Large Volume Ge Research Articles

New ultra-sensitive radioanalytical technologies for new science

Recent developments in radiometric and mass spectrometry technologies have been associated in the radiometric sector mainly with underground operations of large volume Ge detectors, while the mass-spectrometry sector, represented mainly by accelerator mass spectrometry and inductively coupled plasma mass spectrometry has become the most sensitive technique for ultra-low-level analyses of long-lived radionuclides. These new developments have had great impact on investigations of rare nuclear processes and applications of radionuclides in environmental, life and space sciences. New scientific investigations have been carried out therefore which have not been possible before either because of lack of sensitivity or required large sample size.

The Kaguya gamma-ray spectrometer: instrumentation and in-flight performances

A Gamma-Ray Spectrometer (GRS) had been developed as a part of the science payload for the first Japanese lunar explorer, Kaguya. The Kaguya was successfully launched from Tanegashima Space Center on September 14, 2007 and was injected into an orbit around the Moon and the mission ended on June 11, 2009. The Kaguya GRS (hereafter KGRS) has a large-volume Ge semiconductor detector of 252 cc as the main detector and bismuth-germanate and plastic scintillators as an active shielding. The Ge detector achieved an energy resolution of 3.0 keV (FWHM) for 1332 keV gamma ray in ground test despite the use of a mechanical cryocooler and observed gamma rays in energies ranging 0.2 to 12 MeV in lunar orbit. It was the first use of a Ge detector for lunar exploration. During the mission, KGRS participated in geochemical survey and investigated the elemental compositions of subsurface materials of the Moon. In this paper, we summarize the overview of the KGRS describing the design and in-flight performance of the instrument. This paper provides basic information required for reading science articles regarding the KGRS's observation data.

Trends in radiometrics and mass spectrometry technologies: synergy in environmental analyses

The dominant development in the radiometrics techniques for the analysis of short and medium-lived radionuclides in the environment was the utilisation of large volume Ge detectors in underground laboratories with additional anti-cosmic shielding. In the mass spectrometry sector, applications of Accelerator Mass Spectrometry (AMS) and Inductively Coupled Plasma Mass Spectrometry (ICPMS) for the analysis of long-lived radionuclides in the environment are the most important recent achievements. These developments in both sectors did not only considerably decrease the detection limits for several radionuclides (up to several orders of magnitude), but they also enable to decrease sample volumes so that sampling, e.g. of the water column, can be much easier and more effective. Applications of radiometrics and mass spectrometry techniques in isotope oceanography, specifically on the distribution of 3H, 14C, 90Sr, 129I, 137Cs, 239Pu and 240Pu in the water column of the North Pacific and South Indian Oceans are presented and discussed.

Monte Carlo simulation of background characteristics of low-level gamma-spectrometers

Large volume Ge detectors require efficient methods of background reduction if radionuclide analyses at very low-levels are planned. It is advisable therefore to carry out simulations of background characteristics of Ge detectors in advance of installations of low-level Ge-spectrometers either in surface or underground laboratories. We present results of Monte Carlo simulations of background characteristics of Ge detectors in surface laboratories with various lead shielding without and with anticosmic shielding, as well as in underground laboratories operating at different depths.

AstrophysicalSfactor forαcapture onSn117

The cross sections of the $^{117}\mathrm{Sn}$($\ensuremath{\alpha},\ensuremath{\gamma}$) $^{121}\mathrm{Te}$ and $^{117}\mathrm{Sn}$($\ensuremath{\alpha},p$) $^{120}\mathrm{Sb}$ reactions have been measured in the effective center of mass energy from 11.5 to 14.6 MeV. Highly enriched self-supporting $^{117}\mathrm{Sn}$ (90%) foils were bombarded with an $\ensuremath{\alpha}$ beam delivered by the Bucharest IFIN-HH tandem accelerator. The induced activity of $^{121}\mathrm{Te}$ and $^{120}\mathrm{Sb}$ was measured with two large-volume high-purity Ge detectors in close geometry to maximize the detector efficiency. The experimental cross section and astrophysical $S$ factor are compared with statistical model predictions for different global $\ensuremath{\alpha}$-nucleus optical potentials.

