Gamma-ray spectroscopy was revolutionized with the introduction of high energy-resolution semiconductor germanium (Ge) detectors in the early 1960s. This led to the large increase in sensitivity realized by today's arrays of Compton-suppressed Ge detectors. A still larger increase in sensitivity is expected by implementing the new concept of tracking. A tracking array consists of highly segmented Ge detectors (that can cover the full 4π solid-angle) in which γ rays will be identified by measuring and tracking every γ -ray interaction. This article reviews the physics motivation for such detectors and the development of the new technologies involved. The concept of tracking is explained using the example of a proposed array called γ -ray energy tracking array (GRETA).

By deriving the angle between the proton and neutron spin vectors j ជ and j ជ in the shears bands in 198,199 Pb, we present a semiclassical analysis of the B(M 1) and B(E2) transition probabilities as a function of the shears angle. This provides a semiempirical confirmation of the shears mechanism proposed by Frauendorf using the tilted-axis-cranking model. In addition, we propose that the rotational-like behavior observed for these bands may arise from a residual proton-neutron interaction. ͓S0556-2813͑98͒51303-9͔

The lifetime of the 2_+(1) state in 16C has been measured with the recoil distance method using the 9Be(9Be,2p) fusion-evaporation reaction at a beam energy of 40 MeV. The mean lifetime was measured to be 11.7(20) ps corresponding to a B(E2;2_+(1)-->0+) value of 4.15(73)e_2 fm_4 [1.73(30) W.u.], consistent with other even-even closed shell nuclei. Our result does not support an interpretation for "decoupled" valence neutrons.

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...

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