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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...

The electromagnetic dipole response of 138 Ba was measured up to 6.7 MeV excitation energy in a photon scattering experiment. Two Euroball Cluster detectors were used to detect the scattered photons under 94 and 132 degrees. The Cluster at 94 degrees served as a Compton polarimeter. The total observed dipole cross section is in good agreement with previous tagged photon data, but in the present experiment the transition strength and the electric character of most of the transitions has been determined on a state-by-state basis. The data show a concentration of E1 strength around 6 MeV. For one of the strongest excitations M 1 character is suggested. The results of model calculations using the quasiparticle phonon model ͑QPM͒ agree with the observed electric dipole strength distribution. ͓S0556-2813͑99͒51311-3͔