Scintillators on Interplanetary Space Missions

Lanthanum halide scintillators are currently being assessed by ESA for use as remote sensing gamma-ray spectrometers on future planetary missions. Such missions place a raft of constraints and requirements on sensor technology, which invariably stem from environmental and resource issues. In the past, NaI(Tl) and CsI(Tl) based spectrometers offered great advantages in term of device simplicity, reliability and detection efficiency but with poor energy resolution. Lanthanum halides combine these attributes with much higher energy resolution and their extensive deployment in future missions seems assured. In fact, LaBr spectrometers are currently baselined for most missions in planning. In this paper, we describe the needs of remote gamma-ray sensing and efforts by the European Space Agency to develop and qualify the next generation of spectrometers based on LaBr for interplanetary space missions. We contrast their performance to large volume Ge detectors and show that in the context of minimally resourced spacecraft, they represent a more attractive solution.

AstrophysicalSfactor for α-capture onSn112in thep-process energy range

The cross section of the reaction $^{112}\mathrm{Sn}$($\ensuremath{\alpha},\ensuremath{\gamma})^{116}\mathrm{Te}$ has been measured in the energy range of astrophysical interest for the $p$-process. Highly enriched self-supporting $^{112}\mathrm{Sn}$ foils were bombarded with \ensuremath{\alpha} beams in the effective center of mass energy range from 7.59 to 11.42 MeV at the Notre Dame FN Tandem Van de Graaff accelerator. The characteristic activity of $^{116}\mathrm{Te}$ was counted with a pair of large volume Ge clover detectors in close geometry to maximize the detection efficiency. The cross section of the concurrent ($\ensuremath{\alpha},p$) reaction has also been measured. The results are compared with statistical model predictions for different global \ensuremath{\alpha}-nucleus potentials.

Feasibility and design of a solid state gamma-ray detector

For conventional radiation detectors fabricated from compound semi-conductors, the wide disparity between the transport properties of the electron and holes, means that detector performances are limited by the carrier with the poorest mobility-lifetime product (μτ). Finite drift lengths introduce an energy dependent depth term into the charge collection process, which effectively limit maximum detection volume to tens of mm3 – entirely unsuitable for the detection of gamma-rays. The recent introduction of the coplanar-grid charge-sensing techniques has overcome this problem by essentially discarding the carrier with the poorest transport properties, thus permitting high spectral resolution and high detection efficiency. For example, energy resolutions of 2% full-width half-maximum at 662 keV have been demonstrated with coplanar-grid CdZnTe detectors of volumes up to 2 cm3. Further improvements in detector performance and yield are being pursued through refinements in electrode design and material quality. Because coplanar-grid CdZnTe detectors can operate at room temperature, they are ideally suited for applications requiring portability, small size, or low power consumption such as planetary space missions. Other potential applications include well logging, medical diagnostics, and gamma-ray astronomy. We discuss the feasibility and design of a solid state gamma-ray detector based on CdZnTe and compare its performance to a large volume Ge detector. As will be shown, a significant improvement can be made if T1Br is used as the detection medium.

A study of alpha capture cross-sections of 112Sn

The 112Sn(α, γ)116Te reaction cross-section has been measured to test the applicability of statistical models, especially NON-SMOKER in the energy range of importance for the astrophysical p-process nucleosynthesis. The measurements were carried out at the Notre Dame FN Tandem Van de Graaff accelerator by means of the activation method. Enriched self-supporting foils were irradiated with alpha beams over the alpha bombarding energy range of 8MeV to 12MeV in 0.5MeV steps. The induced activity was measured with a pair of large volume Ge Clover detectors in close geometry to maximize the detection efficiency. The preliminary results are compared with recent statistical model predictions using the code NON-SMOKER.

Efficiency calibration of a Ge detector in the 0.1–11.0 MeV region

An efficiency curve was constructed for a large-volume Ge detector located at the thermal-neutron beam line of the JRR-3M reactor at the Japan Atomic Energy Research Institute (JAERI). The calibrations for γ-ray energies below 2754 keV were accomplished with a large number of radioactive sources and above 2754 keV with the 12C(n,γ) and 14N(n,γ) reactions. This detector and its efficiency curve were subsequently used to determine accurately the photon emission probabilities of γ rays from the decays of 56 Co (T 1/2=77 d) and 66 Ga (9.5 h) and from the 35Cl(n,γ) reaction. These radiation sources can serve as secondary standards for efficiency calibrations.

