Shears mechanism and particle-vibration coupling

Macchiavelli, A. O.; Clark, R. M.; Deleplanque, M. A.; Diamond, R.M.; Fallon, P.; Lee, I. Y.; Stephens, F. S.; Vetter, K.

doi:10.1103/physrevc.58.r621

Cited by 53 publications

(39 citation statements)

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“…Experimentally deduced B͑E2͒ values suggest that the nuclei are more spherical than previously thought. In addition, B͑M1͒ values calculated using the alternative phenomenological scheme [7,8] are found to be in good agreement with the experimental B͑M1͒ values for both bands. This Letter therefore provides the first confirmation of the involvement of the shears mechanism in generating these structures in near-spherical tin nuclei.…”

supporting

confidence: 71%

“…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.…”

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confidence: 98%

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Confirmation of the Shears Mechanism in Near-Spherical Tin Nuclei

et al. 1999

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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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confidence: 98%

Confirmation of the Shears Mechanism in Near-Spherical Tin Nuclei

et al. 1999

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“…However, the interpretations are not as firm in these mass regions as in the A ϭ 195 mass region. It may be noted that the high-j intruder orbitals, which are known to play an important role in many highspin phenomena such as superdeformation, also play an important role in generating MR bands.Recent calculations based on the spherical shell model with a reasonable configuration space for protons and neutrons have also been able to reproduce regular MR bands and confirm the shears mechanism of generating magnetic dipole rotational bands [9].A suggestion has recently been made that the shears mechanism may arise due to a residual interaction between the protons and neutrons in the two blades [10]. An interaction of the P 2 ( ) kind can be mediated through the core by particle-vibration coupling acting in second order.…”

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confidence: 87%

Table of Magnetic Dipole Rotational Bands

Amita

Singh

2000

Atomic Data and Nuclear Data Tables

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“…These results suggest that the TAC regime may often get mixed with other modes arising from shape changes with rotational frequency. [16,17] by using the configurations π(i 13/2 h 9/2 )K = 11 − ⊗ ν(i 13/2 ) −2 before and π(i 13/2 h 9/2 )K = 11 − ⊗ ν(i 13/2 ) −4 after the crossing, suggesting the transition from a three-quasiparticle to a five-quasiparticle configuration. The closing neutron-proton blades open up again near I = 22 and a new MR band, having two more neutrons, crosses it.…”

Section: Shape Coexistence Shape Change and Pac-to-tac Transitionmentioning

confidence: 99%

Magnetic rotation — past, present and future

Choudhury

2010

Pramana - J Phys

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Magnetic-dipole rotational (MR) bands were discovered about 15 years ago without any theoretical prediction in contrast to the super-deformed (SD) bands which were predicted long ago. First identification of a quasirotational structure as MR band occurred around 1992 although Kr isotopes probably have the first set of data having the signatures of MR bands as shown by us. Our first compilation of MR bands listed 120 MR bands in 56 nuclides which have now grown to more than 180 bands in 80 nuclides. We have observed new MR bands in the A = 130 mass region in 137 Pr, 139 Nd and 135 Ba nuclei. This led to the observation of the smallest MR bands in 137 Pr, multiple minima in the γ deformation in 135 Ba, coexistence of band structure based on these minima and band crossing of MR bands in A = 130 region. Some of these results have been reviewed in this paper along with theoretical calculations. There are still a number of questions related to MR bands which have not been fully resolved. The role of neutrons/protons in magnetic rotation still needs to be delineated. Do the MR bands follow the I(I + 1) behaviour? Are these structures as regular as normal rotational bands? How important is the existence of deformation for MR bands? We address some of these questions in this paper.

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Shears mechanism and particle-vibration coupling

Cited by 53 publications

References 6 publications

Confirmation of the Shears Mechanism in Near-Spherical Tin Nuclei

Confirmation of the Shears Mechanism in Near-Spherical Tin Nuclei

Table of Magnetic Dipole Rotational Bands

Magnetic rotation — past, present and future

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