IBFFM predictions for low-lying spherical states in the shape-coexisting odd-odd nucleus98Y

Abstract

Extrapolating the /nteraeting Boson--Model treatment from lighter neighboms to 9sy we predict the properties of low-lying states of spherical nature in IBFFM. This odd--odd nucleus which presents one of the rare cases of coexisting spherical and deformed shapes is of fundamental importance for understanding nuclear phase transitions.The odd-odd nucleus 9sy lies between spherical and deformed isotopes: 96y and 97y exhibit patterns of spherical shell-model states, while ggY and t00y have fully deformed structures with well developed rotational bands. Therefore it is a particularly good candidate for coexistence of spherical and deformed configurations. In fact, the first rotational band found in a non-even--even nucleus of the A, 100 region was the very regular band in 98y with the head at 495keV [1-2], while the lower-lying levels seem to have quasiparticle character [3][4]. In this paper we report the application of the Interacting Boson-Fermion-Fermion approach to the low-lying levels in 98y. We are in a particularly difficult situation since very little is known about the spins. Even the ground--state spin and parity was a long--standing problem, and only recently the 0-assignment has been established [4]. In the light of such a situation, we have attempted to predict the properties of low-lying levels in 98y using the parameters from successful IBFM and IBFFM calculations for 97y [5], 97Sr [6] and 9~V [7]. First we have performed IBFM studies for the negative-parity states in the odd-even isotope 97y with the same boson core of U (5) Then IBFM calculations were performed for the positive parity states in the even-odd isotone 97Sr. We have assumed that the u~/~ configuration is completely occupied and it was omitted from the calculations. The neutron quasiparticle parameters were adjusted to 97Sr: e!~712) -e(~'l/2) = 0;9 MeV, e(~/~)z-e(~'l/2)= 1.2 MeV, v (~'1/2) = 0.5, v2(gT/2)= 0.2, v2(aa/2 ) = 0.15. It is assumed that the 3/21 state at 167.1 keV in 97Sr is not of ~ta/2 structure (see also [7]), but of more complex nature as associated with the I = j -1, j -2 anomaly [10]. The boson-neutron interaction strengths Ao ~ = 0.18 MeV, Po v = 0.5 MeV, ho v = 0.4 MeV were adjusted to the levels of gTSr.In the third step we have carried out the IBFFM studies for the lowest-lying states in ~sy which are associated with a spherical shape, using the same boson core. For the quasiparticle parameters and boson-fermion interaction strengths we used the values from the IBFM calculations for the normal-parity states in gTy and 07Sr. The value of Fo is the same as the one used for 98y [7]. For the spin-spin residual interaction strength we employ the estimate Vaa --20]A = 0.2 MeV. The strength of the surface delta interaction is taken as V6 =-0.22 MeV; this value was obtained by requiring that the diagonal matrix element or the ('rgg/2, ugT]2~configuration of 9sy has the same value as in the calculation for 9~y [7].

Structure of theN=59 nucleus97Sr: coexistence of spherical and deformed states

“…It is tempting to interpret the first excited state with U= 3/2 + as the d3/2 single neutron state. However, our BCS as well as recent IBFFM calculations of the oddodd 96Y57 and 98y59 nuclei [24][25][26], indicate that this orbital must lie higher. The nature of the 3/2 + level is therefore more complex (see below), probably containing sizeable admixtures of the g7/2 configuration in addition to the d3/2 single neutron orbital.…”

βdecay of nuclei around90Se: Search for signatures of aN=56subshell closure relevant to theQuinn1, Aprahamian2, Pereira3 et al. 2012Phys. Rev. C

“…Such allowed transitions are not possible from the 98 Sr 60 ground-state 0 + , which decays via forbidden transitions into a daughter's spectrum dominated by negative-parity states. Such states involve the coupling of the p 1/2 proton orbital with s 1/2 , d 3/2 and/or g 7/2 neutron orbitals, coexisting with negative-parity states [32].…”

Study of Intermediate-spin States of $^{98}$Y

“…Shape coexistence in 98 Y has been reported. The spherical nature of the low-lying levels was proposed in a study of the β decay of 98 Sr to 98 Y [1] and was confirmed by calculations using the interacting-boson-fermion-fermion model (IBFFM) framework [15]. It was shown that levels of 98 Y below 500 keV could be described by coupling the πp 1/2 orbit to the lowest-lying spherical neutron levels of the neighboring isotones (N = 59) 97 Sr and 97 Zr.…”