P J Twin

The Clover: a new generation of composite Ge detectors

This paper presents the characteristics of a new generation of composite germanium detectors: the Clover detector. Its development was based on simulation calculations performed with the code GEANT3. An extensive study of its main characteristics involving the photopeak detection e

Construction and use of a three Ge(Li) Compton polarimeter

ciency, the energy resolution and the timing performed with sources and in-beam experiments have been measured. A simple Doppler broadening correction procedure is proposed to improve the in-beam energy resolution of the Clover detector. An other important property, the sensitivity to the linear polarisation of c-rays has been measured. The performances obtained with the associated BGO suppression shield are also reported. All these results compare very well with the initial simulation calculations. ( 1999 Elsevier Science B.V. All rights reserved.

The statistical analysis of γ-ray angular correlation experiments

Abstract A three Ge(Li) Compton polarimeter has been constructed, and its sensitivity to γ-ray linear polarisation determined in the energy range 0.4 to 4.4 MeV. An example is given describing its application to resolve a spin ambiguity for the 4868 keV level in 33 S populated by the reaction 30 Si(α,n) 33 S at an α-particle bombarding energy of 9.8 MeV.

A shape change along the yrast line in 132Ce

Abstract The statistical analysis of γ-ray angular correlation experiments is discussed with the emphasis on experiments where the reaction mechanism does not lead to exactly known alignment. If a model of the reaction producing the alignment is made then it can be used to estimate the most probable value of the alignment and the confidence in that prediction. The inclusion of such estimates in the analysis is described. A procedure for including many types of relevant data into the analysis is described. Particular emphasis is given to the general technique of estimating the error to quote in association with a mixing ratio measurement. A cautious recipe for the estimate is suggested which follows the procedures of linear least squares regression analysis.

Rotational band structures and lifetime measurements in 130Ce

A rotational band has been found in 132Ce which has a high moment of inertia. This band decays by a series of discrete transitions in the spin range (20-40) h(cross) and is more strongly populated above 26 h(cross) than the yrast band known previously. These data are discussed in terms of the predicted shape change expected along the yrast line of 132Ce to a more deformed shape with epsilon 2 approximately 0.35.

Using high-fold data from the new generation of γ-ray detector arrays

The deformed nucleus 130Ce has been studied using the techniques of in-beam gamma -ray spectroscopy. Beams of 16O and 18O were used on targets of 116,117,118Sn to obtain excitation energies, mean lifetimes, spins, parities, mixing ratios and branching ratios. The gamma - gamma coincidence data necessary to determine the decay scheme were obtained using an array of five escape-suppressed spectrometers. The yrast band has been extended to Jpi =24+, or possibly Jpi =26+, with mean lifetimes measured for states up to Jpi =22+. Five new bands have been established and a 150 ns isomer has been identified at an excitation energy of 2454 keV. Backbending is observed due to the alignment of two h11/2 protons at the same rotational frequency in both the yrast band and the bands on the isomer. The gamma band based on the 834 keV (Jpi =2+) state can be identified with a triaxial shape corresponding to gamma approximately=-25 degrees . This band backbends at spin 10+ due to the alignment of two h11/2 protons showing softness of the nucleus and the coexistence of different shapes. The data may be interpreted in terms of the cranking model by assuming a prolate deformation with epsilon 2 approximately=0.25. The measured transition strengths in the yrast band generally show a decrease in epsilon 2 or a change in triaxiality from gamma approximately=0 degrees to gamma approximately=30 degrees . The transition rates measured for the 12+ to 10+ and 14+ to 12+ transitions are bigger than expected and are consistent with an increase in deformation to epsilon 2=0.30-0.35.

Experimental measurements of the relative peak efficiencies of various Compton polarimeters

Abstract The new generation of high-efficiency gamma-ray spectrometers allows the collection of high-fold gamma-ray coincidence data. Some problems associated with the sorting and subsequent analysis of this data are discussed. A method is presented to allow the creation of statistically correct spectra when selection criteria are placed on gamma-ray energies.

Band crossing and triaxiality in 129Ce

Abstract The relative total energy peak efficiencies of conventional γ-ray Compton polarimeters, constructed using both Ge(Li) and NaI(Tl) detectors, and a planar Ge(Li) polarimeter have been experimentally determined for a range of γ-ray energies from 0.66 MeV to 3.27 MeV.

Finite solid angle corrections for NaI(Tl) crystals when pulse height selection is employed

Gamma-ray decays from states in 129Ce have been studied using 16O-induced reactions on 116Sn and 117Sn targets. Previously known positive- and negative-parity bands have been extended to spins of 31/2 and 39/2, respectively. Band crossings are seen for the first time in 129Ce and occur near the same rotational frequency as the h112/ proton band crossing in 130Ce. The signature splittings in the bands are discussed in terms of the cranked shell model with a prolate deformation epsilon 2=0.25. The data show that the one-quasiparticle bands based on the h11/2 (523) 7/2 Nilsson orbit have gamma approximately -15 degrees (signature alpha =+1/2) and gamma approximately -30 degrees ( alpha =-1/2). After the band crossing, the three-quasiparticle bands (including two h11/2 quasiprotons) have gamma approximately 0 degrees for both signatures. The positive-parity states corresponds to gamma approximately 0 degrees for both the one-quasiparticle and three-quasiparticle bands. The branching ratios have been used to deduce the ratio (gK-gR)/Q0 as a function of rotational frequency. This ratio shows an increase when the backbending takes place.

An on-line computer controlled time digitizer for half-life measurements

Abstract Measurements have been made of the relative contributions of different annular sections of a 5″ × 6″ NaI (Tl) crystal to the total energy peak. The results obtained have been used to calculate the correction factors due to finite angle effects in angular correlations when only the counts in tḣe total energy peak are included in the analysis.