Search

Abstract Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new high-efficiency γ -ray spectrometer designed for use in decay spectroscopy experiments with low-energy radioactive ion beams provided by TRIUMF's Isotope Separator and Accelerator (ISAC-I) facility. GRIFFIN is composed of sixteen Compton-suppressed large-volume clover-type high-purity germanium (HPGe) γ -ray detectors combined with a suite of ancillary detection systems and coupled to a custom digital data acquisition system. The infrastructure and detectors of the spectrometer as well as the performance characteristics and the analysis techniques applied to the experimental data are described. [ABSTRACT FROM AUTHOR]

Abstract: The up–down element V ud of the Cabibbo–Kobayashi–Maskawa (CKM) quark mixing matrix is derived from superallowed Fermi decays between isospin T = 1 states using the semi-empirical techniques pioneered by D.H. Wilkinson. Rather than relying directly on the absolute magnitude of theoretical nuclear structure calculations to correct each decay for isospin symmetry breaking effects, these methods extrapolate the experimental ft values to the charge independent limit at where isospin-dependent effects are negligible. The value V ud = 0.97430(29) derived in this work agrees with the value V ud = 0.97425(22) obtained from the most recent shell-model calculations of isospin symmetry breaking that have expanded the shell-model spaces by including select core orbitals. These values are not in agreement with previous shell-model calculations in more restrictive model spaces or the values obtained from recent self-consistent relativistic Hartree and Hartree–Fock calculations based on the random-phase approximation that all underestimate the magnitude of isospin symmetry breaking. [Copyright &y& Elsevier]

Abstract: A general technique that corrects -ray gated decay-curve data for detector pulse pile-up is presented. The method includes corrections for non-zero time-resolution and energy-threshold effects in addition to a special treatment of saturating events due to cosmic rays. This technique is verified through a Monte Carlo simulation and experimental data using radioactive beams of implanted at the center of the -ray spectrometer at the ISAC facility at TRIUMF in Vancouver, Canada. The -decay half-life of obtained from counting 1809-keV -ray photopeaks emitted by the daughter was determined to be following a correction for detector pulse pile-up. This result is in excellent agreement with the result of a previous measurement that employed direct counting and demonstrates the feasibility of high-precision -decay half-life measurements through the use of high-purity germanium -ray detectors. The technique presented here, while motivated by superallowed-Fermi decay studies, is general and can be used for all half-life determinations (e.g. -, -, X-ray, fission) in which a -ray photopeak is used to select the decays of a particular isotope. [Copyright &y& Elsevier]

Abstract: Methods to optimize the performance of the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS), an array of 12 large-volume, 32-fold segmented HPGe clover detectors to be used at the ISAC-II radioactive ion beam facility, have been developed based on GEANT4 Monte Carlo simulations. These methods rely on the segmentation of the outer electrical contacts of the TIGRESS HPGe clovers, and on the 20-fold segmentation of the Compton-suppression shields. The clover segmentation is utilized to make event-by-event decisions as to whether the -ray energy depositions in neighbouring crystals and clovers will be summed. The Compton suppressor segmentation is used to veto events selectively, and to reduce false suppression in experiments with high -ray multiplicity. Procedures to determine the optimal techniques and configurations of the array for a particular experiment, dependent on the expected -ray energies and multiplicities, and the velocity of the recoil ions, are presented. [Copyright &y& Elsevier]

Abstract: Methods of optimizing the performance of an array of Compton-suppressed, segmented HPGe clover detectors have been developed which rely on the physical position sensitivity of both the HPGe crystals and the Compton-suppression shields. These relatively simple analysis procedures promise to improve the precision of experiments with the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS). Suppression schemes will improve the efficiency and peak-to-total ratio of TIGRESS for high γ-ray multiplicity events by taking advantage of the 20-fold segmentation of the Compton-suppression shields, while the use of different summing schemes will improve results for a wide range of experimental conditions. The benefits of these methods are compared for many γ-ray energies and multiplicities using a GEANT4 simulation, and the optimal physical configuration of the TIGRESS array under each set of conditions is determined. [Copyright &y& Elsevier]

