Simple method for true coincidence summing correction

For the correction of losses due to true coincidences summing and edge effect, a simple method which is based on the ratio of a reference single -ray energy to that of cascade energies at near and far geometry is developed. The correction factors for several radioactive sources with simple and complex decay schemes are experimentally determined for three types of germanium detectors. It is shown that coincidence summing can be a complex effect and depends on the individual detector, the counting geometry and on the decay scheme of the radionuclide concerned.     

Correction equations of coincidence summing using75Se radionuclide in the efficiency of HpGe detector

Correction equations of the coincidence-summing effect for efficiencies of HpGe detector based on the decay scheme were developed by considering the summing up to triple coincidence. The correction equations which do not dependent on the kind of the Ge detector are very useful for efficiency calibrations of a Ge detector in the energy region from 60 to 400 keV by using⁷⁵Se radionuclide even with very short source-to-detector distances.     

Design and construction of an ultra-low-background 14-crystal germanium array for high efficiency and coincidence measurements

Physics experiments, environmental surveillance, and treaty verification techniques continue to require increased sensitivity for detecting and quantifying radionuclides of interest. This can be done by detecting a greater fraction of gamma emissions from a sample (higher detection efficiency) and reducing instrument backgrounds. A current effort for increased sensitivity in high resolution gamma spectroscopy will produce an intrinsic germanium (HPGe) array designed for high detection efficiency, ultra-low-background performance, and useful coincidence efficiencies. The system design is optimized to accommodate filter paper samples, e.g. samples collected by the Radionuclide Aerosol Sampler/Analyzer (RASA). The system will provide high sensitivity for weak collections on atmospheric filter samples, as well as offering the potential to gather additional information from more active filters using gamma cascade coincidence detection. The current effort is constructing an ultra-low-background HPGe crystal array consisting of two vacuum cryostats, each housing a hexagonal array of 7 crystals on the order of 70% relative efficiency per crystal. Traditional methods for constructing ultra-low-background detectors are used, including use of materials known to be low in radioactive contaminants, use of ultra pure reagents, clean room assembly, etc. The cryostat will be constructed mainly from copper electroformed into near-final geometry at PNNL. Details of the detector design, simulation of efficiency and coincidence performance, HPGe crystal testing, and progress on cryostat construction are presented.     

Geant4 Monte Carlo radioxenon beta-gamma coincidence efficiency simulation for a phoswich detector

In this work, a Monte Carlo (MC) simulation model is established to accurately characterize a phoswich beta-gamma coincidence detector system. This model can be easily used to predict the beta-gamma coincidence efficiencies of xenon radioisotopes at various stable xenon concentrations in the counting cell. The results demonstrate that there is a significant inverse correlation between beta-gamma coincidence efficiency and stable xenon concentration. The influence of stable xenon concentration on beta-gamma coincidence counting efficiency has been investigated for each individual xenon radioisotope. The results indicate that the effect of stable xenon concentration on beta-gamma coincidence efficiency depends on the xenon radioisotope and its decay modes. The coincidence efficiency of ¹³³Xe with 31.0-keV X-ray decay mode is the most affected one; and then followed by ^131mXe, ¹³³Xe with 81.0-keV gamma-ray decay mode, ^133mXe and finally ¹³⁵Xe. The study also indicates that the gamma absorption by xenon gas plays more of a role in the decrease of beta-gamma coincidence efficiency for ¹³³Xe and ¹³⁵Xe, and that the conversion electron spectrum shifting and broadening plays more of a role in the reduction of beta-gamma coincidence efficiency for the metastable radioxenon of ^131mXe and ^133mXe.     

True coincidence summing corrections in point and extended sources

The true coincidence summing (TCS) effect on the full energy peak (FEP) efficiency calibration of an HPGe detector has been studied as a function of sample-to-detector distance using multi-gamma sources. Analytical method has been used to calculate coincidence correction factors for ¹⁵²Eu, ¹³³Ba, ¹³⁴Cs and ⁶⁰Co for point and extended source geometry at close sample-to-detector distance. Peak and total efficiencies required for this method have been obtained by using MCNP code by using the optimized detector geometry. The correction factors have also been obtained experimentally. The analytical and the experimental correction factors have been found to match within 1–5%. The method has been applied to obtain the activity of the radionuclides (¹⁰⁶Ru, ¹²⁵Sb, ¹³⁴Cs and ¹⁴⁴Ce) present in a fission product sample.     

