Compton Suppression Shield Research Articles

Performance testing of a Compton suppressed coincidence measurements using the Advanced Radionuclide Gamma-spectrOmeter

A Compton-suppressed multidimensional gamma-spectrometer is being developed within the Shallow Underground Laboratory at Pacific Northwest National Laboratory (PNNL, USA). This Advanced Radionuclide Gamma-spectrOmeter system consists of two Broad Energy Germanium detectors, surrounded by a 12-detector sodium iodide Compton suppression shield, situated inside a low-background lead and copper shield, fitted with a cosmic veto background reduction system. The detector has advanced functionality, including operation in single or combined detector mode, coincidence mode and Compton suppressed coincidence mode. Performance testing experiments measuring a neutron irradiated natural uranium sample, have demonstrated order of magnitude improvements in gamma detection sensitivity compared to conventional gamma-spectrometry techniques. Importantly this research demonstrates the feasibility of processing and analysing high-multiplicity datasets and the potential for high-detection sensitivity radionuclide measurements.

Lessons learned with the SAGE spectrometer

The SAGE spectrometer combines a high-efficiency γ-ray detection system with an electron spectrometer. Some of the design features have been known to be problematic and surprises have come up during the early implementation of the spectrometer. Tests related to bismuth germanate Compton-suppression shields, electron detection efficiency and an improved cooling system are discussed in the paper.

AGATA and GRETA: The Future of Gamma-Ray Spectroscopy

High precision gamma-ray spectroscopy is one of the powerful tools used to study the structure of excited nuclear states and it has contributed to the discovery of a wide range of new phenomena. Each major advance in gamma-ray detection has resulted in significant new insights into the structure of nuclei. To date, these advances have culminated in two state-of-the-art 4π arrays of escape-suppressed spectrometers, Euroball in Europe [1] and Gammasphere in the United States [2]. These arrays, which consist of more than a 100 large volume germanium crystals each surrounded by a Compton-suppression shield, have pushed this particular detector technology to its limit, with an efficiency for a 1 MeV gamma ray of about 10% and a peak-to-total ratio of 0.5.

Experimental test of the background rejection, through imaging capability, of a highly segmented AGATA germanium detector

The development of highly segmented germanium detectors as well as the algorithms to identify the position of the interaction within the crystal opens the possibility to locate the γ -ray source using Compton imaging algorithms. While the Compton-suppression shield, coupled to the germanium detector in conventional arrays, works also as an active filter against the γ rays originated outside the target, the new generation of position sensitive γ -ray detector arrays has to fully rely on tracking capabilities for this purpose. In specific experimental conditions, as the ones foreseen at radioactive beam facilities, the ability to discriminate background radiation improves the sensitivity of the gamma spectrometer. In this work we present the results of a measurement performed at the Laboratori Nazionali di Legnaro (LNL) aiming the evaluation of the AGATA detector capabilities to discriminate the origin of the γ rays on an event-by-event basis. It will be shown that, exploiting the Compton scattering formula, it is possible to track back γ rays coming from different positions, assigning them to specific emitting locations. These imaging capabilities are quantified for a single crystal AGATA detector.

Compton-suppression and add-back techniques for the highly segmented TIGRESS HPGe clover detector array

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 γ -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 γ -ray multiplicity. Procedures to determine the optimal techniques and configurations of the array for a particular experiment, dependent on the expected γ -ray energies and multiplicities, and the velocity of the recoil ions, are presented.

Optimization of Compton-suppression and summing schemes for the TIGRESS HPGe detector array

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.

Measured and simulated performance of Compton-suppressed TIGRESS HPGe clover detectors

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 γ -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 1 MeV γ 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.

TIGRESS: TRIUMF-ISAC gamma-ray escape-suppressed spectrometer

The TRIUMF-ISAC gamma-ray escape-suppressed spectrometer (TIGRESS) is a new γ-ray detector array being developed for use at TRIUMF's Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. TIGRESS will comprise 12 32-fold segmented clover-type HPGe detectors coupled with 20-fold segmented modular Compton suppression shields and custom digital signal processing electronics. This paper provides an overview of the TIGRESS project and progress in its development to date.

