Design and construction of a new chamber for measuring the thickness of alpha-particle sources.

The thickness of charged-particle emitting sources can be determined by varying the incidence angle of particles using silicon semiconductor detectors. The differences in energy between the peaks for an alpha emission measured with different incident angles are due to the energy loss of alpha particles in the source. These shifts can thus be used to estimate the source thickness. A new detection chamber has been constructed for this purpose. Its advantage is the registration of the alpha particles emitted from a given source using up to three different detectors simultaneously. Monte Carlo simulation was used to help to determine the best measurement conditions and interpret the results.

An intercomparison of Monte Carlo codes used in gamma-ray spectrometry.

In an intercomparison exercise, the Monte Carlo codes most commonly used in gamma-ray spectrometry today were compared with each other in order to gauge the differences between them in terms of typical applications. No reference was made to experimental data; instead, the aim was to confront the codes with each other, as they were applied to the calculation of full-energy-peak and total efficiencies. Surprising differences between the results of different codes were revealed.

Study of the 243Am decay.

Gamma-ray emission probabilities for 243Am were published in 1996. Three new gamma transitions were then found: 46.84, 98.36 and 102.02keV. In the last evaluation by the Decay Data Evaluation Project, the emission probabilities of 46.84 and 102.02keV transitions were not included. Alpha-gamma coincidence measurements have been now used to check and revise the values for these two transitions and their emission probabilities. The emissions of the 31.13, 43.53, 74.66, 86.71 and 141.89keV transitions were also studied.

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.

Radon in underground waters as a natural analogue to study the escape of CO2 in geological repositories.

Activity concentrations of dissolved (222)Rn and (226)Ra were measured in several underground aquifers, which are candidates for repositories or for the study of analogue natural escapes of CO2. The concentration of both radionuclides in water was determined using liquid scintillation counting. The values obtained for the (222)Rn concentrations varied from 0 to 150 Bq l(-1), while the levels of (226)Ra were in general very low. This indicates that (222)Rn is coming from the decay of the undissolved (226)Ra existing in the rocks and deep layers of the aquifers, being later transported by diffusion in water.

Radioactivity in bottled mineral waters.

Consumption of bottled mineral water is a growing practice and is sometimes a necessity rather than a choice. In this work, a study of the radioactive content of a wide selection of commercial bottled mineral waters for human intake was carried out. The origins of the analyzed waters were very different, coming from various locations in France, Portugal and Spain. Their total alpha and beta activity concentrations were determined and also gamma spectrometry was used to detect some radionuclides. In some cases, the waters presented high values of the total alpha and beta activity concentrations surpassing the reference levels established by the CSN, the Spanish. Regulatory Organization. In these cases, a determination of uranium and 226Ra was also performed by using low-level liquid scintillation counting. The results revealed a strong correlation between radioactive content and dry residue, and lead one to conclude that high radioactive content is mainly related to the mineralization in waters of underground origin.

Improvements to alpha-particle spectrometry techniques.

Improvements to the conventional methods for alpha-particle spectrometry measurement and analysis have recently been implemented in our laboratory. They include the application of corrections for energy drift in long-duration measurements, the development and application of an efficient deconvolution method for complex alpha spectra using the new computer code ALFITeX, an alpha-gamma coincidence system using a dual-parameter multichannel analyzer, and the digitization of this coincidence system.

Measurements of alpha-gamma coincidences with an optimized dual-parameter multichannel system.

Measurements of alpha-gamma coincidences have usually been carried out using a single channel to detect alpha-particles of a given energy, and a multichannel analyser for the detection of the corresponding coincident gamma-rays. An alpha-gamma coincidence chamber coupled to the electronic chain ending with a dual-parameter multichannel analyser has been developed and optimized. This system simultaneously stores alpha-particle, gamma-ray, and alpha-gamma coincidence spectra, which allows a general analysis to be made of the degree of coincidence between each alpha-particle and each gamma-ray emission. With this technique, a two-dimensional spectrum was obtained and analysed using "contour graphics". An application to the study of the decay scheme of (241)Am is described.

