figure 8

The low-energy portion of a 283 keV-gated prompt spectrum produced from the 118 keV gated prompt $\gamma$-$\gamma$ matrix which had a recoil-delayed $\gamma$ time condition of 9 $\mu$s. The spectrum shows no evidence for correlated decays with energies below the 130 keV transition.

Figure 9

Recoil-$\gamma$ time distributions for the delayed (a) 110 keV and (b) 104 keV decays in ${}^{130}$La along with least-squares fit curves.

Figure 10

Spectra produced from a prompt-delayed $\gamma$-$\gamma$ coincidence matrix for ${}^{130}$La. The matrix had an optimum recoil-delayed $\gamma$ correlation time of 2.5 $\mu$s and was background subtracted. (a) Prompt $\gamma$ rays that correlate with delayed 110 keV decays. (b) Delayed $\gamma$ rays that correlate with prompt 137 keV decays.

Figure 11

Background-subtracted planar Ge detector spectrum showing delayed $\gamma$ rays that were in coincidence (within 400 ns) with ${}^{130}$La 110 keV decays recorded by the focal-plane clover detector. Both planar and clover signals correlated with a recoil implantation within 2.5 $\mu$s.

Figure 12

Partial ${}^{130}$La decay scheme produced in this work. The newly observed delayed $\gamma$ rays at 104 and 110 keV are tentatively positioned below the proposed $\left(8^{+}\right)$ isomeric state. The weighted mean of the half-lives measured from the 104 and 110 decay time distributions is adopted as the half-life for the $\left(8^{+}\right)$ state. The 46 keV decay was reported in a previous study [12] but not observed in this work.

Figure 13

The results of potential-energy-surface calculations for the proposed (a) $\pi 3/2^{-}\left[541\right]\otimes \nu 7/2^{+}\left[404\right]$ negative-parity band configuration along with two possible configurations (b) $\pi 3/2^{-}\left[541\right]\otimes \nu 7/2^{-}\left[523\right]$ and (c) $\pi 3/2^{-}\left[541\right]\otimes \nu 9/2^{-}\left[514\right]$ for the positive-parity band in ${}^{132}$Pr.