A simple method for efficiency calibration of HPGe detectors in γ-spectrometric measurements

In this paper a simple, rapid and general method for γ-ray efficiency calibration of Ge detectors for environmental samples is presented. This method is based on the use of an active natural solid sample with several γ-emissions (in our case, ²²⁶Ra) as the calibrating matrix for determining the full energy peak efficiency (FEPE) _c vs γ-emission energy E_γ and the sample height h in a counting cylindrical geometry. The ²²⁶Ra activity concentration is determined by -particle spectrometry, a method that has previously been validated.     

A semi-empirical formula for HPGe detector efficiency calibration

The precision obtainable by instrumental neutron activation analysisdepends strongly on the quality of the calibration of the gamma-ray spectrometerused for measuring the irradiated samples. Even when relative standardizationis employed, practical experience has shown the importance of detector calibration.This is especially relevant when radionuclides of varying activities are involved.The problems often encountered are those of intolerable high dead time andthe occurrence of summed peaks in the gamma-ray spectra. A simple but accuratesemi-empirical formula is presented that could effectively predict the efficiencyof a detector at any source-to-detector distance. Experimental data obtainedat 0, 50, 100, 200 mm source-to-detector distances are used to demonstratethe usefulness of this formulation. In addition, nine elements were analyzedin the NIST Orchard Leaves using absolute standardization including four elementsSm, Ta, Au, and La for which no certified values could be obtained.     

Full energy peak efficiency calibration of HPGe detector for point and extended sources using Monte Carlo code

Monte Carlo simulation is important to get efficiencies for cases where the experimental efficiencies are difficult to get such as for samples with nonstandard geometries and for large samples. In this paper, efficiency of the HPGe detector, routinely used in our lab for a variety of samples, has been computed for point source geometry and its parameters has been optimized to match MCNP and experimental efficiencies within 5% at different sample to detector distances. This optimized geometry was then validated by efficiency transfer to other geometries.     

HPGe detector full-energy peak efficiency calculation for inhomogeneously activated samples

Journal of Radioanalytical and Nuclear Chemistry - We present and evaluate a program for the semi-empirical calculation of the full-energy peak efficiency of a hyper-pure germanium detector, by...     

X-ray fluorescence analysis in environmental radiological surveillance using HPGe detectors

X-ray fluorescence (XRF) has been proven to be a valuable tool for determining trace quantities of heavy metals, such as uranium and lead, in different types of samples. The present paper demonstrates the applicability of XRF spectrometry to measure the concentrations of these heavy metals in samples from natural ore and soil. The values of uranium concentrations in rock from the Peña Blanca uranium ore, in Chihuahua, México, were calculated for the purpose of precertifying the rock powders samples. The comparison with other techniques, such as inductively coupled plasma atomic emission spectrometry, atomic absorption spectrometry, alpha spectrometry and electron microscopy, was used to complete the precertification process, so that the sample powders may be used as secondary standards. The source-sample-detector geometry and the incident angle are the most important factors for obtaining low detection limits. The selected system uses a ⁵⁷Co source of about 0.1 mCi to excite the K X-rays from uranium and lead. X-rays were recorded on a CANBERRA HPGe coaxial detector. The comparative results for two incident angles (90° and 180°) performed previously by other authors show that the best geometry is the backscattering geometry. In the present paper, using EGS4 code system with Monte Carlo simulation, it was possible to determine the location and distribution of background produced by the Compton edge in the optimized geometry. This procedure allowed to find the minimum detectable concentration of uranium and lead, which was experimentally calculated using standards. The possibility of performing in vivo measurements rapidly and easily, as well as the factors affecting accuracy and the minimum detectable concentration in several samples are also discussed.     

Choosing points in potential energy surfaces for fitting polynomial functions: application of permutational symmetry

Using arguments based on permutational symmetry it is shown that it is possible to XXX a minimum potential energy function from high-symmetry to low-symmetry parts of the surface. The method has been applied in X₃, X₄ and AX₄ systems.     

Interpolating moving least-squares methods for fitting potential energy surfaces: Improving efficiency via local approximants

The local interpolating moving least-squares (IMLS) method for constructing potential energy surfaces is investigated. The method retains the advantageous features of the IMLS approach in that the ab initio derivatives are not required and high degree polynomials can be used to provide accurate fits, while at the same time it is much more efficient than the standard IMLS approach because the least-squares solutions need to be calculated only once at the data points. Issues related to the implementation of the local IMLS method are investigated and the accuracy is assessed using HOOH as a test case. It is shown that the local IMLS method is at the same level of accuracy as the standard IMLS method. In addition, the scaling of the method is found to be a power law as a function of number of data points N, N(-q). The results suggest that when fitting only to the energy values for a d-dimensional system by using a Qth degree polynomial the power law exponent q approximately Qd when the energy range fitted is large (e.g., E<100 kcalmol for HOOH), and q>Qd when the energy range fitted is smaller (E<30 kcalmol) and the density of data points is higher. This study demonstrates that the local IMLS method provides an efficient and accurate means for constructing potential energy surfaces.     