Sample quality index to preselect suitable carbonate samples for alpha spectrometry U/Th dating.

Accurate dating of samples by alpha spectrometry presupposes that the dated system behaved as closed and that no allochthonous (232)Th was provided to the system. Therefore the selection of appropriate samples is a fundamental task that will determine the reliability and accuracy or not of the results. In this paper, by means of a thorough characterisation, those petrological and chemical variables that allow the preselection of the most suitable samples to be dated have been identified and used to calculate the sample quality index.

Determination of alpha activity in solid samples by leaching or digestion.

The analysis of solid environmental samples by α-particle spectrometry generally involves processes requiring an initial treatment to destroy the complexes in the matrix, and dissolve the radionuclides of interest. This study compared the activities obtained for several natural radionuclides ((238)U, (234)U, (230)Th, (226)Ra, and (210)Po) in previously well-characterized materials, using two procedures: acid leaching and digestion in a microwave oven. The measured activities were in general very similar for the two procedures, with a strong statistical correlation between the two sets of values.

Application of the Monte Carlo code DETEFF to efficiency calibrations for in situ gamma-ray spectrometry.

We studied the applicability of the Monte Carlo code DETEFF for the efficiency calibration of detectors for in situ gamma-ray spectrometry determinations of ground deposition activity levels. For this purpose, the code DETEFF was applied to a study case, and the calculated (137)Cs activity deposition levels at four sites were compared with published values obtained both by soil sampling and by in situ measurements. The (137)Cs ground deposition levels obtained with DETEFF were found to be equivalent to the results of the study case within the uncertainties involved. The code DETEFF could thus be used for the efficiency calibration of in situ gamma-ray spectrometry for the determination of ground deposition activity using the uniform slab model. It has the advantage of requiring far less simulation time than general Monte Carlo codes adapted for efficiency computation, which is essential for in situ gamma-ray spectrometry where the measurement configuration yields low detection efficiency.

The effect of source chemical composition on the self-attenuation corrections for low-energy gamma-rays in soil samples.

To study the influence of the source's chemical composition on the self-attenuation corrections during the activity quantification of low-energy gamma-ray emitters (< 60keV) in soil samples, we conducted a Monte Carlo-based hypothetical proficiency test in which different analytical laboratories analyze a soil sample, assuming the same density but a different chemical composition. The bias in the activity values was between 4% and 70%, with more than 50% of these results being unacceptable. Our work shows that collecting the detailed chemical composition of the samples is an essential issue to be considered by analytical laboratories that use the Monte Carlo method for the calculation of self-attenuation corrections.

Testing efficiency transfer codes for equivalence.

Four general Monte Carlo codes (GEANT3, PENELOPE, MCNP and EGS4) and five dedicated packages for efficiency determination in gamma-ray spectrometry (ANGLE, DETEFF, GESPECOR, ETNA and EFFTRAN) were checked for equivalence by applying them to the calculation of efficiency transfer (ET) factors for a set of well-defined sample parameters, detector parameters and energies typically encountered in environmental radioactivity measurements. The differences between the results of the different codes never exceeded a few percent and were lower than 2% in the majority of cases.

Study of self-absorption for the determination of gross alpha and beta activities in water and soil samples.

Source thickness is important in determinations of gross alpha and beta activities in soil or water samples because of self-absorption and energy losses. Its influence was studied theoretically, experimentally, and with Monte Carlo simulations. No problems were found for gross beta activity determinations using sources with mass thicknesses up to 25 mg/cm(2). However, to simultaneously determine gross alpha activity, the results appear to indicate that about 10 mg/cm(2) should be the upper limit to avoid problems.

Radioactive content of charcoal.

Coal and charcoal present similar physical and chemical characteristics. There is no standardized procedure to distinguish them. However, their differences in age and origin result in several differences in their natural radionuclide content. Moreover, charcoal can be contaminated with fallout. In this work, activity concentrations of (228)Th, (226)Ra, (40)K, and (137)Cs were determined in charcoal and coal samples in order to obtain evidence to distinguish the two kinds of sample.