Validation of gamma-ray true coincidence summing effects modeled by the Monte Carlo code MCNP-CP

When radionuclides decay by cascading photons, the accuracy of the measured nuclide activity may be affected by true coincidence summing effects. The effects can be quantified by Monte Carlo simulations that can handle correlated γ-and X-ray emissions from a radionuclide. Analysis techniques are also available commercially to correct for the effects due to cascading γ-rays. The MCNP-CP code was used to compute the effects in high purity germanium detectors for several commonly used nuclides and geometries and the results were compared to measurements and an analysis technique. Excellent agreement in true coincidence summing corrections predicted by MCNP-CP and the analysis technique was obtained. In addition, the X-ray true coincidence summing effects were evaluated.     

Total versus effective total efficiency in the computation of coincidence summing corrections in gamma-ray spectrometry of volume sources

For the evaluation of coincidence summing effects for volume sources an effective total efficiency (ETE) is used instead of the common total efficiency (TE). In this paper ETE is computed by the Monte Carlo method. The differences between ETE and TE are analyzed and their origin is discussed. Measured values for the coincidence summing correction factors for a standard solution containing ¹⁵²Eu in a one liter Marinelli beaker are compared with computed values obtained from appropriate values of ETE. It is shown that the procedure for the evaluation of the coincidence effects is reliable. As a consequence it can be concluded that ¹⁵²Eu volume sources can be successfully used for efficiency calibration even in the case of high-efficiency detectors and close source-to-detector distances.     

HPGe detector photopeak efficiency calculation including self-absorption and coincidence corrections for cylindrical sources using compact analytical expressions

Total and full-energy peak efficiencies, coincidence correction factors and the source self-absorption of a p-type coaxial HPGe detector for cylindrical sources have been calculated using direct analytical expressions. In the experiments gamma aqueous sources containing several radionuclides covering the energy range from 60 to 1836 keV were used. By comparison, the theoretical and experimental full-energy peak efficiency values are in good agreement.     

An empirical expression for the full energy peak efficiency of an N-type high purity germanium detector

An empirical expression for the full energy peak efficiency (ε) in terms of the gamma-ray energy (E) and the vertical distance from the detector surface (z) (i.e.,ε=ε(z,E)) has been obtained for an N-type high purity germanium (HPGE) detector using an extended mixed standard radionuclide solution. Comparison of the calculated efficiencies and the experimentally measured values for the energy range of 59.5–1332.5 keV and a source-to-detector distance of 1.2–7.2 cm showed that the theoretical values agree with that of the experiment within 1.5–3.7% standard deviations. This shows a good agreement between the theory and the experiment.     

True coincidence summing correction determination for 214Bi principal gamma lines in NORM samples

The gamma lines 609.3 and 1,120.3 keV are two of the most intensive γ emissions of ²¹⁴Bi, but they have serious true coincidence summing (TCS) effects due to the complex decay schemes with multi-cascading transitions. TCS effects cause inaccurate count rate and hence erroneous results. A simple and easy experimental method for determination of TCS correction of ²¹⁴Bi gamma lines was developed in this work using naturally occurring radioactive material samples. Height efficiency and self attenuation corrections were determined as well. The developed method has been formulated theoretically and validated experimentally. The corrections problems were solved simply with neither additional standard source nor simulation skills.     

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.     

Optimisation design of cylindrical containers for improving the detection efficiency of a high-purity germanium detector using the LabSOCS

The LabSOCS software was used to optimise the geometric dimensions of cylindrical containers and improve the detection efficiency of a high-purity germanium detector for the measurement of environmental radioactivity samples. The optimum ratio of diameter and height for different sample volumes were determined. In addition, the degrees of influence of the sample volume and density when the amount of sample was fixed were compared. Results show that the effect of sample volume on detection efficiency was significantly greater than that of sample density for a given sample amount.     

A Compton-vetoed germanium detector with increased sensitivity at low energies

The difficulty to directly detect plutonium in spent nuclear fuel due to the high Compton background of the fission products motivates the design of a gamma detector with improved sensitivity at low energies. We have built such a detector by operating a thin high-purity Ge detector with a large scintillator Compton veto directly behind it. The Ge detector is thin to absorb just the low-energy Pu radiation of interest while minimizing Compton scattering of high-energy radiation from the fission products. The subsequent scintillator is large so that forward-scattered photons from the Ge detector interact in it at least once to provide an anti-coincidence veto for the Ge detector. For highest sensitivity, additional material in the line of sight is minimized, the radioactive sample is kept thin, and its radiation is collimated. We will discuss the instrument design, and demonstrate the feasibility of the approach with a prototype that employs two large CsI scintillator vetoes. Initial spectra of a thin Cs-137 calibration source show a background suppression of a factor of ~2.5 at ~100 keV, limited by an unexpectedly thick 4 mm dead layer in the Ge detector.     

