Jonathan H. Hamilton

Performance of the Recoil Mass Spectrometer and its Detector Systems at the Holifield Radioactive Ion Beam Facility

The recently commissioned Recoil Mass Spectrometer (RMS) at the Holifield Radioactive Ion Beam Facility (HRIBF) is described. Consisting of a momentum separator followed by an E-D-E Rochester-type mass spectrometer, the RMS is the centerpiece of the nuclear structure endstation at the HRIBF. Designed to transport ions with rigidities near K = 100, the RMS has acceptances of +/- 10% in energy and +/- 4.9% in mass-to-charge ratio. Recent experimental results are used to illustrate the detection capabilities of the RMS, which is compatible with many detectors and devices

Advances in studies of nuclei far from stability

Studies of the structure and decay modes of nuclei far from stability have yielded many new insights which were not seen in nuclei near the line of beta stability. The results of recent experimental and theoretical investigations are presented and the new insights being gained are discussed. Some of the highlights described include ground-state proton radioactivity, beta -delayed 2n, 3n and 2p decays, the coexistence of states built on quite different nuclear shapes in the same nucleus, new regions of strongly deformed nuclei, and the importance of the reinforcement of proton and neutron shell gaps on the shape of a nucleus.

Shape coexistence phenomena in medium mass nuclei

Abstract We review the theoretical interpretation of experimental results on low and high spin states in some even mass A=60−80 nuclei within the complex versions of the Excited Vampir and Fed Vampir variational approaches. The coexistence of oblate and prolate deformed configurations and their mixing at low spin are found to be responsible for the irregular sequences of states and the isomeric decays identified at low excitation energy in even-even nuclei. This behaviour is determined essentially by the isoscalar neutron-proton interaction. At higher spins the coexistence of states with different deformations causes complicated multiple band structures and strong fragmentation of the B(E2) strengths. It also influences the occurence of almost identical bands at normal deformation in some even mass nuclei. In some odd-odd nuclei the shape coexistence furthermore suggests a possible mechanism to populate high spin isomeric states. For the ground state properties of N=Z nuclei the competition between neutron-proton and like-nucleon isovector pairing plays an important role. With increasing angular momentum the neutron-proton alignment dominates the like-nucleon one in the ground bands of odd-odd N=Z nuclei while in the doubly even N=Z nuclei both types of alignment are of comparable importance.

Detailed Ge(Li)-Ge(Li) coincidence studies of levels in152Sm and152Gd

The decay of 12.4 y152Eu to152Sm and152Gd was studied in a high resolution singles measurement and two 4096×2048 Ge(Li)-Ge(Li) coincidence studies. Forty-five gates were analyzed in order to confirm or establish the placement of the γ rays as well as to accurately determine the intensities of the 12 doublets, which included several not previously reported. A new γ-β interband transition, 4γ+→2β+ (561.4 keV), a possible 3γ+→2β+ (423.7 keV) transition, and a new 330.9 keV transition between the 4γ+ level and theIπK=3−0 octupole level were established from coincidence data. Levels at 1757.0 keV in152Sm and 1282.5, 1318.7, and 1692.2 keV in152Gd are now established on the basis of coincidence data. These data also establish for the first time the population in the152Eu decay levels at 1680.0 and 1047.9 keV in152Sm and152Gd, respectively, as well as a new level at 1700.8 keV in152Sm.

Experimental values of internal-conversion coefficients of nuclear transitions: total and K-shell coefficients and L-subshell coefficient ratios

A tabulation is presented of experimentally measured values of internal-conversion coefficients for the K -shell, α K , L -subshells (mostly ratios only, L 1 : L 2 : L 3 ), and for all shells, α total . Results reported prior to November 1965, whose uncertainties are ≦ 25%, are listed and compared with theoretical values. Transition energies and spins an parities of initial and final levels, when known, are included. The theoretical values were obtained from computer interpolation, and in some cases extrapolation, of the tables of Sliv and Band. A brief discussion of the methods of measuring conversion coefficients is given.

A COMPARISON OF THE F-SPIN AND NpNn SCHEMES WITH GLOBAL EMPIRICAL SYSTEMATICS

The limitations of the F-spin and the NpNn schemes in reproducing the overall systematics in the Z=50−82, N=82−126 shell space are reviewed. The neutron-proton excess or isospin T is shown to play an important role. It determines the position of the β-stability line on the N−Z chart, and explains the existence of isotonic (isotopic) multiplets.

Electric Monopole Transitions from Excited 0+ States in 156Gd

Internal conversion electron spectra from the decay of 156 Eu were measured at high resolution with an air-core, β-ray spectrometer. The source of 156 Eu was made through the double neutron capture process by irradiating separated 154 Sm with thermal neutrons and was chemically separated. Our data were combined with recent gamma-ray data to deduce conversion coefficients for 47 transitions. The 0 + states at 1049.4, 1168.1 and 1715.2 keV were confirmed. Spins and parities of other states were also assigned or confirmed. A method of analysis for β-band de-excitations is developed with ratios of E2/E0 matrix elements. The β-ground mixing parameter of 156 Gd is obtained to be 0.014, while the β-γ mixing parameter 0.029 for the 2 + β state and 0.005 for the 4 + β state.

Cold and hot binary and ternary fission yields in the spontaneous fission of 252Cf

The spontaneous fission (SF) of 252Cf has been studied via γ-γ-γ coincidence and light charged particle—γ-γ coincidence with Gammasphere. The yields of correlated Mo-Ba pairs in binary fission with 0–10 neutron emission have been remeasured with an uncompressed cube. The previous hot fission mode with 8–10 neutron emission seen in the Mo-Ba split is found to be smaller than earlier results but still present. New 0n binary SF yields are reported. By gating on the light charged particles detected in ΔE-E detectors and γ-γ coincidence with Gammasphere, the relative yields of correlated pairs in alpha ternary fission with zero to 6n emission are observed for the first time. The peak occurs around the α2n channel. A number of correlated pairs are identified in ternary fission with 10Be as the light charged particle. We observed only cold, 0n10Be and little, if any, hot, xn10Be channels.

810.6–863.6 keV γγ-directional correlation in58Fe

The 810.6–863.6 keV γγ-directional correlation has been measured with an NaI(Tl)-Ge(Li) detector system. The NaI window was set on the 810.6+863.6 keV composite peak. From the (810.6 NaI gate) (863.6 Ge spectrum) data at 90 °, 135 °, and 180 °, we deducedA2=+0.51±0.03 andA4=+0.09±0.04. These results disagree with the values previously obtained from58Co decay (with Nal-NaI detector systems) but agree with the values from γγ(θ) studies following thermal neutron capture by57Fe.

Fine structure in proton emission

Deformations and wave functions of proton‐radioactive nuclei are studied using measured fine structure properties of proton emission and microscopic theoretical models. The experimental data are available for 131Eu and 145Tm decays, as well as for 141gsHo, where an observation of fine structure in proton emission is reported for the first time.