The decay of ¹⁰⁰Rb described in the preceding contribution to this report allowed a considerable extension of the decay scheme of ⁹⁹Sr via the b-delayed neutron decay mode. The decay of the high ground-state spin of I^p=4^- of ¹⁰⁰Rb feeds higher band members than previously observed in the b-decay of the I^p=5/2⁺ ground-state of ⁹⁹Rb [1].

In this region of large deformations many transitions have almost the same energies if one regards a deviation by a few percents as the limit for 'identical' energies. This can be observed irrespective of the involved nuclei are even-even, odd or odd-odd and spins may be integer or half integer. However, some level differences are identical in a much more restrictive sense, i.e. they differ by quantities of the order of 0.1%. The ground-state bands in ⁹⁸Sr and ¹⁰⁰Sr, identical except for the 2 to 0 transition perturbed by a low-lying 0⁺ state in ⁹⁸Sr, were first discussed in [2]. This persists at higher spins [3]. A comparable degree of identity was also reported for the K^p=3/2⁺ bands in the odd-neutron nuclei ⁹⁹Sr and ¹⁰¹Sr [4]. Thus, systematics suggests that moments of inertia for g.s. bands in e-e nuclei and K=3/2 bands in odd-neutron nuclei do depend only on deformation. In the re-assigned 5/2^- band in ⁹⁹Sr [5] the energies of the transitions in the 9/2 to 7/2 to 5/2 cascade are 147.6 and 111.9 keV. The latter is very close to the energy assumed for the 7/2 to 5/2 transition, 111.6 keV, in ¹⁰¹Sr [4]. These strongly perturbed bands also could be identical. In this work, we notice identical transitions in the immediate neighbouring nuclei ⁹⁹Sr and ¹⁰⁰Sr, see fig. 1. First, the 9/2⁺ to 5/2⁺ energy difference in the [411]3/2 g.s. band of ⁹⁹Sr (287.2 keV) has almost the same energy as the 4⁺ to 2⁺ transitions in the even-even ⁹⁸Sr and ¹⁰⁰Sr neighbours, 289.4 and 287.8 keV, respectively. In this case, we note that j_odd=j_even+1/2. Moreover, the 11/2⁺ to 7/2⁺ energy difference of 353.9 keV is also close to the 6 to 4 energy difference of 356.2 keV in the K^p=4^- band in ¹⁰⁰Sr. The energies in this two quasineutron band [411]3/2¤[532]5/2 are compressed with respect to the ground-state band, by an amount which follows the rule of thumb that each odd particle increases the moment of inertia by about 10%. Yet the almost identical energies, now a case where j_odd=j_even-1/2, were unexpected. It is not clear why this situation occurs. We notice that for a K=1/2 band, if the decoupling parameter is a=1 there is a degenerate doublet structure with spacings like in the even-even core. These peculiarities are surely worth to be investigated theoretically. Moreover, the energies of 161.8 and 194.4 keV for ¹⁰⁰Sr are close to the energies of 159.3 and 194.4 keV reported by Durell et al. for the K^p=4^- band in ¹⁰²Zr [6]. This contrasts with the different rigidities of their ground-state bands. Thus breaking of a pair, does not modify as much the moment of inertia of ¹⁰⁰Sr as of ¹⁰²Zr.

Fig. 1: Identical bands in neutron-rich Sr and Zr isotopes

Sr and Zr isotones for N<=62 do not show such identical energies. Nevertheless, the energies for the 9/2 to 7/2 to 5/2 cascade in ¹⁰³Zr are 146.6 and 109.4 keV [7], which are not very different anymore from the ⁹⁹Sr energies of 147.6 and 111.9 keV. We note that while deformations of Sr isotopes are already the largest in this region with 0.40 for ⁹⁸Sr and remain that large with increasing N, deformations of Zr isotopes, e.g. b=0.38 for ¹⁰²Zr increase steadily with N and might approach 0.40 for ¹⁰³Zr. Thus, deformation looks like the key mechanism for the identity of transition energies.

References:

1. B. Pfeiffer et al., Z. Phys. A317 (1984) 123.
2. G. Lhersonneau et al., Z. Phys. A337 (1990) 143.
3. J.H. Hamilton et al., Progress in Particle and Nuclear Physics 35 (1995) 635.
4. G. Lhersonneau et al., Z. Phys. A352 (1995) 293.
5. G. Lhersonneau et al., to be submitted to EPJ
6. J.L. Durell et al., Phys. Rev. C52 (1995) 2306.
7. M.C.A. Hotchkis et al., Nucl. Phys. A530 (1991) 111.