Large HPGe Research Articles

A low background gamma-ray spectrometer with a large well HPGe detector

A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring.

Development of a single-chip digital radiation spectrometer based on ARM Cortex-M7 micro-controller unit

Abstract Digital signal processing is a flexible and effective approach for development of radiation spectroscopy systems. However, the conventional digital spectrometers are built using many hardware components resulted in a more complex and costly system. In this work, a complete digital spectrometer has been implemented in an ARM-based STM32F730 micro-controller. The proposed approach was benefited from a high signal to noise ratio of the trapezoidal filter to extract the particle energies. The recorded pulse height spectra using a large HPGe detector proved the good performance of the implemented system in the experiments. The proposed spectroscopy system provided an energy resolution better than 0.15% at 1332.5 keV along with a maximum nonlinearity of 0.34%, over a wide energy range. The overall results revealed that modern MCUs can be efficiently employed to develop a compact low-powered and cost-effective digital radiation spectrometer.

Hybrid-3Hen --- New Detector for Gammas and Neutrons

Recently, a unique detector system for beta-delayed neutron spectros\-copy, Hybrid-3Hen, was constructed and used in an experiment. This detector is a modification of the neutron detector 3Hen array, which uses \mbox{$^3$He-filled} tubes at 10 atm and high density polyethylene moderator. In Hybrid-3Hen the gamma-ray detection capability was implemented by\break adding two large HPGe clover-type detectors from the CLARION/CARDS array. This modification of 3Hen enabled efficient studies of beta-delayed neutron emission by using neutron--gamma coincidences. The system is equipped with full-digital readout for easy implementation of the slow and fast coincidences between various parts of the entire detection system. This new array was used in measurements at the Holifield Radioactive Ion Beam Facility and proven to be an essential tool to measure the large beta delayed two neutron emission branching ratio from the exotic isotope $^{86}$Ga.

A large HPGe detector for the non-destructive radioassay of an ultra-low-background counting facility

We present the use of a low background counting facility, equipped with a p-type 80% relative efficiency HPGe detector, protected by active and passive shielding, and large enough to count a 10 in photo-multiplier tube (PMT). A GEANT4 Monte-Carlo of this detector was developed and tuned to 3% accuracy. We report the U, Th, and K content in three different types of PMTs used in current neutrino experiments, with accuracies of ~10ppb for U and Th and of ~15ppm for K.

Measurement and energy correction due to charge carrier lifetimes in large HPGe spectrometers

This work addresses the energy spectrum correction due to increased charge carrier collection times in larger HPGe spectrometers. The energy of the radiation interaction is expected to be proportional to the total collected charge. This is increasingly not true with larger HPGe spectrometers. Some charge is lost as the total charge travels from the interaction location to the collection electrode. This path dependent loss of charge results in decreased energy resolution. In HPGe spectrometers, this process is characterized by the charge carrier lifetime constant and is given as an exponential function of the charge carrier collection time divided by this constant. Thus large detectors can experience exponential decrease in energy resolution as charge carrier collection time increases. We studied the effect of charge carrier lifetime on energy resolution for a p-type point contact HPGe spectrometers using pulse shape analysis. We present a method using the rise time to correct for the charge carrier lifetime on a pulse by pulse basis for a given HPGe spectrometer.

Standoff 3D Gamma-Ray Imaging

We present a new standoff imaging technique able to provide 3-dimensional (3D) images of gamma-ray sources distributed in the environment. Unlike standard 3D tomographic methods, this technique does not require the radioactive sources to be bounded within a predefined physical space. In the present implementation, the gamma-ray imaging system is based on two large planar HPGe double sided segmented detectors, which are used in a Compton camera configuration. A LIDAR system is used in conjunction with the gamma-ray imaging system to confine the gamma-ray image space to the interior of physical objects situated within the detection range of the gamma-ray imager. This approach results in superior image contrast and efficient image reconstruction. Results demonstrating the operating principle are reported.

