Evaluation of Monte Carlo simulation of photon counting efficiency for germanium detectors

Photopeak Efficiency and Coincidence Summing Factors for HPGe Detectors Using Bulk Sources

In the field of gamma-ray spectroscopy with high-purity germanium detectors, used to measure the activity when the sample to be measured has low radioactivity, volumetric sources are widely used. To determine the sample activity, the full-energy peak efficiency is needed. Bulk sources are very common for low-level radioactivity measurements in activation analysis and environmental samples. The main problem with these measurements is that coincidence effects are large for close source-detector geometries. Ignoring these effects can lead to an error typically of a factor of 2 in the determination of Co and Y activity, which was used in this work in the calibration process. Extensive experimental readings have been carried out, and bulk gamma aqueous sources containing several radionuclides covering the energy range from 60 to 1,840 keV were used. By comparison, the experimental (present work) and theoretical (published) full-energy peak efficiency values are in good agreement; the overall percentage error is less than 6%.

A numerical method for the calibration of in situ gamma ray spectroscopy systems

High purity germanium in situ gamma ray spectroscopy systems are typically calibrated using pre-calculated tables and empirical formulas to estimate the response of a detector to an exponentially distributed source in a soil matrix. Although this method is effective, it has estimated uncertainties of 10-15%, is limited to only a restricted set of measurement scenarios, and the approach only applies to an exponentially distributed source. In addition, the only soil parameters that can be varied are density and moisture content, while soil attenuation properties are fixed. This paper presents a more flexible method for performing such calibrations. For this new method, a three- or four-dimensional analytical expression is derived that is a combination of a theoretical equation and experimentally measured data. Numerical methods are used to integrate this expression, which approximates the response of a detector to a large variety of source distributions within any soil, concrete, or other matrix. The calculation method is flexible enough to allow for the variation of multiple parameters, including media attenuation properties and the measurement geometry. The method could easily be adapted to horizontally non-uniform sources as well. Detector responses are calculated analytically and Monte Carlo radiation transport simulations are used to verify the results. Results indicate that the method adds an uncertainty of only approximately 5% to the other uncertainties typically associated with the calibration of a detector system.

A method for the quantitative gamma spectroscopic analysis of an unusually shaped unknown source

An unmarked cylindrical device, identified as a ceramic high voltage capacitor, needed its radioactivity assessed so that proper disposal and shipping requirements could be met. Using a high purity germanium detector, naturally occurring 232Th was identified as the source of radioactivity. A series of point source measurements was made along the length of the item's axis using 60Co, having a gamma ray of nearly the same energy as one of the primary 232Th progeny photopeaks. These measurements were then numerically integrated to determine the response of the detector to a line source. A correction for the self shielding of the item was estimated using Monte Carlo simulations. The item was found to contain approximately 1.85 x 10(5) Bq of uniformly distributed 232Th. The overall method has application to any unusually shaped source, with point source measurements performed using an appropriate radionuclide used to establish an overall sensitivity of the detector, including its dead layer, to the radioactivity in a simple geometric representation of the object. An estimation of self shielding from Monte Carlo is then applied to that result.

Intercomparison of efficiency transfer software for gamma-ray spectrometry

The EUROMET project 428 examines efficiency transfer results for Ge gamma-ray spectrometers when the efficiency is known for a reference point source geometry. For this, different methods are used, such as Monte Carlo simulation or semi-empirical computation. The exercise compares the application of these methods to the same selected experimental cases to determine the usage limitations versus the requested accuracy. For carefully examining these results and trying to derive information for improving the computation codes, this study was limited to a few simple cases. The first part concerns the simplest case of geometry transfer, i.e., using point sources for 3 source-to-detector distances: 2, 5 and 20 cm; the second part deals with transfer from point source geometry to cylindrical geometry with three different matrices. The general conclusion is that the deviations between the computed results and the measured efficiencies are mostly within 10%. The quality of the results is rather inhomogeneous and shows that these codes cannot be used directly for metrological purposes. However, most of them are operational for routine measurements when efficiency uncertainties of 5-10% can be sufficient.

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

Direct mathematical methods to calculate total and full-energy peak (photopeak) efficiencies, coincidence correction factors and the source self-absorption of a closed end coaxial HPGe detector for Marinelli beaker sources have been derived. The source self-absorption is determined by calculating the photon path length in the source volume. The attenuation of photons by the Marinelli beaker and the detector cap materials is also calculated. 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.

Monte Carlo calculation of the efficiency calibration curve and coincidence-summing corrections in low-level gamma-ray spectrometry using well-type HPGe detectors

Well-type high-purity germanium (HPGe) detectors are well suited to the analysis of small amounts of environmental samples, as they can combine both low background and high detection efficiency. A low-background well-type detector is installed in the Modane underground Laboratory. In the well geometry, coincidence-summing effects are high and make the construction of the full energy peak efficiency curve a difficult task with an usual calibration standard, especially in the high energy range. Using the GEANT code and taking into account a detailed description of the detector and the source, efficiency curves have been modelled for several filling heights of the vial. With a special routine taking into account the decay schemes of the radionuclides, corrections for coincidence-summing effects that occur when measuring samples containing 238U, 232Th or 134Cs have been computed. The results are found to be in good agreement with the experimental data. It is shown that triple coincidences effect on counting losses accounts for 7-15% of pair coincidences effect in the case of 604 and 796 keV lines of 134Cs.

Coincidence-summing in gamma-ray spectrometry by excitation of matrix X-rays

Sources containing 75Se in a Pb(NO3)2 solution and in water matrices were measured using a well-type and a coaxial HPGe detector. The excitation and detection of characteristic lead X-rays increase coincidence losses from the peaks of 75Se in the case of the Pb(NO3)2 source. Sum peaks between gamma-rays of selenium and lead X-rays are observed in the spectra. The experimental results are in accordance with results of computations carried out with an extension of the GESPECOR Monte Carlo program.