Evidence for “Magnetic Rotation” in Nuclei: Lifetimes of States in theM1 bands of198,199Pb

Abstract

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...

Deformation parameters and transition probabilities for shears bands in Pb isotopes

“…The calculated reduced transition probabilities can be compared with experimental values deduced from recent lifetime measurements [19][20][21][22][23][24]. These experiments employed the multidetector spectrometers Gammasphere and Euroball.…”

“…Reduced M1 transition probabilities, B(M1), for intraband transitions in 193−199 Pb as a function of spin of the initial state. Points (diamonds) mark values from DSAM (RDM) experiments[19][20][21][22][23][24]. Curves are the results of our TAC calculations.…”

Magnetic rotations in198Pb and199Pb within covariant density functional theory

“…In particular, long cascades of M1 transitions were observed first in neutron deficient Pb nuclei in the early 1990s [8][9][10]. Later, in 1997, the lifetime measurements based on the Doppler-shift attenuation method (DSAM) for four M1 bands in the nuclei 198 Pb and 199 Pb provided a clear evidence * sqzhang@pku.edu.cn † mengj@pku.edu.cn for magnetic rotation [11]. Subsequently, another experiment using the recoil distance method (RDM) in 198 Pb together with the results of the DSAM experiment provided further support to the shears mechanism [12].…”

“…The magnetic rotational bands in 198,199 Pb are investigated. The energy spectra, the relation between spin and rotational frequency, deformation parameters and reduced M1 and E2 transition probabilities are calculated and compared with the available data [11,12,61,62] and those obtained from the PQTAC model [18].…”

Magnetic rotation and quasicollective structures in58Fe: Influence of theνg9/2orbital

“…Later, lifetime measurements by Clark et al [40] for four M 1 bands in 198,199 Pb provided clear evidence for magnetic rotation. Numerous magnetic rotational bands have since been observed not only in the mass region A ∼ 190, but also at A ∼ 140, ∼ 110, and ∼ 80.…”