Challenges and techniques to effectively characterize the efficiency of broad-energy germanium detectors at energies less than 45 keV

With the now common availability of large-volume thin-window germanium detectors, it is possible to routinely measure very low energy (<45 keV) gamma and X-rays while maintaining good sensitivity for high-energy gamma rays. The effective calibration of such detectors down to these low energies is often problematic or not possible because of the lack of calibrated sources or knowledge of the source geometry. New methods have been recently developed that extend Canberra’s ISOCS/LabSOCS mathematical efficiency computation methods down to energies as low as 10 keV. Key to these developments is the capability to characterize the efficiency versus spatial location of a detector at the factory and thus eliminate the requirement to have “in the field” low-energy source standards. In this paper, the challenges for performing reliable efficiency characterizations below 45 keV and techniques developed to overcome these challenges are discussed. Response characterization results are presented for various types of low-energy and broad-energy detectors manufactured by Canberra.     

Evaluation of efficiency for in situ gamma spectrometer based upon cerium-doped lanthanum bromide detector dedicated for environmental radiation monitoring

Nuclear power plants that are planned to be constructed in various countries, including Poland, require to setup an environmental radiation monitoring system. Localization of the installation has to be preceded by the studies determining the level of natural background radiation. Presently the in situ gamma spectrometry is widely used for monitoring the natural as well as artificial radionuclides. An analysis and evaluation of parameters of the spectrometric system equipped with scintillation detector made of cerium-doped lanthanum bromide crystal are both the subject of the paper. The main question of the application of any gamma spectrometry system for the radiation monitoring purpose is how its efficiency looks like. Based on the numerical characteristics of the detector the absolute full energy peak efficiency was calculated. The three dimensions characteristics of gamma ray registration efficiency as the function of its energy and considered contaminated area diameter has been also performed. The study of numerical modeling based on MCNP code was performed by the ISOCS/LabSOCS software tool.     

Composite CFC-PU foam ion exchanger in the removal of radioactive cesium

The LabSOCS software performs mathematical efficiency calibration of Ge detectors. Extensive tests have been conducted comparing the LabSOCS efficiency calibration with those from traditional radioactive sources for typical laboratory geometries. For this study, 4 NIST-traceable standards containing 13 different energy lines were counted in 7 different geometries that are typical of laboratory samples. The results from the first 13 detectors produced using this process show that efficiency calibrations generated with LabSOCS have an accuracy of 5.1% SD at low energies and 4.2% SD at high energies.     

Efficiency for close geometries and extended sources of a p-type germanium detector with low-energy sensitivity

Typically, germanium detectors designed to have good sensitivity to low-energy photons and good efficiency at high energies are constructed from n-type crystals with a boron-implanted outer contact. These detectors usually exhibit inferior resolution and peak shape compared to ones made from p-type crystals. To overcome the resolution and peak-shape deficiencies, a new method of construction of a germanium detector element was developed. This has resulted in a gamma-ray detector with high sensitivity to photon energies from 14 keV to 2 MeV, while maintaining good resolution and peak shape over this energy range. Efficiency measurements, done according to the draft IEEE 325-2004 standard, show efficiencies typical of a GMX or n-type detector at low energies. The detectors are of large diameter suitable for counting extended samples such as filter papers. The Gaussian peak shape and good resolution typical of a GEM or p-type are maintained for the high count rates and peak separation needed for activation analysis.     

Validation testing of the Genie 2000 Cascade Summing Correction

Summary To validate the accuracy and precision of the Cascade Summing Correction method, over 800 archived measurements of calibrated sources (filter paper, 20 cm³ liquid scintillation vial, 400 ml beaker and Marinelli beaker) containing cascading (⁸⁸Y and ⁶⁰Co) and non-cascading isotopes from 133 different ISOCS/LabSOCS characterized high purity germanium detectors have been analyzed. Comparing the corrected results for the cascading isotope activities to the known activities shows the method is effective and accurate. Evaluation of the accuracy as a function of the amount of correction reveals a small systematic error for which a variable precision adjustment is recommended. Requirements to filter true coincidence X-rays by are verified.     