Study of Aerosol Deposition at a Wind Exposed Forest Edge Using210Pb and137Cs Soil Inventories

The radionuclide 210Pb derived from gaseous 222Rn present in the atmosphere becomes attached to the same aerosols as the bulk of the main pollutants sulphur and nitrogen. When scavenged from the atmosphere by precipitation, the 210Pb is readily attached to organic matter in the surface horizons of the soil. Inventories of 210Pb in soil can thus be used to measure the spatial variations in wet (or cloud) deposition due to orography averaged over many precipitation events (half-life of 210Pb is 22.3 yr). Similar arguments apply to 137Cs inventories (half-life 30.0 yr), although more care has to be taken in interpreting the results due to possible resuspension and migration of the 137Cs. The method has been applied to quantify the long-term enhancement in aerosol and cloud water (occult) deposition as a consequence of increased aerodynamic roughness at an exposed forest canopy in the Highlands of Scotland. The specific activities of 210Pb and 137Cs in dried soil samples were determined by γ-spectrometry using HpGe detectors, one of which was fitted with a Compton suppression shield. Measurements revealed an average canopy enhancement in deposition of 36 ± 12% relative to the open heathland, in agreement with independent measurements of occult deposition at similar sites and model estimates. Deposition at the exposed edge exceeds that in the open by ≳50%, and thus provides evidence of a pronounced edge effect.

A probabilistic γ-ray tracking method for germanium detectors

A new γ-tracking method based on the Compton-scattering probability of photons in a spectrometer composed of segmented Ge detectors has been developed and evaluated with the GEANT 3 simulation code. The performance of this probabilistic method, in terms of photopeak detection efficiency and peak-to-total ratio calculated for a 4π array composed of planar detectors, is comparable with the ones obtained with the previous γ-tracking methods. Furthermore, the algorithm has been tested successfully with single detection units of other crystal shapes. A simple application is also proposed as a peak-to-total ratio enhancer for bare segmented Ge crystals in replacement of the Compton-suppression shield.

An experimental method for the optimization of anti-Compton spectrometer

Abstract The reduction of the Compton continuum can be achieved using a Compton suppression shield. For the first time, an experimental method is purposed for estimating the optimum dimensions of such a shield. The method can also provide information on the effect of the air gap, source geometry, gamma-ray energy, etc., on the optimum dimension of the active shield. The method employs the measurements of the Compton suppression efficiency in two dimensions using small size scintillation detectors. Two types of scintillation materials; NaI(Tl) and NE-102A plastic scintillators, were examined. The effect of γ-ray energy and source geometry were also investigated using 137 Cs and 60 Co sources with three geometries; point, cylindrical, and Marinelli shapes. The results indicate the importance of both NaI(Tl) and NE-102A guard detectors in surrounding the main detector rather than the distance above it. The ratio between the part of the guard detector above the surface of the main detector to that below it should be about 1:3 for NaI and 1:4 for NE-102A. The optimum dimensions for NaI(Tl) detector were found to be 12.5–20 cm for the height and 15–20 cm for the thickness. The corresponding values for NE-102A are 17.5–25 cm for the height and 25–30 cm for the thickness.

Portable LN 2 gamma-ray spectrometer system

The purpose of this program is to design and demonstrate a highly sensitive, portable spectrometer for detecting and identifying trace amounts of nuclear materials in a broad range of nonproliferation scenarios. A high-efficiency 100% n-type germanium (Ge) detector is combined with a special liquid nitrogen dewar designed to accommodate an active Compton suppression shield. The added collimation capability is required in order to be able to identify localized sources that may be obscured by background radiation. These components are complemented by the addition of a newly developed state-of-the-art multichannel analyzer (MCA) capable of 64×16 000 channel acquisition and electronic Compton suppression. The entire system fits into a watertight case that could be stored below the seat of commercial airliners. The unit has 2.74 keV resolution at 1.33 MeV, 8.5° angular resolution, low-power, >25 h full-time battery life, and operations between 35 keV< E<3 MeV.

Detectors and detection limits in INAA

Detection limit improvement factors have been calculated to predict the improvement in detection limits when replacing a traditional 20% coaxial detector by either a 100% coaxial detector, a well-type detector or by equipping the detector with a Compton suppression shield. It is show that, when measuring sources on the end-cap of the detector, a system with a Compton suppression shield and a 100% detector have almost equal potentials. But when e.g. geometrical consideration larger source-to-end-cap distances are required with coaxial detectors, the well-type detector offers the best outlook for attaining better detection limits. The calculated predictions have been compared with experimental observations and the results agree well.

Test and performance of a BGO Compton-suppression shield for GAMMASPHERE

Bismuth germanate (BGO) Compton-suppresion shields have been constructed to surround the Ge detectors of the GAMMASPHERE array. A shield consists of six hexagonal tapered BGO elements, each coupled to two 1-in. × 1-in. photomultiplier tubes. In addition, a cylindrical BGO detector is placed behind the Ge detector to intercept the forward scattered γ-rays. One hundred ten such shields are planned for the GAMMASPHERE array. Procedures for measuring the performance of these shields have been developed. Large (70%) Ge detectors when used with these shields give a peak-to-total ratio of better than 0.60. To date 85 shields have been tested and approved for use in GAMMASPHERE.