βdecayingT1/2=3.4s isomer in69Ni

A radioactive beam of ${}^{69}\mathrm{Ni}$ was produced by the fragmentation of a 70 MeV/nucleon ${}^{76}\mathrm{Ge}$ beam in a Be target. $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray studies were performed using two thin plastic scintillators and two large-volume Ge detectors following implantation of the ${}^{69}\mathrm{Ni}$ nuclei into a foil of a collection wheel apparatus. A 1296-keV $\ensuremath{\gamma}$-ray transition with a half-life of 3.4(7) s was identified and has been attributed to the decay of the $\ensuremath{\nu}{p}_{1/2}^{\ensuremath{-}1}$ isomeric state in ${}^{69}\mathrm{Ni}.$ The relative population of the low-spin ${J}^{\ensuremath{\pi}}{=1/2}^{\ensuremath{-}}$ isomer to the known high-spin ${J}^{\ensuremath{\pi}}{=(17/2}^{\ensuremath{-}})$ isomer was determined to be 6:1 for the production of ${}^{69}\mathrm{Ni}$ via fragmentation of ${}^{76}\mathrm{Ge},$ based on an upper limit of 36% extracted for the ${}^{69}{\mathrm{Ni}}^{m1}$ $\ensuremath{\beta}$-decay branch to the ${3/2}^{\ensuremath{-}}$ ground state of ${}^{69}\mathrm{Cu}.$ The half-life and branching of the ${}^{69}{\mathrm{Ni}}^{m1}$ $\ensuremath{\beta}$ decay is discussed in light of possible two particle--two hole excitations in the low-energy structure of ${}^{69}\mathrm{Cu}.$

Development of new expanded editions of the Gamma-ray Spectrum Catalogues in electronic format

A program has been developed at the INEL to produce new enhanced editions of the original Gamma-ray Spectrum Catalogues for presentation in electronic format. This program has produced the first version of the Catalogues in CD-ROM format and will offer them on the Internet through the Gamma-ray Spectrometry Center Web Site at INEEL. The original content of the Catalogues has been expanded to include integrated decay scheme drawings, tables of related decay data, and updated material on the techniques of gamma-ray spectrometry. All of the original spectral plots, gamma-ray energy and intensity data, and text were converted to digital format, related decay data from the Evaluated Nuclear Structure Data File (ENSDF) and decay scheme graphics were then added, and all data converted to the Adobe Acrobat (PDF) format. Follow-on versions of the catalogues are being expanded to include prompt neutron capture and n-n' spectra and spectra representing the response of large-volume Ge detectors. Large-volume Ge detector spectra will be calculated, using Monte Carlo and parametric fitting techniques presently under development. Examples of content and details of the advanced technology applied to produce these new volumes will be presented. Services and content of the Gamma-ray Spectrometry Center Web Site will also be described.

Temperature and vacuum measurements on the commercial assembly of a large-volume Ge detector

This work reports on tests which have been extensively made on the assembly of a “dummy” large-volume germanium detector with the aim of measuring temperature gradients along the crystal, as well as in different parts of the cryostat, during the steady cold, room temperature and annealing phases and in the transitions between them. Measurements of the vacuum level and trends of the residual gas composition in the cryostat in the different phases of operation are also reported.

New experimental access to the two-phonon octupole vibration in 208Pb

The feasibility of a new experimental method to identify two-phonon octupole vibrations is investigated for the example of 208Pb. The 2 + member of the 3 I − ⊗ 3 I − multiplet can be excited with the (γ, γ′) reaction and a sizable E1 decay to the 3 I − level serves as a signature of the two-phonon character. Numerical estimates within the quasiparticle-phonon model indicate that such an experiment should be possible with state-of-the-art large volume Ge detectors like the EUROBALL cluster module.

