Lifetimes of states in four of the M1 bands in 198,199 Pb have been determined through a Dopplershift attenuation method measurement performed using the GAMMASPHERE array. The deduced B͑M1͒ values, which are a sensitive probe of the underlying mechanism for generating these sequences, show remarkable agreement with tilted axis cranking (TAC) calculations. The results represent clear evidence for a new concept in nuclear excitations: "magnetic rotation." [S0031-9007(97)02583-0] PACS numbers: 21.10. Tg, 23.20.Js, 23.20.Lv, 27.80.+w The observation of long cascades of magnetic dipole (M1) transitions in the neutron deficient Pb nuclei [1,2] has prompted great interest among nuclear-structure physicists. The properties of the bands are extremely unusual: (i) most of the structures follow the rotational I͑I 1 1͒ rule over many states despite very low deformations, (ii) the levels are linked by strong M1 transitions with weak E2 crossover transitions [typical B͑M1͒͞B͑E2͒ ratios $20 40 ͑m N ͞e b͒ 2 ], and (iii) the ratio ᑣ ͑2͒ ͞B͑E2͒ is roughly an order of magnitude larger than that for normal or superdeformed bands, indicating that a substantial portion of the inertia is generated from effects other than quadrupole collectivity.The initial interpretations of the bands [1,2] suggested that they were based on high-K proton configurations (involving h 9͞2 and i 13͞2 orbitals) which induce a small oblate deformation (b 2 Ӎ 20.1), coupled to neutron holes in the i 13͞2 subshell which carry angular momentum aligned with the collective rotational axis. This picture accounts reasonably for the occurrence of the structures, but how can long, regular cascades, which appear rotational, occur when the nucleus develops only a small deformation?An intuitively appealing description of the behavior of the M1 bands arises naturally from the tilted axis cranking (TAC) model [3], and is schematically illustrated in Fig. 1. Near the band head the proton angular momentum vector, j p , is nearly parallel to the symmetry axis while the neutron angular momentum vector, j n , is perpendicular to it. The total angular momentum vector, J, then lies along a tilted axis at an angle u with respect to the symmetry axis. To generate angular momentum j p and j n gradually align along the direction of J with u remaining approximately constant. Only a small component of the total angular momentum is from collective rotation (denoted by R in Fig. 1). If the spin vectors are long and rigid enough then regular I͑I 1 1͒ sequences are predicted. Since the behavior of j p and j n is reminiscent of the closing of a pair of shears this process has been dubbed the "shears mechanism" [2].The regular sequences of strongly enhanced M1 transitions (sometimes called "shears bands") and the TAC picture suggest a new concept-"magnetic rotation" [4]. This arises as a consequence of breaking the intrinsic rotational symmetry by a long magnetic dipole vector (which rotates about J in the TAC picture described above). This FIG. 1. Schematic representation of the shears...
AN. (1996). Single step links of the superdeformed band in : A measure of the absolute excitation energy, spin and parity of the superdeformed states. Physics Letters B, 380(1-2), 18-23. https://doi.org/10.1016/0370-2693(96)00367-X Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194pb is established at 4877 4-1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the.SD band are also deduced assuming that the SD states have even spin and positive parity.
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