Determination of X-ray mass attenuation coefficients using HPGe detector

A detail analysis of a method to determine the mass attenuation coefficient, μ_m, of low energy X-rays employing HPGe X-ray detector and radioactive sources is described. This method incorporates all the suggestions made by Creagh and Hubbell. An optimum distance is set under the broad beam as well as good geometry arrangement. The photon intensity is measured by gating the channel of the spectrometer at FWHM/photo peak. The selection of the absorber foils, optimum counting times are all done after detailed investigations for the μ_m value. The method is studied in detail using aluminum/nickel as an absorber since these are standard reference materials. From these results, we conclude that the “best value” for μ_m can be obtained for those thicknesses which lie in the transmission (T) range 0.5≥T≥0.02. The measured values of μ_m for Magnesium, Nickel, Copper, Molybdenum and Tantalum and three biological equivalent materials are compared with standard theoretical values.     

HPGe virtual point detector for radioactive disk sources.

Validity of the model of a virtual point detector (implying existence of a point where all interactions virtually occur) was confirmed for measurements of radioactive disk sources with HPGe detectors. The existing correlation of the count rates with the distance between the virtual plain detector and the detector face makes it possible to inter- and extrapolate calibration curves for disk radioactive sources for use at different source-detector face distances. The dependence of the distance between the virtual plane detector and the detector face on the photon energy was studied for sources of various radii.     

Angular response of a HPGe detector to gamma-rays in in situ measurements.

The angular response of a HPGe detector used in in situ measurements was determined empirically as a function of gamma-ray energy at two source-detector geometries--the standard 1 m and the non-standard 0.1 m heights above ground. Values calculated by a Monte Carlo code were found to agree well with the experimental values. The code is thus labor-saving, because it can spare many measurements required for the empirical determination of the angular response in an unknown experimental configuration.     

Measurement of the G-value for 1.5 keV X-rays

Although there are several theoretical predictions of the dependence of the G-value on X-ray energy, measurements have not been made below approximately equal to 7 keV. Using a ferrous sulfate solution modified by the addition of benzoic acid, we have measured the relative G-values for Alk characteristic X-rays (1.5 keV), 238Pu alpha-particles (3.7 MeV), 60Co (1.17 MeV) and 137Cs (0.66 MeV) gamma-rays. This modified ferrous sulfate solution gave a 4-fold increase in sensitivity relative to the conventional solution, making measurements with the Alk X-rays feasible. The relative ferrous-to-ferric conversions as a function of dose were similar for the two gamma-ray energies, yielding G-values of 1.62 and 1.59 mumol J-1 for the 60Co and 137Cs radiations, respectively. The alpha-particle G-value was 0.52 mumol J-1, or 31 per cent of that for the 60Co gamma-rays, in good agreement with previous measurements. The Alk X-rays had a G-value of 0.92 mumol J-1 or 57 per cent of that of the 60Co radiation. This G-value for the 1.5 keV X-rays is within 20 per cent of the values predicted by current theories, and theoretical values are within the error range of our measurement. The consistency between the experimental value reported here and theoretical G-values for ultrasoft X-rays should be valuable for models of radiation action on biological systems.     

Absolute calibration of 60Co by using sum-peak method and an HPGe detector.

The sum-peak method was applied to calibrate four 60Co sources in the range of 25-350kBq. An HPGe-based gamma-ray spectroscopy system was used, and the effects of the angular correlation and pulse pile-up were corrected, and are discussed. The correction for the angular correlation was obtained from a theoretical calculation, while the pulse pile-up effect was corrected using a new extrapolation technique based on the pile-up measurements by varying the shaping time of the amplifier. The determined radioactivities were within 2% deviation from the values reported by the supplier and the associated uncertainties were less than 2%. The observed consistency was obtained under the experimental condition of keeping the total counting rate 相似文献   

Experimental and MC determination of HPGe detector efficiency in the 40-2754 keV energy range for measuring point source geometry with the source-to-detector distance of 25 cm.

A precise model of a 40% relative efficiency p-type HPGe detector was created for photon detection efficiency calculation using the MCNP code. All detector parameters were determined by different experiments. No experimental calibration points were used for the modification of detector parameters. The model was validated by comparing calculated and experimental full energy peak efficiencies in the 40-2754 keV energy range, for point-source geometry with the source-to-detector distance of 25 cm.     

The virtual point detector concept for HPGe planar and semi-planar detectors.

The virtual point detector assumption was validated for planar and semi-planar detectors. The distance of the virtual point detector from the detector face is larger than the thickness of the detector, which means that the virtual detector position is outside the volume of the detector.     

Estimation of background spectrum in a shielded HPGe detector using Monte Carlo simulations

Monte Carlo simulations are powerful tools used to estimate the background γ-radiation detected by high-resolution gamma-ray spectrometry systems with a HPGe (high purity germanium) detector contained inside a lead shield. The purpose of this work was to examine the applicability of Monte Carlo simulations to predict the optimal lead thickness necessary to reduce the background effect in spectrometer measurements. GEANT4 code was applied to simulate the background radiation spectrum at different thicknesses of lead. The simulated results were compared with experimental measurements of background radiation taken at the same shielding thickness. The results show that the background radiation detected depends on the thickness, size and lining of the shield. Simulation showed that 12 cm lead thick is the optimal shielding thickness.     

Monte Carlo determination of full energy peak efficiency for a HPGe detector.

The Monte Carlo method was used to determine full energy peak efficiency of a high-purity germanium (HPGe) co-axial detector within the energy range of 59.5-1836 keV. Plotted ratios of the experimentally derived efficiency data over the fitted values showed oscillations at certain energies attributed to the characteristics of the detector. Results obtained by the Monte Carlo yielded deviations between 0.2 to 12% from the experimental data.     

