First Evidence of Magnetic Rotation in theA=80Region

Abstract

Rotational bands with strong magnetic dipole transitions have been observed in the doubly odd nuclei 82 Rb and 84 Rb. These bands show the characteristic features of magnetic rotation. They are the first evidence of this new kind of nuclear excitation in the A ഠ 80 region. The results are well reproduced within the framework of the tilted axis cranking model on the basis of four-quasiparticle configurations of the type p͑ fp͒-p͑g 2 9͞2 ͒-n͑g 9͞2 ͒. [S0031-9007 (99)09279-0] PACS numbers: 21.10.Re, 23.20.En, 23.20.Lv, 27.50. + e The conventional concept of nuclear rotation is based on the existence of a deformed mass distribution of the nucleus. Regular rotational bands are formed by energy levels that depend on the spin I according to E~I͑I 1 1͒ and are connected by electric quadrupole (E2) transitions [1].Recently, a surprising phenomenon has been observed in nearly spherical Pb isotopes around A 200. While the excited states at low spin show irregular multipletlike structures as expected for nearly spherical nuclei, regular sequences that follow the I͑I 1 1͒ rule evolve at high spin, indicating a rotational mode. The levels of these sequences are linked by strong magnetic dipole (M1) transitions whereas crossover E2 transitions are very weak [2][3][4][5]. The ratios of the transition probabilities are typically in the order of B͑M1͒͞B͑E2͒ ഠ 20 40 ͑m N ͞e b͒ 2 . These observations contradict the common understanding of nuclear physics that only well-deformed nuclei should exhibit rotational bands.A solution of this apparent paradoxon has been given in the framework of the tilted axis cranking (TAC) model [6]. The coupling of angular momentum vectors of a few high-j nucleons is the basic mechanism for generating the total spin I of the nucleus. Few protons occupy orbitals with long spin vectors above a closed shell (high-j particlelike orbitals) while the neutrons fill up a shell except for a few holes (high-j holelike orbitals), or vice versa. A perpendicular coupling of their angular momenta is energetically favored because it maximizes the overlap of the spatial density distribution, which is toruslike for particle orbitals and dumbbell-like for hole orbitals. This coupling results in a substantial component of the magnetic dipole moment, which is transverse to the total spin and gives rise to large M1 transition probabilities of several m 2 N . As the magnetic dipole rotates about the axis of the total angular momentum, this new mode has been called "Magnetic Rotation" [7]. The total angular momentum is increased by the gradual alignment of the individual particle and hole spins along the axis of the total angular momentum. Since this rearrangement of the particle and hole angular momentum vectors resembles the closing of the blades of a pair of shears, it has been called "shears mechanism" [3]. As a consequence of this alignment the transverse component of the magnetic dipole moment decreases and one expects smoothly decreasing B͑M1͒ transition probabilities with increasing spin. Recent results [5] confirm th...

Magnetic dipole sequences inRb83

“…This behavior is typical for magnetic rotation and caused by the gradual alignment of the spins of the involved proton and neutron orbits (shears mechanism). We described the regular negative-parity M1 bands in 82 Rb and 84 Rb within the TAC model on the basis of the lowest-lying four-quasiparticle (4qp) configuration with negative parity: π (fp)π (g 2 9/2 ) ν(g 9/2 ) [32,33]. The good agreement between experimental and calculated characteristics proves the validity of the concept of magnetic rotation for these bands.…”

“…The E2 sequence B is built on top of the sequence of four intense M1 transitions at 100. The M1 sequence D resembles at a first glance the negative-parity M1 bands found in the neighboring odd-odd isotopes 82 Rb and 84 Rb [32,33]. A closer inspection, however, shows remarkable differences.…”

“…i.e., E γ (M1) =hω ∼ J ] [32,33]. The B(M1)/B(E2) ratios determined from branching ratios of transitions within the M1 bands reach values up to 25 (µ N /eb) 2 and decrease smoothly with increasing spin in the range of 13 J 16.…”

Theoretical developments in heavy nuclei

“…The shears bands, or the magnetic rotation phenomena, have already been discovered in several regions of nuclei, see, e.g., recent studies around A≃80 [95], A≃110 [97], A≃140 [98,99], and A≃200 [100,101,102]. Most of such structures were found in light lead isotopes, but in principle, they can occur in any nucleus with the shell structure based on high-j proton (neutron) holes coupled to high-j neutron (proton) particles [93].…”

Interplay between tilted and principal axis rotation

“…In the last two decades, a large number of bands arising from tilted axis rotation (TAR) [1,2] have been identified [3,4,5,6,7,8,9,10,11,12,13,14] in the A ∼200, 140, 110 and 80 regions of the periodic table. In this mode of excitation, the angular momenta of the valence nucleons are along the rotational axis (low Ω orbitals) and the symmetry axis (high Ω orbitals) only and the contribution of the core rotation to the total angular momentum is small (low to moderately deformed nuclei).…”