G. C. Ball

The mass dependence of CsI(Tl) scintillation response to heavy ions

Abstract The response of CsI(Tl) scintillators to heavy ions is investigated as a function of E , A , and Z . In addition to the expected dependence of light output on Z and E , we observe a significant dependence on mass number. A simple parameterization of the quenching in terms of a few physical variables permits characterization of the light output for a variety of nuclear species with a single quenching constant.

Gamma-ray spectroscopy of 126Ba

Abstract States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

Collective properties of 48Cr at high spin

Abstract High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

Rotational bands in 238U

Abstract A thick foil of 238 U was bombarded with 209 Bi beams at 1130 and 1330 MeV, delivered by the TASCC facility at Chalk River Laboratories. Gamma-ray spectroscopy of states populated in multiple Coulomb excitation was performed with the 8π spectrometer, an instrument comprising 20 Compton-suppressed HPGe detectors and 71 BGO ball elements. The event trigger required that 3 BGO elements and 2 HPGe detectors fire in coincidence. The experiment achieved a high degree of sensitivity, the weakest rotational band observed had about 0.16% intensity of the ground state rotational band. Several bands were observed to high spin for the first time, including the γ-vibrational band (spin 27 + ) and the octupole bands with K = 0 (spin 31 − ), K = 1 (spin 28 − ) and K = 2 (spin 25 − ). Results for positive and negative parity vibrational bands are compared with cranked RPA calculations. Although this theory can explain some features of the data, many puzzling aspects remain to be explored.

Multi-particle excitations in the superdeformed 149Gd nucleus

Abstract Six rotational bands built on superdeformed intrinsic configurations have been observed in the 149 Gd nucleus with the Eurogam spectrometer. Orbital configuration assignments have been suggested on the basis of their effective alignments calculated with the Nilsson-Strutinsky cranking model. Most of the excited bands have identical partners in neighboring nuclei including one case differing by four mass units. Measurements of feeding patterns indicate that the 149 Gd yrast superdeformed band is fed over a wider range of angular momentum than other yrast superdeformed bands in this mass region whereas weaker excited bands in the same nucleus are populated in narrower spin windows.

Channel selection for high spin γ-ray spectroscopy studies via total energy measurements in fusion-evaporation reactions

Abstract A channel selection method for high spin γ-ray spectroscopy studies based on the measurement of the total energy of all radiations (both charged particle and γ-ray) emitted in heavy-ion fusion reactions is presented. The method is applicable to all reactions in which charged-particle evaporation from the compound system dominates, and is particularly effective in isolating the weakly populated low particle multiplicity channels that leave the final nucleus with the greatest spin and excitation energy. The method is illustrated using data taken with the 8π γ-ray spectrometer and the miniball 4π charged-particle detector array at the Chalk River Tandem Accelerator Superconducting Cyclotron (TASCC) facility. Channel-to-total ratios are improved over those obtained with charged-particle detection alone by factors as large as 46 without significant loss of statistics for the selected channel.

Origins of intermediate velocity particle production in heavy ion reactions

In this paper, we report for the first time clear distinctions between these prompt processes and an alternative phenomenon of delayed aligned asymmetric breakup that populates the intermediate-velocity zone by a deformation rupture of mainly the heavier of the colliding partners in mass asymmetric collisions. These distinctions were observed experimentally with intermediate-velocity particle correlation analysis of Ni+C and Ni+Au reac

Mass excess and low-lying level structure of

The mass excess of /sup 14/B has been measured to be 23.657 plus or minus 0.030 MeV using the reaction /sup 14/C(/sup 7/Li,)/sup 14/B at E(/sup 7/Li) - 52 MeV ; this shows that /sup 14/B is bound by nearly 1 MeV against neutron emission. Five excited states were also observed at 0.74 plus or minus 0.04, 1.38 plus or minus 0.03, 1.82 plus or minus O.06, 2.08 plus or minus .05, and 2.97 plus or minus 0.04 MeV. The low-lying level structure of /sup 14/B wa s found to be similar to the known negative-parity spectrum of /sup 12/B. (auth)

sup 14

The response of an array of plastic phoswich detectors to ions of 1 ≤ Z ≤ 18 has been measured from EA = 12 to 72 MeV. The detector response has been parameterized by a three parameter fit which includes both quenching and high energy delta-ray effects. The fits have a mean variation of ≤4% with respect to the data.

B

Abstract A search for Gamow-Teller decay branches of 39Ca to excited states in 39K was conducted with a large HPGe detector which observed the decays of 39Ca samples delivered by a helium-jet transport system. A pair of plastic scintillators in front of the HPGe detector were used to reject events due to bremsstrahlung from the 39Ca positrons. With this setup, weak γ-rays could be observed with a degree of sensitivity not previously achieved. The l- forbidden 1 d 3 2 → 2 s 1 2 transition to the first excited state in 39K, was observed with a branching ratio of (2.50 ± 0.27) × 10−5. This result agrees with but is more accurate than the only previous measurement. Thus, the ratio of the l-forbidden Gamow-Teller and isovector M1 matrix elements in mass-39 nuclei continues to disagree with theoretical values obtained by Towner and Khanna from calculations involving core-polarization and meson-exchange currents. This leaves very few possibilities to explain the discrepancy between theory and experiment.