Determination of mass attenuation coefficients of some boron ores at 59.54 keV by using scintillation detector

The mass attenuation coefficients of the 59.54 keV radiation of ²⁴¹Am point source in boron ores such as tincal, ulexite and colemanite were determined experimentally by a scintillation detector and theoretically. Since boron ores contain boron, hydrogen, and a lot of elements, they may be used as shielding against neutrons and gammas simultaneously, e.g. for shielding ²⁴¹Am/Be neutron sources, as they emit both gammas and neutrons.     

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.     

Determination of neutron flux distribution in an Am-Be irradiator using the MCNP.

A neutron irradiator has been assembled at IPEN facilities to perform qualitative-quantitative analysis of many materials using thermal and fast neutrons outside the nuclear reactor premises. To establish the prototype specifications, the neutron flux distribution and the absorbed dose rates were calculated using the MCNP computer code. These theoretical predictions then allow one to discuss the optimum irradiator design and its performance.     

Analytical approach for radioactivity correlation of disc sources with HPGe detector efficiency

The HPGe detector efficiency is measured as a function of source to detector separation using disc sources of ¹³¹I with diameter ranging from 10 to 400 mm. Detector efficiencies are characterized using single photon point-like standard sources at different distances; the calculated efficiencies for disc sources were analyzed by utilizing the double point detector model (DPDM) and the efficiency transfer method. The developed approach provided satisfactory results. The axial variation and radial dependence for disc sources efficiency determination in gamma-ray spectrometry were described with both gamma ray standard sources and measured samples as their extended sources.     

Monte Carlo based geometrical model for efficiency calculation of an n-type HPGe detector

A procedure to optimize the geometrical model of an n-type detector is described. Sixteen lines from seven point sources (²⁴¹Am, ¹³³Ba, ²²Na, ⁶⁰Co, ⁵⁷Co, ¹³⁷Cs and ¹⁵²Eu) placed at three different source-to-detector distances (10, 20 and 30 cm) were used to calibrate a low-background gamma spectrometer between 26 and 1408 keV. Direct Monte Carlo techniques using the MCNPX 2.6 and GEANT 4 9.2 codes, and a semi-empirical procedure were performed to obtain theoretical efficiency curves. Since discrepancies were found between experimental and calculated data using the manufacturer parameters of the detector, a detail study of the crystal dimensions and the geometrical configuration is carried out. The relative deviation with experimental data decreases from a mean value of 18–4%, after the parameters were optimized.     

Dosimetric characterization of a brachytherapy applicator using MCNP5 modelisation and in-phantom measurements

A gynaecological applicator consisting of a metallic intra-uterine tube with a plastic vaginal applicator and an HDR Ir-192 source have been simulated with MCNP5 (Monte Carlo code). A solid phantom has been designed to perform measurements around the applicator with radiochromic films. The isodose curves obtained are compared with curves calculated with the F4MESH tally of MCNP5 with a good agreement. A pinpoint ionization chamber has been used to evaluate dose at some reference points.     

Borehole parametric study for neutron induced capture gamma-ray spectrometry using the MCNP code.

The MCNP Monte Carlo code has been used to simulate neutron transport from an Am-Be source into a granite formation surrounding a borehole. The effects of the moisture and the neutron poison on the thermal neutron flux distribution and the capture by the absorbing elements has been calculated. Thermal and nonthermal captures for certain absorbers having resonance structures in the epithermal and fast energy regions such as W and Si were performed. It is shown that for those absorbers having large resonances in the epithermal regions when they are present in dry formation or when accompanied by neutron poisons the resonance captures may be significant compared to the thermal captures.     

The response of a thermoluminescent dosimeter to low energy protons in the range 30-100 keV.

This study demonstrates the thermoluminescence (TL) response of CaF2:Tm (commercial name TLD-300) to 30-100 keV protons which were generated by means of a Cockcroft-Walton accelerator. The phenomenon in which the total thermoluminescent output from CaF2:Tm (TLD-300) decreases with proton energy from 30 to 100 keV (with increase of LET) can be interpreted by the track structure theory (TST). The analysis of the glow peaks: P2 (131 degrees C), P3 (153.5 degrees C) and P6 (259 degrees C), of TLD-300 show the oscillatory decreasing phenomenon as a function of incident proton energy, which can be interpreted with the TST and the oscillatory emission of electrons in a thermoluminescent dosimeter (TLD) that is caused by resonant or quasi-resonant charge transfer in ion-atom interactions in this TLD-300.     

Monte Carlo simulation of the response of a germanium detector for low-level spectrometry measurements using GEANT4.

In this paper, we will expose the work performed for the adaptation and refinement of the GEANT4 simulation toolkit (originally designed for Monte Carlo simulations in High-Energy Physics) in order to simulate Ge detectors in low-level gamma spectrometry. Special emphasis will be given to show and validate our own algorithms implemented in GEANT4 code for variance reduction and data analysis, that have been used for a quicker and precise efficiency calibration at different source-detector configurations of one of the low-level germanium gamma systems available in our laboratory.     

Precise efficiency calibration of an HPGe detector: source measurements and Monte Carlo calculations with sub-percent precision.

With the goal of measuring precise gamma-ray intensities for short-lived (< 5 s) accelerator-produced activities, we have calibrated the efficiency of an HPGe detector between 53 and 1836keV to sub-percent precision with a combination of source measurements and Monte Carlo calculations. Using known or independently measured detector dimensions, we have achieved both relative and absolute agreement (the latter, to 0.1%) between the calculated and measured efficiencies with only two adjustable detector parameters, the thicknesses of the contact dead layers.