The isotopes 68 74 Ni, of interest both for nuclear physics and astrophysics, have been produced in proton-induced fission of 238 U and ionized in a laser ion guide coupled to an on-line mass separator. Their b decay was studied by means of b-g and g-g spectroscopy. Half-lives have been determined and production cross sections extracted. A partial level scheme is presented for 73 Cu and additional levels for 71 Cu, providing evidence for a sharply lowered position of the p1f 5͞2 orbital as occupancy of the n1g 9͞2 state increases. The latter may have a clear impact on the predicted structure and decay properties of doubly magic 78 Ni. [S0031-9007 (98)07340-2]

In-source resonant ionization laser spectroscopy of the even-A polonium isotopes (192-210,216,218)Po has been performed using the 6p(3)7s (5)S(2) to 6p(3)7p (5)P(2) (λ=843.38  nm) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in (200-210)Po with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.

Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.

New approaches to perform efficient and selective step-wise Resonance Ionization Spectroscopy (RIS) of radioactive atoms in different types of supersonic gas jets are proposed. This novel application results in a major expansion of the In-Gas Laser Ionization and Spectroscopy (IGLIS) method developed at KU Leuven. Implementation of resonance ionization in the supersonic gas jet allows to increase the spectral resolution by one order of magnitude in comparison with the currently performed in-gas-cell ionization spectroscopy. Properties of supersonic beams, obtained from the de Laval-, the spike-, and the free jet nozzles that are important for the reduction of the spectral line broadening mechanisms in cold and low density environments are discussed.Requirements for the laser radiation and for the vacuum pumping system are also examined.Finally, first results of high-resolution spectroscopy in the supersonic free jet are presented for the 327.4 nm 3d 10 4s 2 S 1/2 → 3d 10 4p 2 P 1/2 transition in the stable 63 Cu isotope using an amplified single mode laser radiation.

We describe experiments on Si doping in Al x Ga 1−x N grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2 ϫ 10 20 cm −3 for the entire range of compositions investigated, from x = 0.56 to 1.0. These conditions resulted in donor concentrations of ϳ1 ϫ 10 19 cm −3 up to x = 0.85. Layers of Al x Ga 1−x N up to x = 0.85 show good mobility and low resistivity. In these layers, the activation energy, E a , of Si stays below ϳ25 meV and Si can be considered a shallow donor. For AlN content above x = 0.85 the donor activation energy increases to E a ϳ 250 meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.