A new device for combined Coulomb excitation and isomeric conversion electron spectroscopy with fast fragmentation beams

A new setup has been designed to perform Coulomb excitation experiments with fragmentation beams at intermediate energy and to measure at the same time conversion electrons from isomeric states populated in the fragmentation reaction. The newly designed setup is described and experimental results from a first experiment are shown. Radioactive even–even nuclei in the mass region A ≃ 70 close to the N = Z line were Coulomb excited after fragmentation of an intense primary 78Kr beam and selection in flight with the LISE3 spectrometer at GANIL. The γ rays emitted after Coulomb excitation were detected in an array of four large segmented HPGe clover detectors in a very close geometry. The scattered ions were identified in a stack of highly segmented annular silicon detectors combined with a time-of-flight measurement using beam tracking detectors. Conversion electrons from isomeric 0 2 + states decaying via electric monopole transitions were detected in an array of segmented cooled silicon detectors surrounding a telescope of plastic scintillators. Reduced transitions probabilities B ( E 2 ; 0 1 + → 2 1 + ) were deduced for several stable and radioactive nuclei.

High-sensitivity Compton imaging with position-sensitive Si and Ge detectors

We report on the development of high-sensitivity and compact Compton imaging systems built of large and position-sensitive Si(Li) and HPGe detectors. The primary goal of this effort is to provide improved capabilities in the passive detection of nuclear materials for homeland security. Our detectors are implemented in double-sided strip configuration, which—along with digital signal processing—provides energies and three-dimensional position information of individual γ-ray interactions. γ-Ray tracking algorithms then determine the scattering sequence of the γ-ray, which in turn allows us—employing the Compton scattering formula—to reconstruct a cone of possible incident angles and ultimately an image. This Compton imaging concept enables large-field-of-view γ-ray imaging without the use of a heavy collimator or aperture. The intrinsically high-energy resolution of the detectors used, the excellent position resolution we have demonstrated, both combined with the high efficiency of large-volume detectors is the basis for high Compton imaging sensitivity. These capabilities are being developed to identify and localize potential threat sources and to potentially increase the sensitivity in detecting weak sources out of the midst of natural, medical, or commercial sources. γ-ray imaging provides a new degree of freedom to distinguish between spatial and temporal background fluctuations and compact threat sources.

Double β decay ofMo100to excited final states

A systematic study of the inclusive ($0\ensuremath{\nu}+2\ensuremath{\nu}$) double beta ($\ensuremath{\beta}\ensuremath{\beta}$) decay of $^{100}\mathrm{Mo}$ to various excited final states of $^{100}\mathrm{Ru}$ was performed. Utilizing two large HPGe detectors operated in coincidence, a search for the subsequent deexcitation $\ensuremath{\gamma}\ensuremath{\gamma}$ cascades was conducted. A 1.05-kg sample of isotopically enriched $(98.4%)$$^{100}\mathrm{Mo}$ was investigated for 455 days, yielding an unambiguous observation of the $\ensuremath{\beta}\ensuremath{\beta}$ decay of $^{100}\mathrm{Mo}$ to the ${0}_{1}^{+}$ state (1130.3 keV) of $^{100}\mathrm{Ru}$. This excited final state decays via the ${0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{\mathrm{gs}}^{+}$ sequence (with ${E}_{\ensuremath{\gamma}1}=590.8$ keV and ${E}_{\ensuremath{\gamma}2}=539.5$ keV), and 22 such coincidence events were detected, with a continuous background estimated to be 2.5 events. This counting rate corresponds to a decay half-life for the $\ensuremath{\beta}\ensuremath{\beta}({0}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ transition of ${T}_{1/2}^{(0\ensuremath{\nu}+2\ensuremath{\nu})}=[6.{0}_{\ensuremath{-}1.1}^{+1.9}(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.6(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{20}$ years. Lower limits on decay half-lives were achieved for higher excited final states.