Spline and polynomial models of the efficiency function for Ge gamma-ray detectors in a wide energy range

Summary In one of our recent papers, the applicability of linear parameter functions for fitting the full-energy peak efficiency of n-type Ge gamma-ray detectors has been examined over a wide energy range of 50-8500 keV. In that paper we compared six different analytical functions and showed that higher-order polynomial functions on a log-log scale gave the best performance. However, there is a drawback to using the log-log scale when an additive function of efficiency at different energies or of the inverse efficiency has to be used in a fitting procedure. In the present study, the applicability of higher-order polynomial and spline functions to linear and inverse efficiency, but logarithmic energy scales, is examined.     

Physics-based generation of gamma-ray response functions for CdZnTe detectors

A physics-based approach to gamma-ray response-function generation is presented in which the response of CdZnTe detectors is modeled from first principles. Numerical modeling is used to generate response functions needed for spectrum analysis for general detector configurations (e.g., electrode design, detector materials and geometry, and operating conditions). With numerical modeling, requirements for calibration and characterization are significantly reduced. Elements of the physics-based model, including gamma-ray transport, charge carrier drift and diffusion, and circuit response, are presented. Calculated and experimental gamma-ray spectra are compared for a coplanar-grid CdZnTe detector.     

Identifying and quantifying short-lived fission products from thermal fission of HEU using portable HPGe detectors

Due to the emerging potential for trafficking of special nuclear material, research programs are investigating current capabilities of commercially available portable gamma ray detection systems. Presented in this paper are the results of three different portable high-purity germanium (HPGe) detectors used to identify short-lived fission products generated from thermal neutron interrogation of small samples of highly enriched uranium. Samples were irradiated at the Washington State University Nuclear Radiation Center’s 1 MW TRIGA reactor. The three portable, HPGe detectors used were the ORTEC MicroDetective [1], the ORTEC Detective [2], and the Canberra Falcon [3]. Canberra’s GENIE-2000 software was used to analyze the spectral data collected from each detector. Ultimately, these three portable detectors were able to identify a large range of fission products showing potential for material discrimination.     

A method for self-attenuation and sample-height correction for counting efficiency of HPGe using Marinelli beaker geometry

A procedure for self-attenuation and sample height correction in HPGe gamma spectrometry efficiency has been presented. An MCNP model of an HPGe detector was used to calculate the full energy peak efficiency (FEPE) for a group of different samples with different heights in Marinelli beaker geometry. A proper function has been fitted to the simulation results to obtain the correction function. The function has been used to calculate the FEPE of a spiked soil sample in different sample heights by considering the experimentally known FEPE of another standard solution source. A good agreement between the experiments and calculations have been shown.     

Improvement of the Ca determination accuracy with k 0-INAA using an HPGe coaxial detector with extended energy range efficiency calibration

Journal of Radioanalytical and Nuclear Chemistry - We show that use of the radionuclide 56Co for calibration of the full energy peak efficiency, ε, of a coaxial HPGe detector, in addition to...     

Determining the efficiency of a broad-energy HPGe detector using Monte Carlo simulations

Broad-energy HPGe detectors have a useful range of 3 keV to 3 MeV, making them ideal for the assay of environmental samples. Such measurements however, are hindered by variations in the sample matrix, summing effects, and the Compton continuum. Detectors may be characterised by proprietary software in such a situation, however Monte-Carlo modelling is a useful, inexpensive alternative that also provides greater flexibility when determining the detector response and efficiency during a measurement. In the current work, a full GEANT4 model of a broad-energy HPGe detector is presented, and simulations of various samples are compared to experimental data. These are found to be accurate within 3 % at a confidence level of 95 % for energies from 30 to 3,000 keV, with greater variations below 100 keV due to an increased sensitivity to geometrical inaccuracies.     

Analytical fitting of potential energy surfaces for the Hxy systems

Use of the London-Eyring-Polanyi + 3-Center + power-series (LEP -3C -PS ) analytical potential as a fit to potential energy surfaces (PES ) which are known numerically only are suggested. This analytical fit was performed for the diatomics-in-molecules + 3 Center (DIM -3C ) PES of HCl₂ and HI₂ systems. The HCl₂ analytical LEP -3C-PS potential was used for classical trajectory calculations of the Cl' + HCl → HCl' + Cl reaction. The rate constant obtained from these calculations for T = 358° K is 1.95 X 10⁹ cm³/mol sec which is close to the experimental value of 2.5 10⁹ cm³/mol sec.     

Comparison of efficiency calibration techniques of HPGe detector for radioactivity measurement of soil sample in Marinelli geometry

Journal of Radioanalytical and Nuclear Chemistry - To keep the accredited category for the gamma spectrometry test in our laboratory, the efficiency curves of a HPGe detector for soil sample in...