Application of GEANT4 simulation on calibration of HPGe detectors for cylindrical environmental samples.

The determination of radionuclide activity concentration requires a prior knowledge of the full-energy peak (FEP) efficiency at all photon energies for a given measuring geometry. This problem has been partially solved by using procedures based on Monte Carlo simulations, developed in order to complement the experimental calibration procedures used in gamma-ray measurements of environmental samples. The aim of this article is to apply GEANT4 simulation for calibration of two HPGe detectors, for measurement of liquid and soil-like samples in cylindrical geometry. The efficiencies obtained using a simulation were compared with experimental results, and applied to a realistic measurement. Measurement uncertainties for both simulation and experimental values were estimated in order to see whether the results of the realistic measurement fall within acceptable limits. The trueness of the result was checked using the known activity of the measured samples provided by IAEA.

Primary standardization and determination of gamma ray emission intensities of Ho-166.

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN-CNEN/SP, in São Paulo, for the primary standardization of 166Ho is described. The activity of 166Ho was determined by the efficiency extrapolation technique applied to a 4πß(PC)-γ coincidence system using a gas flow proportional counter in 4π geometry coupled to a 76 × 76 mm NaI(Tl) crystal. The results for the γ-rays intensities at 80.57 and 1379.45 keV were 0.0651(11) and 0.00904(11), respectively.

MCNP APPROACHES FOR DOSE RATES MODELING IN LABORATORY FOR NEUTRON ACTIVATION ANALYSIS AND GAMMA SPECTROMETRY AT OSTRAVA.

The Laboratory for Neutron Activation Analysis and Gamma Spectrometry at the VSB-Technical University of Ostrava was equipped with the neutron generator MP320 operating on the principle of the deuterium-tritium fusion and producing 108 neutrons/s at maximum. To ensure radiation protection of radiation workers and public outside the laboratory, the concrete shielding was designed and its protection efficiency was validated by MCNP simulations. Three approaches to calculate the dose rates were compared. The dose rates were estimated for the ORNL MIRD phantom located at the relevant positions (Tally F6 and *F8) and using the MCNPX mesh tally feature with the new ICRP Publication 116 flux-to-dose conversion factors. It was proven that the Approach II in which the absorbed dose rates due to neutrons for all organs are computed using the cell tally F6 and the photon dose calculation is performed by the *F8 energy deposition tally is the most valuable one.

Gamma-ray detection efficiency of the microchannel plate installed as an ion detector in the low energy particle instrument onboard the GEOTAIL satellite.

A microchannel plate (MCP) assembly has been used as an ion detector in the low energy particle (LEP) instrument onboard the magnetospheric satellite GEOTAIL. Recently the MCP assembly has detected gamma rays emitted from an astronomical object and has been shown to provide unique information of gamma rays if they are intense enough. However, the detection efficiency for gamma rays was not measured before launch, and therefore we could not analyze the LEP data quantitatively. In this article, we report the gamma-ray detection efficiency of the MCP assembly. The measured efficiencies are 1.29%+/-0.71% and 0.21%+/-0.14% for normal incidence 60 and 662 keV gamma rays, respectively. The incident angle dependence is also presented. Our calibration is crucial to study high energy astrophysical phenomena by using the LEP.

Nondestructive characterization of radioactive waste drums by gamma spectrometry: a Monte Carlo technique for efficiency calibration.

A semi-empirical non-destructive technique to assay radioactive waste drums is presented. The technique is based on gamma spectrometry performed using a portable NaI detector and Monte Carlo simulations using the MCNP code in order to derive the gamma ray detector efficiency for the volume source. The derivation of detector efficiency was performed assuming homogeneous distribution of the source activity within the matrix material. Moreover, the MCNP model was used to examine the effect of inhomogeneities in activity distribution, variation of matrix material density, and drum filling height on the accuracy of the technique, and to estimate the measurement bias. The technique was verified by estimating radioactivity levels in 25 drums containing ion exchange resin waste, and comparing the results of the non-destructive method against the analytical results of samples obtained from each drum. Satisfactory agreement between the two assay techniques was observed. The discussed technique represents a cost effective technology that can be used to assay low-activity, low-density waste drums provided the contribution to the gamma ray spectrum can be resolved.

Proficiency testing with uncertainty evaluation for measuring activities per unit mass of 134Cs and 137Cs in brown rice in Japan.

In Japan, we conducted proficiency testing of activity measurement by using high-purity germanium detectors for 134Cs and 137Cs in brown rice grains. Among 176 reported results, 86 % (for 134Cs) and 93 % (for 137Cs) of the results satisfied |En| â‰¦ 1. However, 58 reports for 134Cs and 51 reports for 137Cs had some failures in their evaluations of uncertainties. The proficiency testing was effective to improve the ability of uncertainty evaluation.

