T.E. Drake

Spin response of magnetic dipole transitions in 156Gd and 164Dy

Abstract Intermediate energy proton scattering has been used to probe the spin part of the recently discovered low-lying isovector magnetic dipole transitions in the rotational rare earth nuclei 156 Gd and 164 Dy. A large spin response is found in 164 Dy, whereas in 156 Gd the results are consistent with the picture of a predominantly convective excitation. The results are discussed in the context of the IBA-2 model and recent RPA calculations.

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.

Studies of superdeformation in the gadolinium nuclei

Abstract We have used the 8π γ-ray spectrometer at the Chalk River TASCC facility to study superdeformed rotational bands in the chain of isotopes 145–149 Gd. The five bands already known have been extended to higher spin, while four new excited bands have been discovered. The use of very similar reactions, data-acquisition parameters and data-analysis techniques, combined with the enlarged set of data, has allowed us for the first time to approach the topic of superdeformation in the A = 150 mass region from a comprehensive point of view. Transition energies of neighbouring nuclei were compared to extract effective aligned spins of the added particle. Such a comparison requires knowledge of relative nuclear spins, which we have to assume. Nevertheless, we find that a very illustrative picture emerges making it possible to understand all nine bands in the Gd isotopes within a very simple scheme of orbital assignments.

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.

Rotational bands and shape changes in 124Ba

Abstract High-spin states in 124 Ba were populated via the 94 Zr( 34 S, 4n) 124 Ba and 110 Cd( 16 O, 2n) 124 Ba reactions at 145 MeV and 60 MeV, respectively. The yrast band has been extended to 34 ħ and seven side bands have been observed. Gamma-ray angular distributions with respect to the nuclear spin axis have been used along with directional correlation (DCO) ratios to establish spins and multipolarities. The competing π h 11 2 and ν h 11 2 crossings in this region are investigated and an interpretation of the observed crossings is proposed within the framework of the cranked shell model (CSM) and total routhian surface (TRS) calculations.

The 6−T=1 resonance in 28si via high-resolution inelastic electron scattering

Abstract High-resolution (e,e′) was used to measure the form factor of the 6 − , T =1 resonance in 28 Si. The results disagree with previous experimental results and with theoretical calculations. The role of meson-exchange currents in producing the observed quenching of magnetic strength, and the relevance of (e,e′) to other reactions are briefly discussed.

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.

The triumf low energy pion spectrometer and channel

Abstract A low energy pion spectrometer has been developed for use with the TRIUMF M13 pion channel. The combined channel and spectrometer resolution is presently 1.1. MeV at T = 50 MeV. This is limited by the amount of gas and detector material in the spectrometer in addition to the inherent resolution of the channel. Improvements to both the spectrometer and channel are discussed.

A high-resolution measurement of the photofission spectrum of 232Th near threshold

Abstract The Chalk River bremsstrahlung monochromator at the University of Illinois Microtron Laboratory has been used with a multiwire fission chamber to measure the photofission cross section of 232 Th between 4.95 and 6.76 MeV with a resolution of 12–14 keV. This cross section, which on the average increases exponentially with photon energy, shows three plateaus 5.20 to 5.80, 5.90 to 6.15, and above 6.30 MeV respectively. Structure is observed on eached plateau. In particular there appear to be well-resolved peaks, separated by ≈ 100 keV, at 5.50 and 5.60 MeV on the lowest plateau and other prominent peaks are observed at 5.92, 6.03 and 6.11 MeV on the middle plateau. The closely spaced structure in each region might be interpreted as evidence for the excitation of vibrational states in shallow outer wells of a multiwell potential.

A high resolution bremsstrahlung monochromator for photo-nuclear experiments

Abstract An improved bremsstrahlung monochromator, based on the original University of Illinois design, is being used with the Illinois C.W. microtron for (γ, γ) and (γ, f) measurements. The monochromator includes an electron energy disperser followed by an electron energy analyser operating in the energy loss mode. The disperser, which includes two quadrupoles and one 90° n = 0.5 dipole, focuses a demagnified image of the electrons transmitted by a 2 mm defining slit onto a thin Ni foil, termed the radiator. Electrons scattered by the radiator are analysed by a 180° inclined-plane flat pole-piece magnetic spectrometer and are detected in the focal plane by a hodoscope consisting of a 72-wire proportional gas-counter backed by a 12 segment plane of Ne-102 plastic scintillators. Bremsstrahlung emitted by the Ni foil is incident on a target and photons or fission particles emitted by the target are detected in coincidence with pulses from the scintillators and wire counter. The limit of energy resolution, determined primarily by the 3 mm wire spacing, is 14 keV for 3.5 MeV electrons. The limit of resolving time is set by the 3.5 ns jitter of the plastic scintillator-photomultiplier system. The factors which have contributed to improved resolution are: the use of a proportional gas-counter with closely spaced wires, the use of a reduced image of the defining slits on the radiator, the use of thin radiator foils, 5.4 mg/cm 2 , and the location of the radiator close to the minimum gap between the pole-pieces. The performance of the monochromator is illustrated by data from the resonance fluorescence of 208 Pb and the photofission of 232 Th.