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.     

Improved detector response characterization method in ISOCS and LabSOCS

The In-Situ Object Calibration Software (ISOCS) and the Laboratory Sourceless Calibration Software (LabSOCS) developed and patented by Canberra Industries have found widespread use in the gamma-spectrometry community. Using the ISOCS methodology, one can determine the full energy peak efficiencies of a germanium detector in the 45 keV-7 MeV energy range, for practically any source matrix and geometry. The underlying mathematical techniques used in ISOCS (and LabSOCS) have undergone significant improvements and enhancements since their first release in 1996. One of these improvements is  a spatial response characterization technique that is capable of handling the large variations in efficiency that occurs within a small region. The technique has been in use in ISOCS and LabSOCS releases since 1999, and has significantly improved the overall quality of the close-in and off-axis response characterization for HPGe detectors, especially for Canberra’s Broad Energy Germanium (BEGe) detectors. In this method, the detector response is characterized by creating a set of fine spatial efficiency grids at 15 energies in the 45 keV-7 MeV range. The spatial grids are created in (r,&odash;) space about the detector, with the radius r varying from 0 to 500 meters, and the angle &odash; varying from 0 to π. The reference efficiencies for creating the spatial grids are determined from MCNP calculations using a validated detector model. Once the efficiency grids are created, the detector response can be determined at any arbitrary point within a sphere of 500-meter radius, and at any arbitrary energy within the specified range. Results are presented highlighting the improved performance achieved using the gridding methodology.     

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.     

Calibration frequency for gas proportional counters

A laboratory that employs nuclear detectors should have a calibration procedure, and its practice is detailed in the bibliography, but the frequency of that calibration is not settled in any case. The aim of this work is to establish an optimum frequency for efficiency calibration in an alpha/beta gas proportional counter, related with the results' adequate routine controls. The results obtained in the application of this purpose to a real instrument (D200 Canberra) are also presented.     

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.     

Efficiency calibration of germanium detectors with internal standards

A simple experimental method for the efficiency calibration of germanium detectors especially for environmental samples is presented, using only the natural radionuclides in the sample. The method is based on the fact that for the energy range above 300 keV the full-energy-peak efficiency of a Ge detector can be described in a first order approximation by a linear interpolation curve in the log-log display with errors lying normally under 5%. Photons with different energies which are emitted from one radionuclide yield count rates which are correlated to the corresponding efficiencies. From this correlation one coefficient of the interpolation curve — a first order polynomial — can be calculated. The second coefficient can be obtained by the count rate of⁴⁰K, resulting from KCl, which is mixed homogeneously with the sample. Expecially for environmental samples with large volumes, this method is very useful, because it takes into account the self-absorption of photons in the sample.     

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.     

Stability of metallofullerenes following neutron capture reaction on the metal ion

To achieve the highest possible sensitivity of analysis for environmental samples it is common practice to use both a high efficiency detector and a close measurement geometry with a large sample size (e.g. Marinelli beaker). Under such conditions, the typical efficiency calibration procedure results in a biased activity value for many nuclides due to the true coincidence summing effect. While there are a few methods to correct for this effect with special calibration standards, such calibrations can be both time consuming and expensive. Due to these calibration difficulties, the true coincidence summing effect is often simply ignored. Recently, it has been demonstrated that the coincidence summing correction can be performed mathematically even for voluminous sources. This new method consists of an integration of the coincidence correction factor over the sample volume while taking into account its chemical composition and the container. In this paper, we will discuss the latest approaches for establishing the peak efficiency and peak-to-total efficiency curves, which are required for this method. These approaches have been tested for HPGe detectors of two different relative efficiencies.     

Annealing characteristics of nuclear tracks in glass detectors using optical absorption epectroscopy

The etching and annealing behaviour of heavy ion nuclear tracks have been studied in glass detectors namely sodalime, phosphate and quartz using optical absorption spectroscopy. All these glass detectors were exposed at 90° and 45° for different ions, with collimated beams of varying energies. The absorption difference spectrum was employed to describe the annealing kinetics. A mathematical relation proposed in our laboratory was used to explain the annealing behaviour of radiation damage due to heavy ion beams in glass detectors. The activation energy was obtained from a new formulation and is found to be 0.16 eV in sodalime, 0.56 eV for phosphate and 0.69 eV for quartz glass detectors.     