Parity violation in the γ-decay of polarized 93Tc nuclei in the [formula omitted] isomeric state

We report on a determination of the parity nonconserving (PNC) matrix element in the bound parity doublet 17 − 2 − 17 + 2 of 93Tc. The experiment was carried out at the GSI, Darmstadt and LNL, Legnaro laboratories. The recoil-mass-separated radioactive beam of 93Tc nuclei in the 17 − 2 isomer, following a fusion-evaporation reaction, was polarized by the tilted-foil method and the resulting 0°–180° γ asymmetry with respect to the induced polarization direction was measured by two large-volume Ge detectors. The measured γ asymmetry of 3-σ significance, A γ = 8.4(2.7) · 10 −4, yields a matrix element of |〈 17 − 2 |H pnc | 17 + 2 〉| = 0.59(19)(25) meV . This experimental result is c microscopic calculations based on the DDH “best value” interaction for the nuclear weak Hamiltonian. We discuss our results and their significance with respect to the existing data regarding PNC effects in bound nuclear systems.

Two proton high-spin excitations and dipole bands in192Hg

The192Hg nucleus was populated in the160Gd(36S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of192Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ħ. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i13/2 * h9/2)K=11 andπ (h9/2) K=8 2 , coupled to rotation-aligned quasi-neutron ν(i13/2)n and quasi-proton π(h11/2)2 excitations.

Identification and structure of high spin particle-hole states in89Mo

Particle-gammagamma and gammagamma-coincidences of the reaction Ni-58(Ar-36, 4 pn)89Mo have been used to gain more information about high spin states in 89Mo and to establish the yrast sequence up to 7.6 MeV excitation energy and probable spin 37/2 h. Spins and parities were assigned on the basis of DCO-ratios measured with the OSIRIS spectrometer and a large volume Ge detector placed at 162-degrees to the beam. Furthermore, a gamma-ray angular distribution experiment was carried out using the reaction Ni-58(Cl-35, 3 pn)89Mo. As in the neighboring isotopes 88Mo and Mo-90, the positive-parity high-spin states can be grouped into shell model multiplets characterized by increasing seniorities of proton particles and neutron holes in the 1 g9/2 shell. The negative-parity states can be explained with one nucleon moving in the P1/2 orbit. The energies and wave functions of these states have been deduced by means of the shell model code RITSSCHIL. The 2584 keV (21/2+) is an isomeric state the mean life of which has been estimated from delayed gammagamma-coincidences. (Less)

Rejecting escape events in large volume Ge detectors by a pulse shape selection procedure

The dependence of the response to γ-rays of a large volume Ge detector on the interval width of a selected initial rise pulse slope is investigated. The number of escape events associated with a small pulse slope is found to be greater than the corresponding number of full energy events. An escape event rejection procedure based on the observed correlation between energy deposition and pulse shape is discussed. Such a procedure seems particularly suited for the design of highly granular large volume Ge detector arrays.

The gammasphere

GAMMASPHERE is one of a new generation of gamma ray detector arrays. It consists of 110 Compton-suppressed large volume Ge detectors. The design goal is to achieve high efficiency and peak-to-total value for four to five fold coincidence experiments. Such high-fold coincidence capability will provide new physics opportunities in areas such as high spins, transfer reactions, giant resonances, and astrophysics. The design of the detector and shield has been developed through extensive simulation calculations and an “electronic honeycomb” design was chosen. The electronics and computer systems are capable of operating at 50,000 event/sec. The design and development tasks are being carried out at several laboratories in the U.S. GAMMASPHERE will be sited initially at the 88-Inch Cyclotron at LBL as a national facility. The first experiment is planned in early 1993.

Ballistic deficit correction methods for large Ge detectors

A comparative study of different ballistic deficit correction methods is carried out using three large volume Ge detectors of about 70% efficiency. The present work shows that for spectroscopy systems working with short shaping time constants the gated integrator is superior as it ensures the best energy resolution performance for both high and low γ-ray energies. For longer shaping time constants ( ≥ 2 μs) the Ortec 675 Ge resolution enhancer (the Ortec extension of the Goulding/Landis method) gives the best energy resolution due to a partial compensation of carrier trapping. It is shown that the gated integrator reduces the noise level in comparison to common linear amplifiers with Gaussian shaping. No evidence for its special sensitivity to low frequency noise, at least at short shaping time constants, has been observed. The compensation of ballistic deficit allowed us to determine the intrinsic energy resolution of large volume Ge detectors and to deduce their effective Fano factor.