Abstract: Sensitivity to the locations of -ray interactions within a 32-fold segmented clover-type HPGe detector has been investigated through Compton scattering of -rays from a collimated source. A mean position sensitivity of 0.44mm at an energy of 373keV is deduced by comparing the average pulse shapes for net charge collecting signals, as well as transient induced signals in neighbouring contacts, from 1007 pairs of three-dimensionally localized interaction points. The reconstruction of individual event locations based on a comparison with the measured set of basis waveforms is presented. [Copyright &y& Elsevier]

Recent experiments with large gamma-ray detector arrays coupled to 4π charge-particle detectors have enabled the detailed spectroscopic study of highly- and super-deformed rotational bands in a number of N ≈ Z nucleic in the A ∼ 40 mass region. In the context of the Nilsson scheme, these rotational structures are associated with the shell gaps at large prolate deformation for particle numbers N, Z = 16, 18, 20, while in the spherical shell model context they are described in terms of multi-particle multi-hole excitations across the N, Z = 20 spherical shell gap. In analogy with rotational bands in heavy nuclei, these highly collective structures involve active valence particles in two major shells for both protons and neutrons, yet their valence space dimension remains small enough to be approached from a shell-model perspective. These rotational bands thus provide an ideal opportunity for detailed comparison between experimental data and both microscopic and macroscopic models of collective motion in nuclei. [Copyright &y& Elsevier]

High-resolution γ-ray spectroscopy is one of the most powerful and versatile experimental techniques in low-energy nuclear physics research. With the continuing development of hyper-pure germanium (HPGe) detector technology, including multi-crystal detectors, contact segmentation, and digital signal processing techniques, large γ-ray detector arrays will continue to play a major role in the experimental programs at existing and future radioactive ion beam facilities. This paper provides an overview of recent progress in, and future plans for, the development of large γ-ray spectrometers at such facilities, including the recent commissioning of the 8π spectrometer at ISAC-I and the proposed TRIUMF-ISAC gamma-ray escape suppressed spectrometer array for the ISAC-II facility. [Copyright &y& Elsevier]

Abstract Gamma–gamma angular correlation measurements are a powerful tool for identifying the angular momentum (spin) of excited nuclear states involved in a γ -ray cascade, and for measuring the multipole orders and mixing ratios of transitions. Though the physical angular correlations are fully calculable from first principles, experimental effects can make the extraction of coefficients and thus conclusions about spins and mixing ratios difficult. In this article we present data analysis techniques developed for the clover detectors of the GRIFFIN spectrometer at TRIUMF-ISAC combined with GEANT4 simulations in order to extract accurate experimental results. [ABSTRACT FROM AUTHOR]

Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new experimental facility for radioactive decay studies at the TRIUMF-ISAC laboratory. This article describes the details of the custom designed GRIFFIN digital data acquisition system. The features of the system that will enable high-precision half-life and branching ratio measurements with levels of uncertainty better than 0.05% are described. The system has demonstrated the ability to effectively collect signals from High-purity germanium crystals at counting rates up to 50 kHz while maintaining good energy resolution, detection efficiency and spectral quality. [ABSTRACT FROM AUTHOR]

The Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new experimental facility for radioactive decay studies at the TRIUMF-ISAC laboratory. The performance of the 16 high-purity germanium (HPGe) clover detectors that will make up the GRIFFIN spectrometer is reported. The energy resolution, efficiency, timing resolution, crosstalk and preamplifier properties of each crystal were measured using a combination of analog and digital data acquisition techniques. The absolute efficiency and add-back factors are determined for the energy range of 80–3450 keV. The detectors show excellent performance with an average over all 64 crystals of a FWHM energy resolution of 1.89(6) keV and relative efficiency with respect to a 3 in . × 3 in . NaI detector of 41(1)% at 1.3 MeV. [ABSTRACT FROM AUTHOR]