A sobering assessment of small‐molecule force field methods for low energy conformer predictions

We have carried out a large scale computational investigation to assess the utility of common small‐molecule force fields for computational screening of low energy conformers of typical organic molecules. Using statistical analyses on the energies and relative rankings of up to 250 diverse conformers of 700 different molecular structures, we find that energies from widely used classical force fields (MMFF94, UFF, and GAFF) show unconditionally poor energy and rank correlation with semiempirical (PM7) and Kohn–Sham density functional theory (DFT) energies calculated at PM7 and DFT optimized geometries. In contrast, semiempirical PM7 calculations show significantly better correlation with DFT calculations and generally better geometries. With these results, we make recommendations to more reliably carry out conformer screening.     

Assessment of the suitability of Monte Carlo simulation for activity measurements of extended sources

Monte Carlo simulations can be a powerful tool in calibrating high-resolution gamma-ray spectrometry based on high pure germanium (HPGe) detectors. The purpose of this work is to examine the applicability of Monte Carlo simulations for the computation of the efficiency transfer in various measurement geometries on the basis of the detected efficiency for point source geometry. For this, GEANT4 code was applied for the computation of the detection efficiency for incident gamma energy of radionuclide placed at different distances from HPGe detector from 50 to 2,000 keV in addition for volume sources of different compositions and densities. The experimental efficiency curves were compared with the prediction of the GEANT4 code. Efficiency is computed at discrete values of point and volume sources in different distances to derive new efficiencies values for other distances.     

Mathematical efficiency calibration methods for high quality laboratory based gamma spectrometry systems

The efficiency calibration of laboratory based gamma spectrometry systems typically involves the purchase or construction of calibration samples that are supposed to represent the geometries of the unknown samples to be measured. For complete and correct calibrations, these sample containers must span the operational range of the system, which at times can include difficult configurations of size, density, matrix, and source distribution. The efficiency calibration of a system is dependent not only on the detector, but on the radiation attenuation factors in the detector–source configuration, and therefore is invalid unless all parameters of the sample assay condition are identical to the calibration condition. An alternative to source-based calibrations is to mathematically model the efficiency response of a given detector–sample configuration. In this approach, the measurement system is calibrated using physically accurate models whose parameters can generally be easily measured. Using modeled efficiencies, systems can be quickly adapted to changing sample containers and detector configurations. This paper explores the advantages of using mathematically computed efficiencies in place of traditional source-based measured efficiencies for laboratory samples, focusing specifically on the possibility of sample optimization for a given detector, uncertainty estimation, and cascade summing corrections.     

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.     

Theoretical study and rate constant calculation for the reactions of SH (SD) with Cl2, Br2, and BrCl

The mechanisms of the SH (SD) radicals with Cl2 (R1), Br2 (R2), and BrCl (R3) are investigated theoretically, and the rate constants are calculated using a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6-311G(d,p) and MPW1K/6-311G(d,p) levels. Higher-level energies are obtained at the approximate QCISD(T)/6-311++G(3df, 2pd) level using the MP2 geometries as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MPW1K geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of these reactions, which indicate that the reactions may proceed via an indirect mechanism. The enthalpies of formation for the species XSH/XSD (X = Cl and Br) are evaluated using hydrogenation working reactions method. By canonical variational transition-state theory (CVT), the rate constants of SH and SD radicals with Cl2, Br2, and BrCl are calculated over a wide temperature range of 200-2000 K at the a-QCISD(T)/6-311++G(3df, 2pd)//MP2/6-311G(d, p) level. Good agreement between the calculated and experimental rate constants is obtained in the measured temperature range. Our calculations show that for SH (SD) + BrCl reaction bromine abstraction (R3a or R3a') leading to the formation of BrSH (BrSD) + Cl in a barrierless process dominants the reaction with the branching ratios for channels 3a and 3a' of 99% at 298 K, which is quite different from the experimental result of k3a'/k3' = 54 +/- 10%. Negative activation energies are found at the higher level for the SH + Br2 and SH + BrCl (Br-abstraction) reactions; as a result, the rate constants show a slightly negative temperature dependence, which is consistent with the determination in the literature. The kinetic isotope effects for the three reactions are "inverse". The values of kH/kD are 0.88, 0.91, and 0.69 at room temperature, respectively, and they increase as the temperature increases.     