Low-spin states of 66Zn excited in the decay

Abstract Gamma-rays from the 66 Ga (β + + e) 66 Zn decay have been investigated with a Ge(Li) spectrometer in a Compton-suppression shield. In preceding work the energies of 34 strong lines had been obtained with very high precision. By internal calibration the energies of many other lines observed in the present work, and thus of most excitation energies, could also be measured with good accuracy. The 141 observed transitions have been placed in a 66 Zn level scheme containing 33 states. Additional new information has been obtained on γ-ray branching ratios, log ft values, spins and parities.

Precision energy determination of gamma-rays from the 66Ga( β++ ϵ) 66Zn decay

The energies of 29 γ-rays from the 66Ga( β + + ϵ) 66Zn decay have been measured with Ge(Li) detectors, of which one in a Compton suppression shield. Energy calibration standards were provided by 46Sc, 56Co, 65Zn, 110Ag m and 124Sb radioactive sources. The resulting excitation energies of 18 66Zn levels, obtained in a least-squares fit, then provide the energies for 26 strong lines ( E γ = 0.83−4.81 MeV) with a final precision of better than 10 ppm. Of particular importance are the five E γ = 3.76−4.81 MeV lines, which now can serve as calibration standards in this hitherto uncovered high-energy region.

The Pittsburgh-Florida State Universities γ detection array

A γ detector array for studying high-spin structure in nuclei has been constructed by combining detectors from two universities. The system contains 11 high-resolution Ge detectors with BGO Compton-suppression shields and 28 low-resolution BGO scintillators to provide multiplicity and sum energy information. A unique feature of the array is the heterogeneous nature of the detectors which were not originally purchased for such a system.

Compton-suppression tests on Ge and BGO prototype detectors for GAMMASPHERE

Measurements are described of the Compton suppression of two prototype detectors for GAMMASPHERE. The prototypes consist of a large (75% efficient) n-type germanium detector with a BGO Compton-suppression shield. A peak-to-total ration of 0.678(2) is achieved with a 60Co source. This is the best ratio reported to date for a detector of this kind. The results are compared with those obtained from Monte Carlo simulations. The simulations can predict the observed peak-to-total ratios if a significant background from scattered photons is included.

Compton-suppression spectrometer for in-beam coincidence measurements and proposal for a detector configuration consisting of systems with a large solid angle

A γ-γ coincidence apparatus consisting of a Compton-suppression spectrometer with a large solid angle of 54 msr and a single Ge detector is described, and examples of experimental results obtained in in-beam spectroscopy of heavy-ion induced reactions are presented. Furthermore, a γ-γ coincidence Compton-suppression spectrometer consisting of three veto systems for three Ge-detectors is described. By using one integrated Compton-suppression shield consisting of a NaI(Tl) detector optimally divided into four parts for the Ge detectors, an extraordinarily large solid angle of 100 msr is obtained for each of Compton-suppression spectrometer. At the same time the total solid angle of the NaI(Tl) veto shield is minimized, which is the central problem when measuring γ cascades of high multiplicity.

The 25Mg(p, γ)26Al reaction; branchings, energies and lifetimes

The γ-decay has been measured of 75 25Mg(p, γ)26Al resonances in the Ep = 0.31–1.84 MeV region with Ge detectors at 90° and 55°, the latter in a Compton suppression shield. Several of these resonances had not been observed previously and of about half the decay had not been measured. The spectra have been corrected for summing effects, for the instrumental tails of lower-energy resonances, for the Breit-Wigner tails of neighbouring broad resonances, and for direct capture. On the average about 29 primaries are observed per resonance (with a maximum of 48); 74 primaries are of the resonance → resonance type. Altogether 109 secondary states are excited in the decay, of which 50 had been seen in previous (p, γ) work. The total number of observed decay γ-rays from non-resonance levels amounts to 548, with a maximum number of branches per level of 20. Accurate energies have been measured for all states. The errors amount to ΔEx = 0.03–0.26 keV for the non-resonance levels and to 0.04–0.4 keV for the resonances (with ΔEx = 2 keV for four very broad resonances). As a byproduct lifetimes (or lifetime limits) have been measured from the observed Doppler shifts for 32 levels for which no previous measurements existed.