Monte Carlo simulation versus semiemperical calculation of autoabsorption factors for semiconductor detector calibration in complex geometries

Calibrating GESPECOR model of computing the full-energy peak efficiency of coaxial high-purity germanium detectors by Monte Carlo simulation

In this work, a classical approach was used for calibrating the GESPECOR detector model for computing the full-energy peak efficiency of p-type coaxial HPGe detectors that is based on the use of linear least squares optimization. The key element of the work is the multiplicative model developed for approximating the values of the full-energy peak efficiency provided by GESPECOR code. It was linearized using the logarithmic transformation to allow an easy use of the linear least squares optimization. A procedure was also developed to estimate the optimal values of the parameters, describing the p-type coaxial HPGe detectors. Its application to a Canberra detector GC3018 showed that it is possible to determine accurate values of the full-energy peak efficiency computed by GESPECOR code using the optimized parameter values.

Influence of the detector on calculation of self-attenuation correction factor of volume sources

Using Monte Carlo simulation and experiment, this study shows that the self-attenuation correction factors (SACFs) for identical samples measured with different detectors are not equal. The Geant4 toolkit is used to study and the following conclusions are drawn: compared with the unattenuated case, when γ-rays with different energies are attenuated by the matrix and container, the distribution of γ-rays on the surface of the entrance window of the detector is distorted. As a consequence, the SACF should be modified to account for both the self-attenuation of the sample, and the corresponding changes in detector efficiency as a result of the changes in the spatial distribution and to the energy spectra caused by the self-attenuation of the source matrix. This paper demonstrates that within certain energy ranges, and for a certain range of detectors, the required SACF modifications are not negligible. Detector parameters should therefore be considered and included when calculating the SACF.

Monitoring system of oil by-products interface in pipelines using the gamma radiation attenuation

This paper presents a methodology to precise identify the interface region, which is formed in the transport of petroleum by-products in polyducts, using gamma densitometry. The simulated geometry is compose for a collimated ¹³⁷Cs source and a NaI(Tl) detector to measure the transmitted beam. The modeling was validated experimentally on stratified flow regime using water and oil. The different volume fractions were calculated using the MCNPX code in order to determine the region interface with an accuracy of 1%.

Optimization of the marinelli beaker dimensions using genetic algorithm

A computational code, based on the genetic algorithm and MCNPX version 2.6 code was developed and used to investigate the effects of some important parameters of HPGe detector (such as Al cap thickness, dead-layer thickness and Ge hole size) on optimum dimensions of marinelli beaker. In addition, the effects of detector material on optimal beaker dimensions were also investigated. Finally, the optimized beaker dimensions at various beaker volumes (300, 500, 700, 1000 and 1500 cm³) were determined for some conventional Ge detectors with different crystal sizes (16 sizes). These sets of data then were used to drive mathematical formulas (obtained by best fitting to data sets). The results showed that, there is no meaningful correlation between the optimum dimensions of the beaker and each of the dead-layer thickness, Al cap thickness and the Ge-crystal hole size. On the other hand, the optimum beaker radius increases with decreasing the density of the detector material while the beaker height decreases.

Determination of HPGe detector response using MCNP5 for 20–150keV X-rays

The Monte Carlo method was used to calculate the efficiency, escape and Compton curves of a planar high-purity germanium detector (HPGe) in the 20-150 keV energy. These curves were used for the determination of photons spectra produced by an X-ray machine in order to allow a precise characterization of photon beams applied to medical diagnosis. The detector was modeled with the MCNP5 computer code and validated by comparison with experimental data. The air kerma calculated after the spectra stripping was compared with ionization chamber measurements.

Monte Carlo simulation of the self-absorption corrections for natural samples in gamma-ray spectrometry

Gamma-ray self-attenuation corrections in the energy range 60-2000 keV were evaluated by means of Monte Carlo calculations for environmental samples in a cylindrical measuring geometry. The dependence of the full-energy peak efficiency on the sample density was obtained for some particular photon energies and, as a result, the corresponding self-attenuation correction factors were obtained. The calculations were performed by assuming that natural materials have mass attenuation coefficients very similar to those of water in the energy range studied. Three different HpGe coaxial detectors were considered: an n-type detector with 44.3% relative efficiency and two p-type detectors of relative efficiencies 20.0% and 30.5%. Our calculations were in very good agreement with the self-attenuation correction factors obtained experimentally by other workers for environmental samples of different densities. This work demonstrates the reliability of Monte Carlo calculations for correcting photon self-attenuation in natural samples. The results also show that the corresponding correction factors are essentially unaffected by the specific coaxial detector used.

Transfer of the efficiency calibration of Germanium gamma-ray detectors using the GESPECOR software

The GESPECOR software was extended to incorporate procedures for the computation of the efficiency transfer factor for cases of practical interest: (a) sources with identical geometry, but different matrices, (b) sources with similar (but not identical) geometry; and (c) transfer from a point source to a volume source. Fast and accurate algorithms. based on correlated sampling, were implemented for solving the first two cases. A procedure to take into account the imperfect charge collection in the detector was implemented.

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.