A software package using a mesh-grid method for simulating HPGe detector efficiencies

Traditional ways of determining the absolute full-energy peak efficiencies of high-purity germanium (HPGe) detectors are often time consuming, cost prohibitive, or not feasible. A software package, KMESS (Kevin’s Mesh Efficiency Simulator Software), was developed to assist in predicting these efficiencies. It uses a semi-empirical mesh-grid method and works for arbitrary source shapes and counting geometries. The model assumes that any gamma-ray source shape can be treated as a large enough collection of point sources. The code is readily adaptable, has a web-based graphical front-end, and could easily be coupled to a 3D scanner. As will be shown, this software can estimate absolute full-energy peak efficiencies with good accuracy in reasonable computation times. It has applications to the field of gamma-ray spectroscopy because it is a quick and accurate way to assist in performing quantitative analyses using HPGe detectors.     

Validation of true coincidence summing correction in Genie 2000 V3.2

A new cascade summing correction method with the algorithm extended to include true coincidence summing effects from low-energy gamma-rays, KX-rays from Electron Capture and Internal Conversion, and to include the 511 keV positron annihilation photons has been developed and implemented in Genie 2000 V3.2 released in 2009. To validate the accuracy and precision of the extended correction method, measurements of calibrated sources containing cascading nuclides from various types of ISOCS/LabSOCS characterized High Purity Germanium detectors have been analyzed. Validation of the true coincidence summing correction factors for the extended correction method has been made by comparison to the results from the Monte Carlo code MCNP-CP. In addition, comparison between the measured and the known activities of the cascading nuclides was performed, which shows that the method is effective and accurate.     

Development of high efficiency, multi-element CdZnTe detectors for portable measurement applications

We describe the development of detector arrays and electronics for large-volume, hand-held CdZnTe detectors with the same counting efficiency as portable NaI(Tl) detectors presently used for nuclear material measurement applications. The pulse-height resolution of the multi-element detectors is at least three times better than NaI(Tl) over a wide energy range (from 100 keV to several MeV), enabling more accurate measurements of gamma-rays emitted by special nuclear material. Arrays of up to eight coplanar grid detectors can be combined to make detectors ranging in size from 4 to 14 cm³. Because the number of spectroscopy channels is small, low-power, hand-held detectors can be manufactured with conventional printed circuit board technology, thus keeping the cost of multi-element detectors to a minimum. The design and performance of an 8-element detector is presented.     

Total efficiency of gamma-ray detection by germanium semiconductor detectors

The total efficiency of γ-ray detection by Ge(Li) semiconductors is calculated for the energy range 0.1 to 10 MeV, for cylindrical detectors with radii from 0.25 cm to 2.5 cm and thicknesses from 0.1 cm to 1 cm, at distances between the source and detector ranging from 2 cm to 20 cm.     

Characterization of HPGe gamma spectrometers by geant4 Monte Carlo simulations

Two coaxial and a low-energy HPGe detector were characterized with Monte Carlo simulations, using the geant4 toolkit. The geometry of the detectors, including the dimensions of the crystal and the internal structural parts, were initially taken from the factory specifications and from X-ray radiographies, and were later fine-tuned. The detector response functions, with special emphasis on the absolute full-energy peak efficiencies and peak-to-total ratios, were calculated and compared to experimental data taken at different measurement geometries. Between 150 keV and 11 MeV an agreement within 1–2 standard deviation has been achieved, whereas systematic deviations were experienced at lower energies.     

Energiebestimmung von Alphateilchen mit Festkörperspurdetektoren

Recently a method using solid state track detectors for the energy determination of alpha-particles has been developed in the Central Institute for Nuclear Research at Rossendorf. The method shows a good accuracy and has an energy resolution which is sufficient for the application in several disciplines. The method is based on a determination of the particle ranges in the detector material. The particle energy can be calculated by the known energy—range relationship. In the energy range from 3 to 6 MeV, which is the energy region of the most alpha-emitters, the energy resolution is 50 keV. The accuracy under optimum conditions is about ±50 keV. The efficiency of the method depends on the particle energy and the energy resolution desired. The method can be used for the energy determination of alpha-particles above 2.7 MeV.     

