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...

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High-spin states were populated in 108 Cd using the 96 Zr( 16 O,4n) reaction at a beam energy of 72 MeV. Two magnetic dipole bands have been observed, both of which contain weak E2 crossover transitions. Lifetimes for the stronger of the two bands were measured via the Doppler shift attenuation method. The configuration assignment for this band has been determined from comparison with tilted axis cranking model calculations to be ͓g 9/2 Ϫ3 g 7/2 ͔ ͓h 11/2 (g 7/2 d 5/2 ) 1 ͔ and ͓g 9/2 Ϫ3 g 7/2 ͔ ͓h 11/2 3 (g 7/2 d 5/2 ) 1 ͔, before and after the h 11/2 2 band crossing, respectively. The deduced B(M 1) strengths decrease rapidly with increasing spin after the h 11/2 2 alignment, which is a characteristic behavior of a shears band.

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