Nuclear structure of 122Xe studied via high-statistics β/EC-decay

The nuclear structure of 122Xe has been investigated with measurements of the β/EC decay of 122Cs with the 8π γ-ray spectrometer at the TRIUMF-ISAC facility. The data collected have enabled the determination of relative B(E2) values of some low-energy transitions, and the in-band transitions of the excited 0 bands have been observed. As a result, the 2 rotational band members for the 0+2 and 0 + 3 states have been firmly identified.


Introduction
Nuclei in the Z > 50, N < 82 region exhibit a very smooth evolution of collectivity; however, collectivity in this region is poorly characterized because of a general lack of high-quality spectroscopic data for low-spin states. It is the characterization of low-spin states, e.g., relative and absolute B(E2) decay strengths and the occurrence of E0 decays, that are essential is describing the collective nature of these nuclei.
The excited 0 + 3 states in 124−132 Xe isotopes [1] are very strongly populated in ( 3 He, n) reactions, suggesting that there are important proton subshell gaps influencing the low-lying structure of these isotopes. The strongly populated 0 + states, which are candidates for the main fragments of the proton pairing vibration, appear to have a similar deformation as the ground state band based on the similarity of the spacing of their possible rotational band members. The assigned band members, however, are generally based on energy considerations since, in most cases, the in-band transitions have remained unobserved. In nuclei with Z ≤ 50, there is ample evidence for shape coexistence [2,3] of the bands that are also strongly populated in the ( 3 He, n) reactions. Shape coexistence with mixing of the configurations would lead to strong E0 transitions, and E0 transitions are observed in 118−124 Xe [4] and in 114−126 Te [5], but they are not fully characterized. To date, no evidence for shape coexistence in the Xe isotopes has been found, but it remains an interesting question if the Xe isotopes might manifest shape coexistence given their proximity to the Z = 50 closed shell.
The Xe isotopes were thought to be γ-soft nuclei or, in the language of the IBM, possess O(6) symmetry. Howa e-mail: bjigmedd@uoguelph.ca ever, recent Coulomb excitation work, in particular 124 Xe [6] showed that O(6) was badly broken while on 128 Xe [7] showed that 128 Xe is not an E(5) nucleus as previously suggested. The B(E2) values determined in Ref. [6] were refined with highly sensitive γ-ray measurements with the 8π spectrometer following 124 Cs β decay [8]. The 0 + 3 level identified as the proton pairing vibration based on previous ( 3 He,n) work [6] was shown to have a nearly identical absolute value of deformation, |β| as the ground state band. Key observations that enabled this determination in the βdecay measurement were the in-band 2 + → 0 + transitions that established firmly the 2 + rotational band members.
The systematics of the excited 0 + bands are shown in Fig. 1, where in 120 Xe and 122 Xe there are no candidates for the 2 + members of the 0 + 3 bands. This deficiency makes characterizing their behaviors difficult, and thus they need to be investigated further. In order to provide detailed spectroscopic data, especially of weak lowenergy decay branches and possible key E2 transitions, a high-statistics 122 Cs β + /EC-decay experiment was performed at the TRIUMF-ISAC facility.

Experimental detail
The experiment to study the β + /EC of 122 Cs was performed at the TRIUMF-ISAC facility located in Vancouver, B.C., Canada. A 65-μA, 500-MeV proton beam was delivered to the ISAC facility and bombarded a thick nat Ta foil target. Products of the spallation reaction diffused to the surface of the Ta target foils, were ionized with a Re surfaceion source, and passed through a magnetic mass separator that was set to select singly-charged A = 122 ions. The high-intensity beam of 1.   of 122 Cs in the 8 − isomeric state with a 3.7 minute half life was delivered to the center of the 8π γ-ray spectrometer [9][10][11][12] and implanted into a FeO-coated mylar tape. The 8π spectrometer consisted of 20 high-purity Ge detectors surrounded by bismuth-germanate (BGO) Comptonsuppression shields. The average source-to-Ge-detector distance was approximately 14 cm. A BC-422Q fast plastic scintillator with a solid angle of approximately 20% of 4π was located immediately behind the beam deposition point, while the 5 Si(Li) detectors of the PACES array for conversion-electron detection were positioned upstream and aligned to the beam-spot position. The average source-to-Si-detector distance was 3 cm. More details of the 8π spectrometer are given in Refs. [10,12], and PACES is described in Refs. [12,13]. Two sets of data were collected for short-and long-half-life decays in repeated cycles. Each set of data was collected in a mixed trigger mode involving scaled-down γ-ray and e − singles, and γ − γ and γ − e − coincidences. The 8π Ge efficiency was measured using standard radioactive sources of 133 Ba, 152 Eu, 56 Co, and 60 Co. The data were sorted into γ-ray and e − spectra, and γ − γ and γ − e − randombackground-subtracted coincidence matrices. Analyses of the matrices and fitting of the spectra were performed with the Radware package [14].

Results and discussions
The data collected enables the observation of hundreds of new transitions including those that feed the 0 + 2 and 0 + 3 states. As an example of data quality, a portion of the level scheme of the low-lying states of 122 Xe is presented in Fig. 2. Newly observed transitions are indicated in blue colored arrows and numbers while new levels, with their energies and assigned spins, are shown in red. Examples of coincidence γ-ray spectra that clearly show newly observed transitions are given in Figs. 3, 4, and 5. The peaks in the spectra are labelled with γ-ray energies and newly observed γ rays are presented with blue colored numbers. The relative B(E2) values for transitions draining the 2 + 3 and 2 + 4 levels were determined via   were determined by taking a gate on the strongest feeding γ ray and dividing the peak intensity by the total intensity out of the level of interest. In Fig. 6, the relative B(E2) values obtained from this work are given in red numbers inside the brackets. As seen in Fig. 6, the in-band 2 + →0 + transitions have B(E2) values that strongly dominate the out-of-band transitions. This fact, together with the energies of the 2 + states, leaves little doubt that the 2 + band members have been firmly identified. The energy spacing of the 2 + band members suggests that the excited 0 + bands have similar deformation, but as shown in Ref. [8] this can be deceiving and the 0 + 2 band in 124 Xe, in particular, was found to have a significantly larger |β| value than the ground-state band. Absolute B(E2) values are required to make any conclu-sions in 122 Xe. It is suggested that the 0 + band at 1717-keV is the analogue of the proton-pairing vibrational 0 + 3 bands in the heavier Xe isotopes.

Conclusions
This work is part of a systematic study of the Xe isotopes with high-statistics β decay. We have firmly identified for the first time the 2 + rotational band members of the 0 + 2 and 0 + 3 states in 122 Xe. The rotational spacing is similar that of the ground state band. The analysis is going on and, to date, we have observed more than 160 new transitions and more than 100 new levels that expands dramatically the level scheme for 122 Xe. The analysis has been concentrated on the γ-ray data but the conversion electron data will be also analyzed to provide a comprehensive picture of the low-lying states of 122 Xe.