J. X. Saladin

The ICEBall: a multiple element array for in-beam internal conversion electron spectroscopy

Abstract The construction and testing of a multiple-element mini-orange spectrometer array, the Pitt ICEBall, is described. The performance of the array in tests with sources and in in-beam studies is discussed, and the questions of gamma-ray, delta-ray and secondary electron background are addressed. The ICEBall has a maximum efficiency of 18% of 4π, in a compact package capable of inclusion within large gamma-detector arrays such as the Oak Ridge Spin Spectrometer.

Forking and Unusual Decay Out of Superdeformed Bands in 83Zr

Two superdeformed (SD) bands extending over eight to eleven transitions have been identified in Zr-83, The quadrupole moment of the more intense band was determined by the Residual Doppler Shift Method and is consistent with a quadrupole deformation of beta(2) approximate to 0.5. The large quadrupole moment and population intensity of the yrast SD band (approximate to 5%) in Zr isotopes relative to their isotones in Sr and Y nuclei suggest the presence of a large SD shell gap at proton number Z = 40, At the decay-out points, the Routhians of the SD bands reapproach that of the positive parity normally-deformed states which may be the reason why both of these bands feed mainly (approximate to 85%) into the positive-parity yrast band.

Superdeformed Bands in 80Sr and the Evolution of Deformation in Sr Isotopes

Four superdeformed bands are reported in Sr-80, extending known superdeformation in the Sr-38 series down to N = 42. The characteristics of these bands are discussed. Residual Doppler shifts were measured and average transition quadrupole moments (Q(t)) inferred for these new bands, These Q(t) values are compared to Q(t) values obtained for previously identified superdeformed bands in Sr81-83. The low Q(t) of 2.7(-0.6)(+0.7) eb obtained for the yrast band in Sr-80 indicates a reduction in the deformation of yrast superdeformed bands in the series Sr80-83 with decreasing N, and possibly the onset of triaxiality in superdeformed shapes

Band Structure in 79Y and the Question of T=O Pairing

Gamma rays in the N=Z + 1 nucleus Y-79 were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental setup consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma rays were found to form a strongly coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N=Z and N=Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T=0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. [S0556-2813(98)50612-7].

Electromagnetic properties of excited bands

Abstract In this paper the predictions of various microscopic and phenomenological models for the electromagnetic properties of nuclei are compared with experiment. The discussion centers on five case studies and illustrates that there often exist certain key matrix elements which permit to differentiate between various models.

The FIGARO Facility at Los Alamos: Capabilities and First Results

A new beam line at the fast neutron spallation source at Los Alamos Neutron Science Center has been constructed for studies of neutron-induced reactions producing gamma rays, internal conversion electrons or neutrons. This facility, called FIGARO (Fast neutron-induced GAmma-Ray Observer), follows on the great successes of GEANIE (described in other contributions to this conference), by detecting de-excitation gamma rays with high-resolution germanium detectors. FIGARO has fewer gamma-ray detectors than GEANIE, but instead offers other features including: extremely good collimation of the neutron beam for background reduction, a flexible experimental area to optimize detection efficiency and to allow evaluation of other detectors such as ICEBALL-II for internal conversion electrons, inclusion of neutron detectors for the study of neutron-gamma coincidences, beam time to relieve the scheduling pressure on GEANIE, and a PC-based data acquisition system. Our initial measurements include level density studies through 59Co(n, xgamma) reactions to complement our previous 59Co(n, xalpha) measurements, reaction studies of MeV neutrons on 99Tc with the goal of determining cross sections relevant to transmutation and neutron transport in the design of facilities to incinerate nuclear waste, and an assessment of measuring internal conversion electrons, rather than gamma rays, produced by neutron excitation of actinides.

Nuclear physics with fast neutrons at LANSCE/WNR: GEANIE and FIGARO

GEANIE is an array of 26 HpGe detectors used to study nuclear reaction dynamics and structure following reactions with high-energy (1<En<200 MeV) neutrons, for both basic and applied research projects. Studies have included the measurement of (n,xn) partial cross sections as a function of En for a variety of nuclei, particularly actinides. More recently, studies of n-induced fission-fragment distributions and nuclear structure in the actinide region have been started. A second beam line and experimental station (FIGARO) have been set up to complement and extend this program. Research conducted on this second beam line includes the use of conversion electron spectroscopy to explore nuclear structure, using the University of Pittsburgh ICEBall II array.

Band Structure in 79Y and the Question of T=0 Pairing

Excited states in the N = Z+1 nucleus Y-79 Were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental set up consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma-rays were found to form a strongly-coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N = Z and N = Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T = 0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. (Less)