Measurements on prototype Ge and BGO detectors for the Eurogam array

doi:10.1016/0168-9002(92)90084-H

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 313, Issues 1–2, 1 March 1992, Pages 37-49

https://doi.org/10.1016/0168-9002(92)90084-H Get rights and content

Abstract

This paper presents the results of tests, both with sources and in-beam experiments, on prototype Ge and BGO detectors of the Eurogam array. Timing and energy resolution data are reported along with results for various methods of ballistic deficit correction. The first measurements with the new “shared suppression” technique are presented.

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A bismuth-germanate (BGO) anti-Compton shield (ACS) for Compex germanium detector modules was designed, built, and characterised. BGO performance was evaluated for light output and energy resolution. The response of the ACS shield and its impact on germanium detector spectra was measured for standard ⁶⁰Co, ¹³³Ba, and ¹³⁷Cs sources. The results were compared to comprehensive Geant4 simulations. The $\sim 70 %$ improvement of the peak-to-total ratio for the ¹³⁷Cs 662-keV $γ$ -ray peak is in good agreement with expectations from the simulations. A ¹³³Ba source exemplifies decay schemes of odd- $A$ superheavy nuclei. Congruence between measured and simulated spectra indicates that the Compex plus ACS multi-detector module is well understood.
Experimental observation of the M1 scissors mode in 254No
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The 254No ions were separated in flight in the gas-filled recoil separator RITU [24,25], and implanted in a pair of double-sided silicon strip detectors (DSSDs) placed at the focal plane of RITU. Prompt γ rays were detected by the Jurogam II germanium-detector array [26], consisting of 15 Compton-suppressed coaxial detectors [27,28] and 24 clover-type detectors [29]. The prompt γ rays were selected in delayed coincidence with recoils detected in the DSSDs.
We present the first experimental evidence of the scissors mode in the superheavy nucleus ²⁵⁴No produced in the ²⁰⁸Pb(⁴⁸Ca, $2 n γ$ ))²⁵⁴No reaction. The spectrum of γ rays emitted by the excited ²⁵⁴No nuclei shows an enhanced γ-ray yield for transition energies of ≈2.5 MeV. By measuring the linear polarization properties of the emitted γ rays, we confirm that the transitions in the enhancement region are predominantly of magnetic-dipole character, characteristic for the scissors mode. To further characterize the enhanced γ-ray yield, simulations of the electromagnetic decay of ²⁵⁴No were performed. The observed enhancement is reproduced by including an M1 component in the γ strength function with total strength $B (M 1 ↑) = 11.8 (19) μ_{N}^{2}$ . This is in good agreement with the integrated M1 strength from sum-rule estimates and new calculations within the quasi-particle random-phase approximation presented here. Our results provide a stringent test of phenomenological formulae for the scissors mode currently used in stellar nucleosynthesis calculations. We find that those formulae are not satisfactory, and we recommend using sum-rule estimates assuming a rigid-body moment of inertia instead for describing the scissors mode in superheavy nuclei.
Evidence against the wobbling nature of low-spin bands in 135Pr
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The electromagnetic character of the $Δ I = 1$ transitions connecting the 1- to 0-phonon and the 2- to 1-phonon wobbling bands should be dominated by an E2 component, due to the collective motion of the entire nuclear charge. In the present work it is shown, based on combined angular correlation and linear polarization measurements, that the mixing ratios of all analyzed connecting transitions between low-lying bands in ¹³⁵Pr interpreted as 0-, 1-, and 2-phonon wobbling bands, have absolute values smaller than one. This indicates predominant M1 magnetic character, which is incompatible with the proposed wobbling nature. All experimental observables are instead in good agreement with quasiparticle-plus-triaxial-rotor model calculations, which describe the bands as resulting from a rapid re-alignment of the total angular momentum from the short to the intermediate nuclear axis.
Lifetime measurements of excited states in 169,171,173Os: Persistence of anomalous B(E2) ratios in transitional rare earth nuclei in the presence of a decoupled i<inf>13/2</inf> valence neutron
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Citation Excerpt :
Prompt γ rays at the target position were detected by the JUROGAM II germanium detector array consisting of 15 Eurogam Phase I-type [21] and 24 Euroball clover [22] escape-suppressed detectors, with a total photopeak efficiency of ∼6% at 1.3 MeV.
Lifetimes of low-lying excited states in the $ν i_{13 / 2^{+}}$ bands of the neutron-deficient osmium isotopes ^169,171,173Os have been measured for the first time using the recoil-distance Doppler shift and recoil-isomer tagging techniques. An unusually low value is observed for the ratio $B (E 2; 21 / 2^{+} \to 17 / 2^{+}) / B (E 2; 17 / 2^{+} \to 13 / 2^{+})$ in ¹⁶⁹Os, similar to the “anomalously” low values of the ratio $B (E 2; 4_{1}^{+} \to 2_{1}^{+}) / B (E 2; 2_{1}^{+} \to 0_{gs}^{+})$ previously observed in several transitional rare-earth nuclides with even numbers of neutrons and protons, including the neighbouring ^168,170Os. Furthermore, the evolution of $B (E 2; 21 / 2^{+} \to 17 / 2^{+}) / B (E 2; 17 / 2^{+} \to 13 / 2^{+})$ with increasing neutron number in the odd-mass isotopic chain ^169,171,173Os is observed to follow the same trend as observed previously in the even-even Os isotopes. These findings indicate that the possible quantum phase transition from a seniority conserving structure to a collective regime as a function of neutron number suggested for the even-even systems is maintained in these odd-mass osmium nuclei, with the odd valence neutron merely acting as a “spectator”. As for the even-even nuclei, the phenomenon is highly unexpected for nuclei that are not situated near closed shells.
Lifetime measurements of excited states in 163W and the implications for the anomalous B(E2) ratios in transitional nuclei
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2019, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Abstract

References (6)

Nucl. Instr. and Meth.

Nucl. Instr. and Meth.

Cited by (256)

Anti-Compton shield for COMPEX germanium detector modules

Experimental observation of the M1 scissors mode in <sup>254</sup>No

Evidence against the wobbling nature of low-spin bands in <sup>135</sup>Pr

Lifetime measurements of excited states in <sup>169,171,173</sup>Os: Persistence of anomalous B(E2) ratios in transitional rare earth nuclei in the presence of a decoupled i<inf>13/2</inf> valence neutron

Lifetime measurements of excited states in <sup>163</sup>W and the implications for the anomalous B(E2) ratios in transitional nuclei

A time-of-flight correction procedure for fast-timing data of recoils with varying implantation positions at a spectrometer focal plane