ANGLE: A PC-code for semiconductor detector efficiency calculations

A broadly applicable, flexible and user-friendly PC-code (ANGLE) for calculations of semiconductor detector full energy peak efficiencies ( _p ) is presented. The physical model behind is the concept of the effective solid angle . Written in Pascal, and operating in windows and menus data manipulation mode, ANGLE yields the efficiencies for: (1) HPGe true- and closed-end coaxial (bothn- andp-types), (2) Ge(Li) open- and closed-end, (3) planar LEPD and (4) well-type detectors. Supposing coaxial positioning, cylindrical or Marinelli sources can be treated, regardless of their dimensions (this includes point, disk and ring sources, bulky samples and infinite geometrics). Possible displacement between source and detector axes is treated in our another work, relative to this one. ANGLE input parameters are: (1) reference efficiency curve for the detector used (i.e., efficiency vs. -energy for calibrated point sources at a reference distance), (2) detector type and configuration (active body and inactive layers, end cap, windows, housing, shielding, (3) source data (dimension and composition of both container and active material), (4) source-detector geometry (distance, intercepting layers and their composition) and (5) some computational data (Gauss integration coefficients). Gamma-attenuation is calculated upon an extensive (per element and per energy) data file. In the output, efficiency vs. -energy is found, both in forms of tables and graphs. In routine applications accuracies of 3–4% are achieved (not worse than 7% for the most unfavourable geometries). Computation times when using recent PC models are of the order of minutes. ANGLE frame is also easily adjustable to other semiempirical or Monte Carlo models for efficiency calculations.     

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.     

Methods for achieving ultra-low backgrounds in above-ground germanium detector systems

The authors have previously shown how to achieve a 40-fold reduction in system background for above-ground germanium detectors. Such systems are generally sufficient for examining small hydrogenous samples or samples of non-hydrogenous materials such as soil, glass, metals, etc. However, samples with a large hydrogen content yield a seriously degraded background due to a number of neutron-induced interactions, such as Compton scatter following hydrogen capture of thermalized neutrons. A study was performed to better understand the sources of thermal neutron flux in above-ground detector systems. The effects of different quantities of hydrogen in the sample were also examined. Methods are presented for additional lowering of system background for both hydrogenous and non-hydrogenous samples.     

A new method for assessing the efficiency of stabilizers in polyolefins exposed to chlorinated water media

The chlorine used as disinfectant in tap water degrades most materials, including polyethylene. The most adequate (functional) test method, the pressure test, is complicated and expensive because the chlorinated aqueous media (Cl₂ or ClO₂ in water) are unstable and they undergo reactions that are dependent on the pH. A new method which assesses the protection efficiency of phenolic antioxidants in polyolefins was developed. The method uses a liquid hydrocarbon analogue, squalane, in which antioxidants are dissolved. The organic phase was dispersed in the aqueous chlorinated phase (containing 10 ppm of either Cl₂ or ClO₂; pH = 6.8) at 70 °C by intense stirring. The depletion of antioxidant (Irganox 1010) was monitored by standard DSC determination of the oxidation induction time. It was shown that 300 min of exposure was sufficient to obtain useful data.     

Determination of 134Cs activity with the sum-peak method using a well-type germanium detector

Journal of Radioanalytical and Nuclear Chemistry - The new sum peak method was applied to the samples containing both 137Cs and 134Cs using a well-type HPGe detector. The standard sources...     

A reliable and accurate procedure for preparing low-activity efficiency calibration standards for germanium gamma-ray spectrometers

An empirical procedure is described which is readily capable of producing reliable and accurate efficiency calibration sources of low activity for high-resolution -ray spectrometers. Known activities of a multiisotopic solution are adsorbed onto a mixture of cation and anion-exchange resins and chromatographic-grade cellulose. The dried powder that results can be mixed with any desired solid natural matrix material to produce a homogeneous, extended geometry, efficiency calibration source. Results are presented which enable the method to be critically evaluated.