Parameterization of detector efficiency for the standardization of NAa with stable low flux reactors

With SLOWPOKE and MNS reactors which have reproducible neutron fluxes, the standardization of multielement NAA can be reduced to measuring activation constants once for all elements and then determining relative detection efficiencies for new detectors and counting geometries. In this work, a method has been developed for the parameterization of the efficiency of gemanium detectors. The gamma-ray detection efficiency was measured as a function of energy and distance for three detectors. The variation with distance was found to follow a modified EID law, within 1%, for point sources 1 mm to 250 mm from the detector. A model, including coincidence summing corrections, was developed to calculate efficiency for NAA samples; it requires 16 measured parameters. Tests showed that the calculated relative detection efficiencies are accurate to better than 3% for close counting geometries and sample volumes up to a few millilitres. Areas of possible improvement to the accurarcy of the method are suggested.     

Methods and software for predicting germanium detector absolute full-energy peak efficiencies

High-purity germanium (HPGe) and lithium drifted germanium (Ge(Li)) detectors have been the detector of choice for high resolution gamma-ray spectroscopy for many years. This is primarily due to the superior energy resolution that germanium detectors present over other gamma-ray detectors. In order to perform quantitative analyses with germanium detectors, such as activity determination or nuclide identification, one must know the absolute full-energy peak efficiency at the desired gamma-ray energy. Many different methods and computer codes have been developed throughout history in an effort to predict these efficiencies using minimal or no experimental observations. A review of these methods and the computer codes that utilize them is presented.     

A simple mathematical method to determine the efficiencies of log-conical detectors

A new type of photon detector, log-conical, is proposed. The average path length traveled by an incident photon of arbitrary energy as well as the geometrical solid angle are calculated in a mathematical expression to determine the efficiencies of this detector for an arbitrarily positioned isotropic radiating point source. The off-axis effect of the source position was analyzed to demonstrate the powerful capability of the proposed method. The results are compared with those obtained using a standard 3″×3″ cylindrical detector of the same volume in order to show the enhanced efficiency of the log-conical detector.     

A contactless conductivity detector for capillary electrophoresis: effects of the detection cell geometry on the detector performance

The effect of the gap between the electrodes and of their width on the behavior of a capacitively wired contactless conductivity detector was studied. The results obtained have indicated that the detector response can be qualitatively described by a model based on the concept of the effective electrode width which is a complex parameter determined by the gap between the electrodes, the frequency of the input signal and the conductivity of the test solution. The detector sensitivity and the effect on the separation efficiency depend on the difference between the effective and geometric electrode widths. Higher detection sensitivities have been attained for detectors with wide electrodes operating at lower frequencies, however, better separation efficiencies have been achieved using detectors with narrow electrodes and higher operational frequencies. The noise increases with decreasing gap between the electrodes and increasing frequency, especially with detectors employing narrow electrodes.     

Effect of chelated mineral supplementation on the absorption of Cu, Fe, K, Mn and Zn in horse hair

The effects of gamma-irradiation at a dose range of 5-56 Mrad on the fission track registration efficiency of the Tuffak polycarbonate track detector from dry (K _dry) as well as the solution media (K _wet) are investigated. Fission tracks were recorded in the gamma-irradiated detectors along with the unirradiated ones by an electrodeposited source of ²⁵²Cf and also separately from a ²⁵²Cf solution. The fission track registration efficiencies for the irradiated detectors were determined in comparison with those of the unirradiated detectors. The results show that while the K _dry values for the irradiated detectors decrease by about 2-10%, the corresponding K _wet values decrease by about 3-20% as compared to unirradiated detectors . Further, the bulk etch rates were found to increase with the increase in gamma dose. The changes induced in the gamma-irradiated detectors as a function of dose have been studied also by thermogravimetry (TG).     

