Lifetime measurements in ${}^{92}\mathrm{Mo}$: Investigation of seniority conservation in the $N=50$ isotones

Excited states in the yrast and negative parity bands in ${}^{92}\mathrm{Mo}$ were populated in two different experiments using the ${}^{90}\mathrm{Zr}(\alpha ,2n){}^{92}\mathrm{Mo}$ and ${}^{93}\mathrm{Nb}(p,2n){}^{92}\mathrm{Mo}$ fusion-evaporation reactions at the Cologne FN Tandem accelerator and measured using a hybrid setup of high purity germanium and lanthanum bromide detectors. Lifetimes of the excited $2_1^{+}, 4_1^{+}, 6_1^{+}, 8_1^{+}, 5_1^{-}, 7_1^{-}$, and $9_1^{-}$ states were measured using the $\gamma \text{−}\gamma$ fast-timing technique. The newly measured lifetime of the $4_1^{+}$ state differs from the recently published value measured using the recoil distance Doppler shift method. Experimental $B\left(E2\right)$ strengths of excited states in ${}^{92}\mathrm{Mo}$ are used to predict theoretical $B\left(E2\right)$ values in the $N=50$ isotones from ${}^{93}\mathrm{Tc}$ up to ${}^{95}\mathrm{Rh}$ using semiempirical calculations in the single-$j$ orbital $0g_{9/2}$ for the protons.

Figure 1

Calibrated mean time-walk characteristic of the setup used for EXP1 (top). The residuum of the fit and the $1\sigma$ uncertainty interval (bottom).

Figure 2

Calibrated mean time-walk characteristic of the setup used for EXP2 (top). The residuum of the fit and the $1\sigma$ uncertainty interval (bottom).

Figure 3

Coincidence spectra of both experiments for multiplicity three data with Ge-LaBr-LaBr coincidences shown in black and Ge-LaBr-Ge coincidences shown in red. Transitions that were used within the analysis are labeled. The insets show a zoom of the low-energy part.

Figure 4

Partial level scheme showing important states and transitions used within the analysis. The transition energies are given in keV. The arrows thickness approximately emphasize the relative intensities of the transitions observed with the ${}^{90}\mathrm{Zr}(\alpha ,2n){}^{92}\mathrm{Mo}$ reaction (EXP1). Adopted from Refs. [29, 30].

Figure 5

Time spectra used for the measurement of the lifetime of the $6_1^{+}$ state (a) and for lifetime of the $5_1^{-}$ state (b). The procedure is shown for the data from EXP1. The term for the random distributed background was set to a constant value, which was determined via integration and also as parameter from a fit, leading to consistent results. The data from EXP2 was processed in the same way.

Figure 6

Background correction procedure for the analysis of the $4_1^{+}$ state measured in EXP1. (a) Resulting spectra after gating on the 773 keV decay transition. (b) Respective spectra after gating on the 330 keV feeder transition. The LaBr spectra are shown in black and corresponding gated Ge spectra for monitoring in red. (c) and (d) The corresponding background interpolation (black), the uncorrected centroid ${\mathrm{C}}_{\exp }$ is shown in red, the interpolated centroid ${\mathrm{C}}_{\mathrm{BG}}$ is marked with $\otimes$.

Figure 7

Background correction procedure for the analysis of the $4_1^{+}$ state measured in EXP2. (a) Resulting spectra after gating on the 773 keV decay transition. (b) Respective spectra after gating on the 244 keV feeder transition. The LaBr spectra are shown in black and corresponding gated Ge spectra for monitoring in red. (c) and (d) The corresponding background interpolation (black), the uncorrected centroid ${\mathrm{C}}_{\exp }$ is shown in red, the interpolated centroid ${\mathrm{C}}_{\mathrm{BG}}$ is marked with $\otimes$.

Figure 8

The resulting time distributions, centroids (dashed lines) and gate information used to measure the $4_1^{+}$ state for EXP1 (top) and EXP2 (bottom). For the upper time distribution, matrices with Ge gates on 148 keV and 1510 keV were added to increase the statistic. Since the multiplicity condition was set to only validate coincidences containing exactly two LaBr hits and one Ge hit the two energy conditions are mutual excluding/exclusive and no risk of double counting exists.