Spectroscopy and lifetime measurements of states in ${}^{76}\mathrm{Kr}$ populated in ${}^{76}\mathrm{Rb}$ decay

The properties of the excited states of ${}^{76}\mathrm{Kr}$, populated in the decay of ${}^{76}\mathrm{Rb}$ produced at ISOLDE isotopic separator, have been investigated by using K-conversion electron, γ-ray, and γ-γ coincidence measurements. The lifetimes of several levels have been measured by means of the advanced time-delayed $\beta \gamma \gamma (t)$ method. The identification of 19 new levels and 55 transitions and the information deduced on spin and/or parity of the observed states from K-conversion coefficients, log ft values and decay properties have led to an improved knowledge of the level scheme. In particular, six $1^{-}$ states have been definitely identified and $J^{\pi }=2^{-}$ has been assigned to the 2227 keV level, on which a band proposed to have negative-parity and even-spin had been previously observed. The strengths of the E0 and E2 transitions deexciting the $0_2^{+}$ and $0_3^{+}$ states (the latter is here identified with the state at 1598 keV) have been measured. The experimental data concerning the $0_2^{+}$ state have been analyzed in a simple two states mixing model and in the framework of the proton-neutron interacting boson model.

Figure 1

Portion of the γ-ray spectrum in coincidence with the 1768-keV transition showing evidence for the new level at 2192 keV. It is an example of $\beta \gamma \gamma$ coincident spectrum recorded in two Ge detectors and gated by β-events. A partial decay scheme shown in the figure illustrates the position of the gating 1768 keV transition and the 355, 378, and 424 keV γ-rays. The latter deexcited known states in ${}^{76}\mathrm{Kr}$. The 511 keV peak is due to the ${\beta }^{+}$ decay process.

Figure 2

Selected results from the $\beta \gamma \gamma$ coincidences using β-Ge-Ge and β-Ge-${\text{BaF}}_2$ detectors. Top panel: portion of the Ge energy spectrum in coincidence with the 345 keV transition. Middle and bottom panels: portion of the coincident Ge and ${\text{BaF}}_2$ energy spectra, respectively, gated on the 917 keV transition in Ge. The latter spectrum shows good separation of the 345, 424, 511, and 883 keV peaks. This is crucial in order to select the 345–424, 917–424, 917–345 time-delayed coincidences, which were used to determine the lifetime of the $0^{+}$ 770 keV level; see text for details.

Figure 3

Selected results from the $\beta \gamma \gamma$ coincidences using β-Ge-Ge and β-Ge-${\text{BaF}}_2$ detectors. Top panel: portion of the Ge energy spectrum in coincidence with the 1174 keV transition. Middle and bottom panels: portion of the coincident Ge and ${\text{BaF}}_2$ energy spectra, respectively, gated on the 973 keV transition in Ge. The latter spectrum shows good separation of the 355, 424, 511, and 1174 keV peaks. These spectra help to visualize the lifetime determination of the $0^{+}$ 1598 keV state, see text for discussion.

Figure 4

Time-delayed $\beta \gamma$(t) spectra gated by the fast response β and ${\text{BaF}}_2$ detectors with the following gates: β-917(Ge)-424(${\text{BaF}}_2$) in the top panel and β-1263(Ge)-424(${\text{BaF}}_2$) in the bottom panel. The difference in the centroids is due to the lifetime of the $0^{+}$ 770 keV level. The broken line marks the centroid of the bottom spectrum, from which the centroid of top spectrum is visibly shifted to the right. The observed time-shift is 51(18) ps and it corresponds to a half-life of the 770 keV level of 35(12) ps; see text for discussion. Time scale is 26.1 ps/ch.

Figure 5

The calculated value of ${\alpha }_K$ as a function of $\left|\delta \right|$ is compared to the experimental one for the 453 keV transition deexciting the 3024 keV level. The error bar corresponds to one standard deviation.

Figure 6

Relevant section of an electron-energy spectrum showing the K- and L-conversion lines of the 770 keV, $0_2^{+}\to 0_1^{+}$ transition. The two close lying K-770 keV and K-767 keV transitions (the latter deexcites the 3024 keV level) are clearly resolved by the fitting procedure.

Figure 7

Relevant sections of electron (a) and γ (b) energy spectra showing the region around 800 keV. In the electron spectrum, in addition to the conversion lines of the 797 keV, $2_2^{+}\to 2_1^{+}$ transition and 822 keV transition, deexciting the 2104 keV level, a line at 814 keV is present with no counterpart in the γ spectrum. It has been identified as the K-conversion line of the $0_3^{+}\to 0_2^{+}$ transition (see text).

Figure 8

Partial level scheme of ${}^{76}\mathrm{Kr}$ showing the decay of the states populated in the ${\beta }^{+}$-decay of ${}^{76}\mathrm{Rb}$. The feeding percentage and the associated log ft values are reported on the right-hand side of the figure up to 2600 keV.

Figure 9

Same as for Fig. 8 except for levels above 2600 keV.

Figure 10

Values of ${\rho }^2$ calculated as a function of the deformation parameter ${\beta }_2$ for different values of the squared mixing amplitude a${}^2$, having assumed ${\beta }_1=0.33$. The horizontal lines indicate the experimental value together with the ±σ statistical uncertainty.