Development of a microtomography system at the Lund sub-micron beamline

A system for ion beam tomography has been implemented in conjunction with the new sub-micron beamline at the Lund Nuclear Microprobe laboratory. The first version of the system is dedicated to STIM (scanning transmission ion microscopy) tomography for three-dimensional mass density analysis, but future development will allow also PIXE (particle induced X-ray emission) tomography for elemental analysis of micrometer-sized samples. The PIXE tomography will gain from the 8-element large area HPGe X-ray detector (100mm2 for each element) at the low beam currents inherited with a sub-micrometer beam spot size.In this work, the first results are presented together with a detailed description of the developed system, including computer control of the sample rotation. The reconstruction technique, as well as potential tomography applications is briefly discussed.

Ultra-sensitive radionuclide spectrometry: Radiometrics and mass spectrometry synergy

Recent developments in radiometrics and mass spectrometry techniques for ultra-sensitive analysis of radionuclides in the marine environment are reviewed. In the radiometrics sector the dominant development has been the utilization of large HPGe detectors in underground laboratories with anti-cosmic or anti-Compton shielding for the analysis of short and medium-lived radionuclides in the environment. In the mass spectrometry sector, applications of inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS) for the analysis of long-lived radionuclides in the environment are the most important recent achievements. The recent developments do 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. A comparison of radiometrics and mass spectrometry results for the analysis of radionuclides in the marine environment shows a reasonable agreement - within quoted uncertainties, for wide range of activities and different sample matrices analyzed.

Characterization of a new large area HPGe X-ray detector for low beam current application

In the new sub-micron beamline at the Lund Nuclear Microprobe facility a special large area HPGe X-ray detector has been installed. It consists of eight 100-mm 2-size detector elements arranged in an annular formation around the beam entrance hole. Different parameters like FWHM, FWTM, peak-to-background ratio and peak-to-valley ratio for each individual detector element as well as their sum spectra have been investigated. The data handling, i.e. how eight parallel signals are individually stored and summed together, has been discussed. The possibility to use the detector for simultaneous measurement of high and low energy photons, i.e. PIXE combined with PIGE has been demonstrated.

Blue: a database for high-fold γ-ray coincidence data

Optimal sensitivity in large HPGe γ-ray spectrometers such as Gammasphere and Euroball is achieved with 4-fold γ-ray coincidences or higher. Analysis of coincidence data at these folds is difficult as storage constraints make histogram-based techniques impractical. To address the problem of efficient storage of and access to high-fold coincidence data we have developed Blue, a specialized database program. Each coincidence event in this database is stored in its native fold eliminating statistical artifacts which can be introduced by unfolding a high-fold event to several lower-fold events. Partition of the dataset is based on the density distribution of coincidence events which makes data access efficient. A simple yet flexible query system is implemented by using a tree-like index for these partitions. This program is capable of executing a variety of queries, such as taking gates on a multi-fold database, at timescales which are interactive (s) or near-interactive (min). Simple compression techniques are employed so that databases with sizes of 10 9 coincidences can easily be stored in a space of 10 Gb or less.

Measurement of Capture Gamma Rays from the Broad 53-keV and the Narrow 35-keV Neutron Resonances of 23Na

The capture γ rays from the broad 53-keV and the narrow 35-keV p-wave neutron resonances of 23Na were measured using a large anti-Compton HPGe γ-ray spectrometer. A neutron time-of-flight method was adopted with a 1.5-ns wide pulsed-neutron source by the 7Li(p, n)7Be reaction. The standard capture cross sections of 197Au and a pulse-height weighting technique were employed to determine the number of neutrons incident on a capture sample. The numbers of observed primary γ-ray transitions were 15 and 13 for the 53- and 35-keV resonances, respectively. The partial capture kernels and the partial radiative widths corresponding to these transitions were derived for the first time. The total radiative widths of these resonances were also derived. The present results are compared with measurements by other authors and evaluations. Moreover, a new 4,909-keV state in 24Na was found and its spin and branching ratio were determined from the analysis of the γ-ray transitions from the narrow 35-keV resonance.