Standardization of 222Rn by LSC and comparison with alpha- and gamma-spectrometry.

Liquid scintillation counting (LSC) was used for the measurement of 222Rn in equilibrium with its daughters, with detection efficiency close to 5. The appropriate corrections were considered, including one related to the probability that the 165-micros half-life 214Po decays during the dead time of the counter initiated by the disintegration of his parent nuclide, 214Bi. The dead-time determination of a commercial LS counter is also presented using a 222Rn standard source. The LSC 222Rn sources were prepared by transfer of 222Rn produced by a solid 226Ra source into LSC cocktail frozen at 77K, flame-sealed afterwards. They were measured using the LNHB triple coincidence counter with adjustable extending-type dead-time unit, between 8 and 100 micros; two different procedures were used to calculate an effective dead time and then to deduce the counting rate extrapolated to zero dead-time value. The LSC results were compared with those obtained by cryogenic alpha-particle spectrometry (LNHB system) and by gamma-ray spectrometry for the same radon source in the LSC vial; the geometry transfer coefficient was calculated using the ETNA software. Measurement results and uncertainties are discussed.

Application of PENELOPE code to the efficiency calibration of coaxial germanium detectors.

The code PENELOPE is applied here to the efficiency calibration exercise described in the EUROMET project 428 [Lépy et al., 2001. Appl. Radiat. Isot. 55, 493]. We evaluated the peak efficiencies for a coaxial HPGe detector in the range 60-2000 keV, for point sources located at various distances from the detector and for a cylindrical box containing two different matrixes: a low-density silica and a hydrochloric acid solution. After an optimization of several detector parameters, the results were in good agreement with the experimental values.

Development of gamma-ray emitting test sources for portal monitors.

A new set of gamma-ray emitting test sources was designed by the National Institute of Standards and Technology (NIST) for testing of radiation portal monitors for homeland security applications. This paper provides a description of the construction and characterization of these new sources that were designed and built to meet the American National Standard Institute (ANSI) N42.35-2004 and N42.38 standards' requirements.

Standardization of 99mTc.

The radioactivity of 99mTc was standardized by the 4piPC-gamma coincidence method with two different modes. One is using coincidences between (119.5-142.6)keV conversion electrons and K X-rays, and the other is coincidences between the 2.13 keV conversion electrons and 140.5 keVgamma-rays. The background of the K X-ray peak and the sensitivity of the proportional counter (PC) to 140 keV gamma-rays were the main sources of uncertainties in the first case and low detection efficiency for conversion electrons in the second case. General coincidence equations were written, with specific forms, for the three measurement variants, including literature variant. Comparison with the ionization chamber calibration is reported.

Primary standardization of 72Ga.

The activity of 72Ga sources produced by irradiation at the IEA-R1 reactor have been performed in a 4pibeta-gamma coincidence system by using the extrapolation technique. The measurements were undertaken selecting two windows in the gamma-channel, in order to check the consistency of the results. A Monte Carlo calculation was performed in order to predict the behavior of the observed activity as a function of the 4pibeta detector efficiency and the results were compared to experimental values.

Standardization of 241Am solution.

The standardization of 241 Am solution has been undertaken using a 4pibeta-gamma coincidence system. The 4pi proportional counter used for alpha detection has a 0.1 mm thick Al window in the outside wall in order to minimize gamma-ray attenuation. The extrapolation technique was applied to determine the activity of the solution. The variation of alpha efficiency has been made using external absorbers and by the electronic discrimination. The results from the two methods were compared and are in agreement within the experimental uncertainty.

Standardization of 152Eu and 154Eu by 4pibeta-4pigamma coincidence method and 4pi(beta+gamma) integral counting.

The 4pibeta-4pigamma coincidence counting and 4pi(beta+gamma) integral counting techniques were applied for the standardization of 152Eu and 154Eu. In these techniques, the beta-detector is composed of two thin plastic scintillators sandwiching the source coupled with a slender photomultiplier tube. This beta-detector was inserted into a large well-type NaI(Tl) scintillation detector for gamma-ray detection, making a 4pibeta-4pigamma detector configuration. The results obtained by the above two techniques were in good agreement and consistent with the results of international comparisons.

Standardisation of 54Mn and 65Zn using a software coincidence counting system.

The activities of 54Mn and 65Zn have been determined by 4pi(PC)-gamma coincidence counting, with efficiency variation performed by the conventional method of altering the self-absorption in the sources as well as by the computer discrimination method. The standardisation of 65Zn presents some complications requiring optimisation of the gamma-ray energy window settings to achieve a linear efficiency-extrapolation curve. Determination of these optimal settings by the conventional coincidence method is a tedious task. These difficulties have been reduced by the utilisation of a software coincidence counting system that records time and amplitude information of individual pulses from coincidence measurements, where the coincidence parameters are set after the data collection process has completed, facilitating multiple data evaluations on a single data set. The optimal gamma-ray energy window settings for the 65Zn standardisation were derived from the results of the 54Mn standardisation, as well as from studies of the 65Zn data itself. The setting of the PC channel thresholds for K and both (K+L) electrons is also discussed. The results are compared with those attained using conventional coincidence counting.