Assessments on energy and efficiency calibration of an alpha spectrometry system using standard sources

Studies and evaluations of the main parameters of a high resolution multi-chamber alpha spectrometer and its full calibration using standard sources of ²⁴¹Am, ²³³U and ²⁴⁴Cm are presented for the validation of the complex method of characterization of an alpha spectrometer dedicated for alpha radioactive concentration measurements of various environmental samples. The resolution and efficiency were determined at all possible source-detector distances for all eight solid state ORTEC ULTRA-AS detectors. In addition, the solid angles associated to different measurement geometries and the repeatability of the results were assessed. A detailed inter-comparison of the results was performed drawing important conclusions regarding the quality of the alpha detector response to the alpha radiation of the standard sources used in measurements.     

Effect of cascade coincidences on the efficiency calibration of a gamma-X detector

The sensitivity on n-type gamma-X detectors for low-energy X- and -rays calls for coincidence corrections in the efficiency calibration that do not apply to the calibration of p-type detectors. Corrections were calculated for the effect of cascade coincidences between -rays, X-rays, annihilation radiation, and bremsstrahlung, for 15 radionuclides frequently used for efficiency calibration. Experimental results are presented for a -X detector with 37% relative efficiency at distances from 0.9 to 17.5 cm. After coincidence correction smooth efficiency curves were found for the energy range 12 to 2750 keV, even for the position closest to the detector.     

Ion-pair dissociation dynamics of H2S in the photon energy range 15.26-15.55 eV

The H(+) velocity map images from the ion-pair dissociation of H(2)S + hν → SH(-)(X(1)Σ(+), υ = 0, 1) + H(+) have been measured at the excitation energies 15.259, 15.395, and 15.547 eV, respectively. The experimental results show that most of the available energies are transformed into the translational energies. The angular distributions of the fragments SH(-)(X(1)Σ(+), υ = 0) indicate that the dissociation occurs via pure parallel transition with limiting anisotropy parameter of +2. Because the ion-pair dissociation usually occurs via the predissociation of Rydberg states, this suggests that the ion cores of the excited Rydberg states have linear geometries. The geometries and electronic structures of the linear H(2)S(+) have been calculated employing the quantum chemistry calculation method at the CASPT2/avqz level. The electronic structures for the ion-pair states have been calculated at the CASSCF/avtz level, which indicates that the equilibrium geometries of the ion-pair states have bent geometries.     

{\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs24 Efficiency of germanium detectors as a function of energy and incident geometry: Comparison of measurements and calculations \par }

Summary {\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs24 The use of HPGe detectors in counting situations where the sample is not easily reproduced has increased the use of models to determine the counting efficiency for the specific geometry. The accuracy of these simulations of the germanium detector response relies on detailed knowledge of the performance of the detector. Several different types of detectors were measured at different energies using a pencil beam of gamma-rays. These measurements showed that the dead layer was not uniform from detector to detector. This and the construction details were used to calculate the efficiency for several detectors. \par }     

Systematic study of the performance of density functional theory methods for prediction of energies and geometries of organoselenium compounds

A variety of density functional theory (DFT) methods are paired with Pople basis sets of varying sizes and evaluated for use with organoselenium compounds. The ability of each method to predict reliable geometries and energies is determined through comparison with quadratic configuration interaction with single and double excitations (QCISD) results. The recommended procedure for accurate prediction of energies and geometries is to use the B3PW91 functional with the 6-311G(2df,p) basis set. The B3PW91/6-31G(d,p) level of theory gives almost identical geometries as larger basis sets, so geometries can be predicted at this level for computational efficiency.     

Diamond detectors for synchrotron radiation X-ray applications

Due to its unique physical properties, diamond is a very appealing material for the development of electronic devices and sensors. Its wide band gap (5.5 eV) endows diamond based devices with low thermal noise, low dark current levels and, in the case of radiation detectors, high visible-to-X-ray signal discrimination (visible blindness) as well as high sensitivity to energies greater than the band gap. Furthermore, due to its radiation hardness diamond is very interesting for applications in extreme environments, or as monitor of high fluency radiation beams. In this work the use of diamond based detectors for X-ray sensing is discussed. On purpose, some photo-conductors based on different diamond types have been tested at the DAFNE-L synchrotron radiation laboratory at Frascati. X-ray sensitivity spectra, linearity and stability of the response of these diamond devices have been measured in order to evidence the promising performance of such devices.