Abstract: The SPICE spectrometer is a new in-beam electron spectrometer designed to operate in conjunction with the TIGRESS HPGe Clover array at TRIUMF-ISAC. The spectrometer consists of a large area, annular, segmented lithium-drifted silicon electron detector shielded from the target by a photon shield. A permanent magnetic lens directs electrons around the photon shield to the detector. Experiments will be performed utilising Coulomb excitation, inelastic-scattering, transfer and fusion–evaporation reactions using stable and radioactive ion beams with suitable heavy-ion detection. Good detection efficiency can be achieved in a large energy range up to 3500keV electron energy using several magnetic lens designs which are quickly interchangeable. COMSOL and Geant4 simulations have been used to maximise the detection efficiency. In addition, the simulations have guided the design of components to minimise the contributions from various sources of backgrounds. [Copyright &y& Elsevier]

Abstract: The wave-function composition for the low-lying states in 29Na was explored by measuring their electromagnetic properties using the Coulomb-excitation technique. A beam of 29Na ions, postaccelerated to 70 MeV, bombarded a 110Pd target with a rate of up to 600 particles per second at the recently commissioned ISAC-II facility at TRIUMF. Six segmented HPGe clover detectors of the TIGRESS γ-ray spectrometer were used to detect deexcitation γ rays in coincidence with scattered or recoiling charged particles in the segmented silicon detector, BAMBINO. The reduced transition matrix element in 29Na was derived to be 0.237(21) e b from the measured γ-ray yields for both projectile and target. This first-time measured value is consistent with the most recent Monte Carlo shell-model calculation, indicating a significant admixture of both sd and pf components in the wave function, and also providing evidence for the narrowing of the neutron shell gap from for stable nuclei to for 29Na. [Copyright &y& Elsevier]

Abstract: Tests of the performance of a 32-fold segmented HPGe clover detector coupled to a 20-fold segmented Compton-suppression shield, which form a prototype element of the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS), have been made. Peak-to-total ratios and relative efficiencies have been measured for a variety of -ray energies. These measurements were used to validate a GEANT4 simulation of the TIGRESS detectors, which was then used to create a simulation of the full 12-detector array. Predictions of the expected performance of TIGRESS are presented. These predictions indicate that TIGRESS will be capable, for single 1MeV rays, of absolute detection efficiencies of 17% and 9.4%, and peak-to-total ratios of 54% and 61% for the “high-efficiency” and “optimized peak-to-total” configurations of the array, respectively. [Copyright &y& Elsevier]

Abstract: The diffusion-limited release of radioactive 120Xe from several thin metal foils abruptly heated to temperatures of order 1000°C while in a vacuum is characterized and interpreted in terms of impurity atom diffusion coefficients. The 120Xe impurities were produced in situ from the decay of 120Cs implanted into Ta, Pt, and Zr foils from a 30keV mass-separated ion beam. The temperature dependence of the diffusion coefficients extracted from our data is well described by Arrhenius relations, for which best-fit parameters are presented. The fastest release of 120Xe for a given set of implantation conditions and temperature is observed for Zr, although its reactivity may prove problematic for its use as a catcher foil. A number of convenient analytic formulae for the diffusion-limited release of impurity atoms are presented. [Copyright &y& Elsevier]

Abstract: Techniques have been developed for on-line collection and transfer to a measurement cell of rare noble gas isotopes. A low-energy isotope separator beam is stopped in a foil. After collection, the foil is heated, and the noble gas atoms are cryopumped to a cold finger and then transferred with a buffer gas to a measurement cell. This has been tested with 120Xe produced from a 120Cs beam at the TRIUMF ISAC facility, and greater than 40% efficiency for transfer from the foil to the cell has been demonstrated. The technique will be used for electric dipole moment measurements with radon isotopes and may also be useful for other measurements with gaseous targets including laser spectroscopy. [Copyright &y& Elsevier]