Correlation between low-lyingM1 andE2 strength in heavy rare earth nuclei

Abstract

Magnetic dipole excitation strengths attributable to the scissors mode in even-A rare earth nuclei between A=140 -190 are collected and presented assuming the excitation energy as an additional signature for the fragments of this mode. In this whole mass region the total M1 strength is found to be proportional to the quantity B(E2;0, + -+2, +)/Z as has been known before for the Nd and the Sm nuclei. In the upper half of the N= 82 -126 major shell the M1 and the E2 strengths do not saturate. Both quantities exhibit a rather monotonous increase towards midshell.PACS number(s): 21.10. Re, 23.20.g, 25.20.Dc, 27.70.+q One of the most exciting findings in nuclear spectroscopy in the last decade is the observation of strong low-lying magnetic dipole excitations in deformed nuclei which are frequently referred to as a scissors mode [1,2]. A crucial proof of the "scissors" character of this M1 mode was the discovery of the so-called "8 law" by Ziegler et al. [3] and its confirmation in Refs. [4 -6]. In various nuclear models such as microscopical, algebraical, geometrical, and phenomenological models a large effort has been made to predict the energy and the excitation strength of the scissors mode. Nearly all of these models are consistent with a quadratic dependence of its excitation strength on the deformation parameter 8 [7 -18].Thus the 8' law implies a saturation of the excitation strength of the scissors mode in light rare earth nuclei toward midshell as the deformation 8 saturates. This effect is actually observed.Contrary to the prediction of simple collective models such as the axially symmetric two rotor model which predicts a single scissors mode state, in real nuclei the scissors mode turns out to be fragmented into several states. This fragmentation considerably complicates the detection and identification of the scissors mode.Since the prediction of the scissors mode [1] in the late seventies and its discovery [2] in 1984 in a high resolution electron scattering experiment the nuclei of the rare earth region have been systematically investigated by means of the nuclear resonance fiuorescence (NRF) technique [19 -21] which is particularly well suited to study dipole excitations from the ground state. Recent NRF experiments have provided (y, y') data on heavier nuclei [22 -25]. It is the purpose of this paper to collect and discuss the available data for the even-A rare earth nuclei focusing on their dependence on the nuclear deformation and the numbers of nucleons.We will first discuss how one can obtain information on the scissors mode from a (y, y') NRF experiment. Observables in NRF experiments on even-A nuclei are the excitation energy F, the spin J of the excited level, the branching ratio R,"~=B(ml; I;~2+, )/B(ml; I; -+0t+), and the absolute dipole transition strengths B(m1;0, +~1;). If a Compton polarimeter is used as is discussed in Refs. [26 -28] information about the parities m of the excited states can be obtained. NRF measurements using a Compton polarimeter have been performed [6,29 -31] f...

Shell model description of “mixed-symmetry” states in Mo

“…The 1 + mode is expected to be dominantly excited by the isovector part of the M1 operator indicating its isovector character. The large M1 transition strength and its close correlation [23][24][25][26] to the collective E2 excitation strength in deformed nuclei is usually considered an indication of the collective nature of the 1 + sc state. Another state with spin different from J π = 1 + but of similar isovector character, the one-quadrupole phonon 2 + ms state, has been identified from M1 strengths, too.…”

Parity assignments inYb172,174using polarized photons and theKquantum number in rare earth nuclei

“…For triaxial shapes the Alaga rules do not hold and in this case the assignment of positive parity from the branching ratio is lacking a basis. If the stable nuclei in the A ≈ 170 mass region would indeed exhibit a more pronounced triaxiality than the lighter stable rare earth nuclei, then the previous compilations [6,7] of the total scissors mode's M 1 excitation strength using parity assignments on the basis of decay branching ratios might contain a systematic error for nuclei with mass numbers A ≈ 170.…”

A novel decay channel of the 1+scissors mode: coupling to the vibrationalβ-excitation

“…Therefore, it is desired to study those properties of the scissors mode that arise entirely from finite number effects of valence particles, such as electromagnetic transition rates to other intrinsic excitations. The scissors mode [1][2][3][4] is a lowlying magnetic dipole excitation, typically observed in deformed nuclei [5][6][7]. Within a geometrical picture the scissors mode is a counter oscillation of the deformed proton against the deformed neutron body in the intrinsic frame.…”