Confirmation of the Shears Mechanism in Near-Spherical Tin Nuclei

Abstract

Lifetimes of states of a magnetic dipole band in each of the nuclei 106,108 Sn have been obtained using the Doppler shift attenuation method. The deduced B͑M1͒ transition rates show the characteristic behavior associated with the shears mechanism. A simplified semiclassical analysis yields B͑M1͒ values in qualitative agreement with those expected for previously assigned configurations. The results suggest extremely low deformations for these dipole structures. In particular, the 106 Sn band appears to be the first example of almost pure magnetic rotation in a spherical nucleus. PACS numbers: 21.10.Tg, 23.20.Lv, 25.70.Gh, 27.60. + j Considerable evidence has now been collated for the existence of "rotational-like" magnetic dipole ͑M1͒ bands in the lead and tin regions (e.g., [1][2][3]). It has been proposed that the mechanism responsible for the existence of such structures in weakly deformed nuclei is the shears mechanism [4]. This mechanism is so called by analogy with the closing of a pair of shears since nearly all the angular momentum is generated by the gradual alignment of proton and neutron spin vectors (j p and j n ), which are initially coupled perpendicularly at the bandhead, with the total angular momentum vector, I (Fig. 1). This behavior has been discussed in terms of the tilted axis cranking (TAC) model [4][5][6] and the configurations and properties of the observed bands have been described, with some success, within the model [1,3]. The model introduces a new form of quantized rotation, "magnetic rotation" [4], where the rotational symmetry breaking arises from the anisotropic arrangement of nucleon currents. A more phenomenological description of the shears mechanism has recently been presented in terms of the coupling of two long vectors, j n and j p [7,8]. Their interaction is mediated by an effective quadrupole force attributed to particlevibration coupling. Both this and the TAC model predict a definitive signature characteristic of the shears mechanism, namely, that the B͑M1͒ transition rates between the levels in the bands, which are proportional to the square of the perpendicular component of the magnetic dipole vector (Fig. 1), should decrease markedly with increasing angular momentum. This behavior cannot be reproduced within the standard Dönau and Frauendorf cranking formalism [9]. The shears picture has been confirmed in the lead region through studies of the behavior of the B͑M1͒ values extracted from lifetime measurements [10,11] and a measurement of the g factor of the bandhead of a magnetic dipole band in 193 Pb, which confirmed the initial perpendicular coupling of the component proton and neutron spin vectors [12]. However, it is not clear if the shears mechanism will be active in the light tin region, where the spin vectors are expected to be shorter than in the lead region, due to the lower-j orbitals involved. Furthermore, the nuclei have a small prolate deformation in contrast to the oblate deformation, e 2 ഠ 20.1, of the lead nuclei. The behavior of the B͑M1͒ values shoul...

Shape coexistence and strongly coupled bands inSb118

“…Such bands have been observed in even-even 106,108 Sn [6,7], odd-A 105 Sn [8], and odd-odd 108 Sb [9]. In the odd-odd Sb isotopes, the yrast band of 108 Sb (N = 57) has been interpreted as the magnetic rotational band in terms of the tilted axis cranking (TAC) model [9].…”

“…Another interesting area in this mass region is the phenomenon of shear bands. Such bands have been observed in even-even 106,108 Sn [6,7], odd-A 105 Sn [8], and odd-odd 108 Sb [9]. It is of particular interest to investigate the shear mechanism along an isotopic chain in this region.…”

Specific features and symmetries for magnetic and chiral bands in nuclei

“…Indeed, the life time measurements indicated that the spin sequence form a stretched dipole band (B(M 1) ∼ 3 − 6µ 2 N ) and the E2 transition are almost missing (B(E2) ∼ 0.1(eb) 2 ). The decreasing behavior of the M1 strength with increasing spin was also experimentally confirmed [21,23,24]. The magnetic bands are characterized by quite unusual features:i) They are ∆I = 1 sequences of states of similar parity; ii) Despite the very low deformation, the energies follow the I(I + 1) rule; iii) The levels are linked by strong M1 transition rates with weak E2 crossover transitions (the typical B(M1)/B(E2) ratio ≤ 20 − 40(µ N /eb) 2 ); iv) the ratio J (2) /B(E2) is about an order of magnitude larger than that for normal or super-deformed bands.…”

“…• High-spin states were populated in 136 Pm by the 105 Pd( 35 Cl,2p2n) reaction, where the 35 Cl beam was accelerated to 173 MeV [157]. The chiral-twin candidate bands earlier observed in 136 Pm, have been extended to high spins [ I= (21)], using the Gammasphere γ -ray spectrometer and the Microball charged-particle detector array. The rotational alignments and B(M 1)/B(E2) ratios confirm that both sequences have the πh 11/2 ⊗ νh 11/2 configuration.…”

“…Indeed, there are several experimental data sets showing that the dipole bands have large values for B(M 1) ∼ 0.5 − 2µ 2 N , and very small values of B(E2) ∼ 0.1(eb) 2 (see for example Ref. [21]), compared with the typical values for well deformed nuclei. The states are different from the scissors like ones, exhibiting instead a shears character.…”