The ¹⁰⁰Rb activity was produced by fission of natural uranium induced by 600 MeV protons and on-line mass separated at the ISOLDE facility. The precise deformation of the b-decay daughter nucleus ¹⁰⁰Sr from the lifetime of its 2⁺ state at 129 keV has been reported in [1]. In the same experiment, a 85 ns lifetime was observed for the 1619 keV [2]. Recently, prompt-fission studies lead to identification of further members of the ground-state band [3]. Therefore, we have re-analysed the data and have considerably extended the level scheme of ¹⁰⁰Sr [4].

The systematics of N=62 isotones can be drawn over a large number of nuclides and is surprisingly smooth, see fig. 1. Several low-spin levels of collective character can be followed from the spherical ¹⁰⁸Pd to the strongly deformed ¹⁰⁰Sr, owing to the gradual evolution of the energies and of their decay branchings. Systematics strongly supports the presence of an excited 0⁺ state for which the 938 keV level is a very good candidate. This level can be regarded as the head of the b-vibration band. At somewhat higher energy, two 2⁺ states are expected corresponding to the 2⁺ state of the b-vibration and the head of the g-band. Two experimental levels at 1257 and 1315 keV have been found to have a ground-state transition and have been assigned as 2⁺ states. The extrapolation of energies favour the 1257 keV and 1315 keV levels as members of the b- and g-bands, respectively. The 3⁺ state associated with the g-vibration is expected at energies between 1500 and 1600 keV. Several levels with decays to the 2⁺ and 4⁺ g.s. band members and a transition from the 1619 keV level are possible 3⁺ states in this energy region. A bit higher some other levels with a fairly strong branch to the 417 keV 4⁺ state could be the 4⁺ states belonging to the b- and g-bands. Systematics is, however, not able to provide predictions which could be associated with a single of very few levels.

Fig. 1: Level systematics for N = 62 isotones

The 1619 keV isomeric level has been interpreted as a I^p=4^- state, based on considerations of hindrances of its decay to the 4⁺ level of the ground-state band and the available Nilsson orbitals near the Fermi level for N=62 [2]. The [411]3/2¤[532]5/2 neutron configuration indeed involves the lowest quasiparticles for N=61 and N=63 in this region [5,6]. A similar level has been reported in ¹⁰²Zr by prompt fission, leading to observation of a band up to I^p=9^- [7]. The lifetime of this band head at 1821 keV has not been measured. The postulated configuration has been reproduced by Quantum Monte Carlo calculations for ¹⁰⁰Sr and ¹⁰²Zr [8]. The decrease of pairing confirms numerous calculations of decay properties by the QRPA model [9]. The Gallagher rule favours coupling of antiparallel intrinsic spins. Thus, a 1^- level is expected below the 4^- spin triplet state. Its decay could proceed easily to the ground state by a [532]5/2 to [411]3/2 single-particle transition. Such a level has not been identified in ¹⁰²Zr [7]. The 1257 keV level in ¹⁰⁰Sr has a strong ground-state branch, but on basis on systematics we have assumed it to be a collective 2⁺ state. Anyway, quasiparticle states are lying very low in ¹⁰⁰Sr. Assuming that some of the levels fed from the 1619 keV level could be of this nature we have been searching for patterns of hindrances, but could not find convincing evidence for K-assignments. Finally, we mention that our choice of placing the 194 keV transition was based on the analogy with ¹⁰²Zr, which implies that one observes the 6^- to 5^- to 4^- levels of the band.

References:

1. G. Lhersonneau et al., Z. Phys. A337 (1990) 143.
2. B. Pfeiffer et al., Z. Phys. A353 (1995) 1.
3. J.H. Hamilton et al., Progress in Particle and Nuclear Physics 35 (1995) 635.
4. G. Lhersonneau et al., to be submitted to EPJ
5. G. Lhersonneau et al., Phys. Rev. C51 (1995) 1211.
6. G. Lhersonneau et al., Phys. Rev. C54 (1996) 1592.
7. J.L. Durell et al., Phys. Rev. C52 (1995) 2306.
8. R. Capote et al., J. Phys. G24 (1998) 1113.
9. G. Lhersonneau et al., Research Reports in Physics, Springer-Verlag, 58 (1988).