Absolute counting of 188Re radiopharmaceuticals.

The Institute of Nuclear Energy Research (INER) has provided 188Re radiopharmaceuticals for hospitals in Taiwan. To enhance the accuracy of commercial radionuclide calibrators used by radiopharmacies and hospitals, and to ensure that patients receive proper doses of these radiopharmaceuticals, it is very important to standardize this nuclide. The 4pibeta-gamma coincidence counting method was used to standardize the mass activities of 188Re in this study. At the same time, three well type ionization chambers, Centronic IG11-N20, Centronic IG11-A20, and ISOCAL-IV, were calibrated by the standardized solutions of the nuclide. In this research, the calibration figures of ISOCAL-IV for the nuclide were consistent with the results of National Physical Laboratory (NPL, UK). The outcome implied that the results of coincidence counting did mutually agree. On the other hand, the radionuclide calibrator in the radiopharmacy was producing measurement errors of about 20% when using the manufacturers recommended calibration setting: an accurate correction factor has now been determined in this study.

Kr-81m calibration factor for the npl ionisation chamber.

A general method has been developed for the measurement of the activity concentration of 81mKr gas. Due to its short half-life, 13.1s, this gas has to be eluted from a 81Rb/81mKr generator. The 81Rb parent has a half-life of about 4.6 h. The calibration was done in two steps: firstly, a gamma-ray spectrometer was calibrated using 51Cr and 139Ce sources, nuclides with gamma-ray energies bracketing that of 81mKr (190.5 keV). The measurement geometry was equivalent to that of the 81mKr measurement; the sources were inserted into two collimated PTFE tubes in front of the gamma-ray detector. Secondly, a calibration factor for the NPL radionuclide calibrator was determined with a specially designed ionisation chamber insert. The 81mKr gas passed in front of the gamma-ray detector in PTFE tubing before and after entering the ionisation chamber. The calibration factor for 81mKr in the radionuclide calibrator with this geometry was independent of the gas flow rate within determined limits. The analytical calculations of the activity determination, uncertainties and measurement criteria are discussed.

Activity measurement of a 176Lu sample using coincidence peaks and Monte Carlo simulations.

We present the results of the activity measurement of a sample of the rare isotope 176Lu, obtained by gamma-ray spectrometry through the use of 22 peaks in an energy spectrum acquired with a 300 cc well-type HPGe detector located in the Modane underground facility below the French Alps. Of the above peaks, 17 are coincidence peaks between two or three gamma-rays or X-rays. True coincidence summing corrections were made using our Coincal code. Monte Carlo simulations were carried out for calibration purposes.

Assessment of measurement result uncertainty in determination of (210)Pb with the focus on matrix composition effect in gamma-ray spectrometry.

Reference materials were used to assess measurement result uncertainty in determination of (210)Pb by gamma-ray spectrometry, liquid scintillation counting, or indirectly by alpha-particle spectrometry, using its daughter (210)Po in radioactive equilibrium. Combined standard uncertainties of (210)Pb massic activities obtained by liquid scintillation counting are in the range 2-12%, depending on matrices and massic activity values. They are in the range 1-3% for the measurement of its daughter (210)Po using alpha-particle spectrometry. Three approaches (direct computation of counting efficiency and efficiency transfer approaches based on the computation and, respectively, experimental determination of the efficiency transfer factors) were applied for the evaluation of (210)Pb using gamma-ray spectrometry. Combined standard uncertainties of gamma-ray spectrometry results were found in the range 2-17%. The effect of matrix composition on self-attenuation was investigated and a detailed assessment of uncertainty components was performed.

A novel method for the activity measurement of large-area beta reference sources.

A novel method has been developed for the activity measurement of large-area beta reference sources. It makes use of two emission rate measurements and is based on the weak dependence between the source activity and the activity distribution for a given value of transmission coefficient. The method was checked experimentally by measuring the activity of two ((60)Co and (137)Cs) large-area reference sources constructed from anodized aluminum foils. Measurement results were compared with the activity values measured by gamma spectrometry. For each source, they agree within one standard uncertainty and also agree within the same limits with the certified values of the source activity.

Determination of impurities in (124)I samples by high resolution gamma spectrometry.

(124)I is a radionuclide used in the diagnosis of tumors. The National Health Agency requires identification and activity measurement of impurities. Using gamma spectrometry with an efficiency calibrated high-purity germanium detector, impurities (125)I and (126)I in an (1)(24)I production sample were identified. Activity ratios of (125)I and (126)I to (124)I were approximately 0.5% and 98%, respectively.