On the deexcitation of phonon excitations in nonspherical even-even nuclei by M 1 transitions
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CLEI Theory
On the Deexcitation of Phonon Excitations in Nonspherical Even–Even Nuclei by M1 Transitions A. M. Demidov, L. I. Govor, V. A. Kurkin, and I. V. Mikhailov Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia Received October 20, 2005; in final form, May 24, 2006
Abstract—The reasons for the violation of sign invariability for the parameter of mixture of M 1 and E2 multipoles in the deexcitation of levels of the β- and γ-rotational bands to levels of the ground-state rotational band are considered for N = 90–114 nonspherical even–even nuclei. PACS numbers : 23.20.Gq, 23.20.Lv, 25.40.Fq, 27.70.+q DOI: 10.1134/S1063778807020032
1. INTRODUCTION In the present analysis of experimental results, emphasis is placed on the sign of the mixing pa rameter for E2 and M 1 multipoles, δ = E2/M 1. Our consideration is based on the two-quasiparticle– phonon model. Specifically, we analyze γ transitions from the first K π = 0+ and K π = 2+ single-phonon states, for which present-day experimental data are the most comprehensive and for which there are theoretical calculations of the wave-function structure. In nonspherical nuclei, the first K π = 2+ states are usually γ-vibrational ones, whereas, in N = 90–114 nuclei, there are a few single-phonon K π = 0+ levels in the energy range between 1 and 2 МeV. All of the latter or, at least, the most collective of them (it is not always the lowest) can be thought to be β-vibrational. For our purposes, it is of importance that each of these levels is treated as a superposition of twoquasiparticle neutron and proton states. Since the rules that we formulate for the deexcitation of levels are independent of the degree to which these levels are collective, we retain here the term “β-vibrational” levels for the first K π = 0+ states in all of the specific nuclei considered in our study. In N = 90–114 nuclei, quadrupole moments determining the probability of E2 transitions from levels of the β- and γ-rotational bands to levels of the ground-state rotational band are positive (these nuclei are prolate along the symmetry axis); therefore, the sign of the parameter δ is determined by M 1 transitions—that is, by the sign of the magnetic moment for the specific states of nucleons involved in these transitions. This sign is negative for a neutron in Ωn = Λn + 1/2 ≡ Λn ↑ Nilsson’s state, but it is positive for the Ωn = Λn − 1/2 ≡ Λn ↓ state, as well as for any proton state (with the exception of the Ωp =
Λp − 1/2 ≡ Λp ↓ state at Λp = 1). Thus, a negative sign of δ indicates that, in the transition being considered, M 1 radiation is predominantly determined by neutrons in states of magnetic moment µ < 0— that is, in the Ωn = Λn + 1/2 state—or by Ωp = 1/2 protons at Λp = 1. In [1], we applied the same approach in considering M 1 transitions in N = 90–114 nuclei. As twoquasiparticle configurations for β- and γ-phonon excitations, we employed sets found according to the semimicroscopic description of the structure of deformed nuclei in [2]. For the ground state, we too
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