Comparison of portable detectors for uranium enrichment measurements

Uranium enrichment and holdup measurements require a detector capable of accurately obtaining the 186-keV peak area. NaI detectors have been widely used for these tasks. However, for recycled uranium, the interference of the 239-keV peak from the ²³²U decay chain challenges the capabilities of the NaI detectors to accurately extract the area of the 186-keV peak. Using CZT detectors, which have much better resolution than the NaI detectors, has temporarily solved this interference problem. However, the CZT detectors have setbacks in that they are generally small and have low efficiencies, which require long acquisition times for reasonable statistics. Recently, two new types of scintillator detectors have become available commercially, LaCl₃(Ce) and LaBr₃(Ce). These cerium-doped lanthanum halide detectors, with comparable resolution but better efficiency than the CZT detectors, appear to permanently solve the interference problem for recycled uranium measurements. In this report, we compare the uranium enrichment measurement performances of a portable NaI detector, a large coplanar-grid CZT detector, and a LaBr₃ detector.     

Proton stopping powers: Binary encounter calculations based on accurate speed distributions for target electrons

Atomic and molecular electronic stopping powers for medium energy protons (≈ 10 keV-10 MeV) have been calculated using the binary-encounter approximation in conjunction with (1) either an energy or maximum impact parameter cut-off based on minimum excitation energies; and (2) ab initio electronic speed distributions. The maximum impact parameter approach yields good agreement with experiment for inert gases and closed-shell polyatomic molecules comprised of first-row atoms.     

Study of silicon detectors for high resolution radioxenon measurements

Measurement of radioactive xenon in the atmosphere is one of several techniques to detect nuclear weapons testing, typically using either scintillator based coincidence beta/gamma detectors or germanium based gamma only detectors. Silicon detectors have a number of potential advantages over these detectors (high resolution, low background, sensitive to photons and electrons) and are explored in this work as a possible alternative. Using energy resolutions from measurements and detection efficiencies from simulations of characteristic electron and photon energies, the minimum detectable concentration for Xe isotopes was estimated for several possible detector geometries. Test coincidence spectra were acquired with a prototype detector.     

Comparison of the response of four aerosol detectors used with ultra high pressure liquid chromatography

The responses of four different types of aerosol detectors have been evaluated and compared to establish their potential use as a universal detector in conjunction with ultra high pressure liquid chromatography (UHPLC). Two charged-aerosol detectors, namely Corona CAD and Corona Ultra, and also two different types of light-scattering detectors (an evaporative light scattering detector, and a nano-quantity analyte detector [NQAD]) were evaluated. The responses of these detectors were systematically investigated under changing experimental and instrumental parameters, such as the mobile phase flow-rate, analyte concentration, mobile phase composition, nebulizer temperature, evaporator temperature, evaporator gas flow-rate and instrumental signal filtering after detection. It was found that these parameters exerted non-linear effects on the responses of the aerosol detectors and must therefore be considered when designing analytical separation conditions, particularly when gradient elution is performed. Identical reversed-phase gradient separations were compared on all four aerosol detectors and further compared with UV detection at 200 nm. The aerosol detectors were able to detect all 11 analytes in a test set comprising species having a variety of physicochemical properties, whilst UV detection was applicable only to those analytes containing chromophores. The reproducibility of the detector response for 11 analytes over 10 consecutive separations was found to be approximately 5% for the charged-aerosol detectors and approximately 11% for the light-scattering detectors. The tested analytes included semi-volatile species which exhibited a more variable response on the aerosol detectors. Peak efficiencies were generally better on the aerosol detectors in comparison to UV detection and particularly so for the light-scattering detectors which exhibited efficiencies of around 110,000 plates per metre. Limits of detection were calculated using different mobile phase compositions and the NQAD detector was found to be the most sensitive (LOD of 10 ng/mL), followed by the Corona CAD (76 ng/mL), then UV detection at 200 nm (178 ng/mL) using an injection volume of 25 μL.