No abstract

A superdeformed rotational band has been identified in 36Ar, linked to known low-spin states, and observed to its high-spin termination at Ipi = 16(+). Cranked Nilsson-Strutinsky and spherical shell model calculations assign the band to a configuration in which four pf-shell orbitals are occupied, leading to a low-spin deformation beta(2) approximately 0.45. Two major shells are active for both protons and neutrons, yet the valence space remains small enough to be confronted with the shell model. This band thus provides an ideal case to study the microscopic structure of collective rotational motion.

The binding energies of even-even and odd-odd NϭZ nuclei are compared. After correcting for the symmetry energy we find that the lowest Tϭ1 state in odd-odd NϭZ nuclei is as bound as the ground state in the neighboring even-even nucleus, thus providing evidence for isovector np pairing. However, Tϭ0 states in odd-odd NϭZ nuclei are several MeV less bound than the even-even ground states. We associate this difference with the Tϭ1 pair gap and conclude from the analysis of binding energy differences and blocking arguments that there is no evidence for an isoscalar ͑deuteronlike͒ pair condensate in NϭZ nuclei.

The transition quadrupole moments Q t for the superdeformed band in 40 Ca have been determined through thin-target Doppler-shift attenuation analyses. A best-fit value of Q t ϭ1.30Ϯ0.05 e b is obtained when a single value is assumed for the entire band. Fitting separate quadrupole moments for in-band transitions decaying from the high-spin states and the presumably admixed low-spin states results in Q t (high)ϭ1.81 Ϫ0.26 ϩ0.41 e b and Q t (low)ϭ1.18 Ϫ0.05 ϩ0.06 e b, respectively. Q t values extracted for individual transitions in a Doppler-broadened line-shape analysis also indicate smaller Q t values at lower spins. These results are consistent with the interpretation of this band as an eight-particle-eight-hole superdeformed band with a significant admixture of less-collective configurations at low spins.

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