[en] The perturbations of γ-γ directional correlations from nuclei oriented by heavy-ion induced reactions, as observed with a multidetector array, are employed to measure the magnetic dipole moments of sub-nanosecond excited states in neutron-deficient nuclei. The technique is compared with other techniques for measuring g factors of short-lived excited states. Some representative results, which use transient hyperfine fields to measure g factors at high spin and static hyperfine fields to measure g factors at low spin, are presented

[en] Gyromagnetic ratios were measured in the (1(2))⁺ [411] ground-state band of ¹⁶⁹Tm by the transient field technique in such a manner that the extracted g-factors are independent of assumptions concerning the strength and velocity dependence of the transient field. Precise γ-ray branching intensities and multipolarity mixing ratios were determined from measured particle-γ-ray angular correlations. The electromagnetic properties of the low-lying natural parity states in ¹⁶⁹Tm are compared with particle-rotor calculations based on the Woods-Saxon potential and the implications of the measured magnetic moments for the calibration of the transient field strength are discussed

[en] Full text: Magnetic moment (or g-factor) measurements of short-lived states in neutron-deficient nuclei present a number of experimental challenges which require the development of new procedures and techniques. We have developed a perturbed DCO (Directional Correlations from Oriented nuclei) technique for measuring the g-factors of unstable nuclei which has a number of advantages over alternative methods. States in ¹⁸⁰-¹⁸⁴Pt were populated by 145 MeV ²⁹Si induced reactions on to a thick ^natGd target which allowed all three Pt nuclei to be created simultaneously as well as serving as a ferromagnetic host. The target was polarized perpendicular the gamma-ray detection plane by a compact electromagnet specifically designed to fit within the CAESAR array of Compton suppressed Ge detectors. Comprehensive gamma-gamma correlation data containing unperturbed and perturbed DCO information were collected for all (21) two-detector combinations in CAESAR. From the data we have determined the relative g-factors of the low-lying 2⁺ and 4⁺ states in the ground state bands of ¹⁸⁰Pt, ¹⁸²Pt and ¹⁸⁴Pt, as well as the average g factors of the continuum at high spins above the yrast line. Along with a description of the technique, the nuclear structure implications of the results will be discussed in terms of shape-coexistence models of the low-spin states and the effects of aligned particles at high spin

[en] When the dipole-dipole energy transfer, Rb 25s_1/2+Rb 33s_1/2→Rb 24p_1/2+Rb 34p_3/2, resonant at fields of 3.0 and 3.4 V/cm, is observed in a cold Rydberg gas, the resonances are broader than expected for this dipole-dipole interaction alone. Using a variant of the Ramsey interference method, in which the system is detuned from the resonance, we observe a density-dependent dephasing which we attribute to the inhomogeneities in the always resonant dipole-dipole excitation exchange interactions Rb 25s_1/2+Rb 24p_1/2→Rb 24p_1/2+Rb 25s_1/2 and Rb 33s_1/2+Rb 34p_3/2→Rb 34p_3/2+Rb 33s_1/2. Since the dephasing rate is comparable to the observed widths, it appears that all of these interactions contribute to the observed linewidths

[en] Full text: A technique based on measuring the perturbations of γ-γ correlations following heavy-ion induced reactions has been used to study gyromagnetic ratios at low and high spin in several nuclear near A=80. The nuclei of interest, ⁷⁸Kr and ⁸¹Sr, were populated following ²⁸Si induced reactions on a natural iron foil that served as both the target and as a ferromagnetic host in which the spins of the excited nuclei were perturbed by intense hyperfine fields. In the A=80 region, the nuclei are soft and have states at similar excitation energies that are associated with different nuclear deformations. For example, in ⁸¹Sr rotational bands with prolate and oblate deformations co-exist. This paper will discuss these and related structures in the light of the new gyromagnetic ratio data

[en] The 1/2[411] ground state band (gsb) of the nucleus ¹⁶⁹Tm provides a good example of a rotational band near the strong-coupling (pure K) limit of the particle-rotor model. Although it has been much studied, and has been utilised as a probe nucleus to calibrate the TF for rare-earth ions in Fe and Gd hosts, there has been no precise, independent measurement of the g-factors above spin 7/2⁺. States of interest in the gsb of ¹⁶⁹Tm were Coulomb excited up to the 21/2⁺ state using beams of 170, 190 and 220 MeV ⁵⁸Ni from the ANU 14UD Pelletron Accelerator. In all measurements the recoil-implanted Tm nuclei traversed the same 5.3 mg/cm² thick Gd foil (cooled to ∼90 K) and stopped in a backing layer of Cu. The Gd foils were polarized perpendicular to the γ-ray detection plane by a small electromagnet which produced an external magnetic field of 0.08 T. The direction of the polarizing field was automatically reversed approximately every 15 minutes. Precession measurements for the particle-γ angular distributions were measured using 2 pairs of HP Ge detectors placed at ±60 deg C and ±120 deg C to the beam direction. Backscattered ⁵⁸Ni ions were detected in coincidence with the de-exciting γ-rays from excited states using an annular silicon surface barrier counter subtending an angular range from 150 deg C to 166 deg C. Fed precessions of the most strongly populated states were measured to a relative precision of better than 2%. Unperturbed particle-γ-ray angular distributions were determined at all three beam energies. The detector pair in the backward quadrant remained at -120 deg C to serve as monitors while the forward detectors were placed, in turn at various angles. The data are being analysed to determine the relative g-factors of the individual states in the gsb, independent of the TF calibration. Our data will also enable critical evaluation of the velocity dependence of the TF, without relying on model-based g-factors

[en] Cold dense samples of Rydberg atoms spontaneously evolve into ultracold plasmas. Initially some cold ions are formed by black body photo ionization and collisions, and they trap electrons freed later. It appears that these trapped electrons initiate an avalanche of ionization of the remaining Rydberg atoms. How quickly the avalanche occurs is determined in part by how rapidly energy is supplied to the system

[en] By photoionizing cold, trapped atoms it is possible to produce ultracold plasmas with temperatures in the vicinity of 1 K, roughly 4 orders of magnitude colder than conventional cold plasmas. After the first photoelectrons leave, the resulting positive charge traps the remaining electrons in the plasma. Monitoring the dynamics of the expansion of these plasmas shows explicitly the flow of energy from electrons to the ionic motion, which is manifested as the expansion of the plasma. The electron energy can either be their initial energy from photoionization or can come from the energy redistribution inherent in recombination and superelastic scattering from recombined Rydberg atoms. If the cold atoms are excited to Rydberg states instead of being photoionized, the resulting cold Rydberg gas quickly evolves into an ultracold plasma. After a few percent of the atoms are ionized by collisions or blackbody radiation, electrons are trapped by the resulting positive charge, and they quickly lead to ionization of the Rydberg atoms, forming a plasma. While the source of this energy is not clear, a likely candidate is superelastic scattering, also thought to be important for the expansion of deliberately made plasmas