Absolute measurement of 192Ir

The radioactivity of 192Ir has been measured with several different techniques. The conventional 4 πβ– γ coincidence results were confirmed by the 4 πβ– γ(Ge) spectroscopic coincidence method, in which a large HPGe detector [120% cf. 3″×3″ NaI(Tl)] was used to avoid low γ-efficiency problems. A well-type NaI(Tl) crystal was applied for the 4 πγ counting, which was very effective for such multi- γ emitting nuclides. These results were in good agreement, and those of preliminary relative measurements with ionization chambers and γ-spectrometry were also in reasonable agreement.

Application of reference materials in the accurate calibration of the detection efficiency of a low-level gamma-ray spectrometry assembly for environmental samples

A new assembly dedicated for the low-level ψ-ray spectrometric measurements of environmental samples have been installed recently at the International Atomic Energy Agency's Laboratories, Seibersdorf. Calibration of the detection efficiency was performed by a set of 14 IAEA Certified Reference Materials, a standardized solution of134Cs, solutions of 2 pure potassium salts, and 2 mixed radionuclide gamma reference standard solutions. These materials, in 1.3 liter Marinelli beakers, were measured by a large HPGe detector. The high precision and accuracy of the calibration procedure is shown by the high significance achieved in the linear fits of the efficiency results, which are traceable to certified and standard reference materials. Small biases between some calibrands were detected. A new corrected result is the 1.37±0.05 Bq/kg activity concentration of137Cs in IAEA-A-14 Milk Powder.

Confirmation of the l-forbidden Gamow-Teller decay branch of 39Ca

A search for Gamow-Teller decay branches of 39Ca to excited states in 39K was conducted with a large HPGe detector which observed the decays of 39Ca samples delivered by a helium-jet transport system. A pair of plastic scintillators in front of the HPGe detector were used to reject events due to bremsstrahlung from the 39Ca positrons. With this setup, weak γ-rays could be observed with a degree of sensitivity not previously achieved. The l- forbidden 1 d 3 2 → 2 s 1 2 transition to the first excited state in 39K, was observed with a branching ratio of (2.50 ± 0.27) × 10 −5. This result agrees with but is more accurate than the only previous measurement. Thus, the ratio of the l-forbidden Gamow-Teller and isovector M1 matrix elements in mass-39 nuclei continues to disagree with theoretical values obtained by Towner and Khanna from calculations involving core-polarization and meson-exchange currents. This leaves very few possibilities to explain the discrepancy between theory and experiment.

Performance of extrapolated leading edge timing for fast coincidence with a large-volume HPGe detector

The performance of the extrapolated leading edge type of time pickoff used with a large HPGe coaxial detector is studied. The time resolution using such a type of time pickoff has been found to be comparable with that obtained by employing the commonly used amplitude and rise-time compensation technique. For a BaF 2−HPGe (27% efficiency) coincidence setup, the values of FWHM and FWTM are 3.2 and 7.0 ns, respectively, when 60Co photopeaks are selected.

HPGe detector preamplifier output pulse simulator

The pulse simulator described here can be used to simulate the output pulses of a large HPGe detector preamplifier combination. A number of such simulators can be used to obtain pulses having a fixed time origin but with randomly varying risetime. These simulators can replace large HPGe detectors at the time of setting up of electronic modules in a multidetector fast coincidence experiment using an accelerator beam. Thus the unnecessary exposure of detectors to radiation during the preparatory phase can be avoided and the entire accelerator beam time can be used for acquisition of experimental data. The minimum and maximum values of risetime can be set to have a spread within a band of 60 to 600 ns and the pulse amplitude is adjustable from 0 to −2 V. The FW (0.01) M of the pulse height distribution is 1 mV for a pulse height of 2 V, which is much less than that of a photopeak obtained with large HPGe detectors.

Performance of an intrinsic germanium three-crystal ultralow background gamma-ray spectrometer

A γ-ray spectrometer with three large HpGe crystals totalling 560 cm 3 in one cryostat was fabricated with a view to achieving ultralow background count rates in the energy region below 4 MeV. At 2 MeV, the continuum background is 1.7 × 10 −3 counts per keV per 1000 h per cm 3, making this one of the lowest background, large systems at the present time. Only passive shielding is used. Residual activities intrinsic to the detector and shielding materials have been identified with respect to their origin from fission and cosmogenic sources.