Dosimetry formalism and calibration procedure for electronic brachytherapy sources in terms of absorbed dose to water.

The LNE-LNHB has developed a methodology to standardize electronic brachytherapy sources in terms of absorbed dose to water. It is based on the measurement of the air-kerma rate at a given distance from the source and the Monte Carlo calculation of a conversion factor. This factor converts the air-kerma in measurement conditions into absorbed dose to water at a 1 cm reference depth in a water phantom. As a first application, the method was used to calibrate a Zeiss INTRABEAM system equipped with its 4 cm diameter spherical applicator. The absorbed-dose rate value obtained in the current study was found significantly higher than that provided by the manufacturer in line with the observations already reported by a few other teams.

Comparative study of two drying techniques used in radioactive source preparation: freeze-drying and evaporation using hot dry nitrogen jets.

Quantitative solid sources are used widely in the field of radionuclide metrology. With the aim to improve the detection efficiency for electrons and x-rays, a comparative study between two source drying techniques has been undertaken at LNE-Laboratoire National Henri Becquerel (LNE-LNHB, France). In this paper, freeze-drying using commercial equipment is compared with a system of drying using hot jets of nitrogen developed at Institute for Reference Materials and Measurements (IRMM, Belgium). In order to characterize the influence of self-absorption, the detection efficiencies for (51)Cr sources have been measured by coincidence counting and photon spectrometry.

Study of scattering effects in low-energy gamma-ray spectrometry.

The photon-scattering effect was studied in the low-energy range 15-80keV with planar and coaxial germanium detectors. Different experimental tests have been conducted with a point source, progressively reducing the matter around the radioactive deposit, to investigate the origin and characteristics of the different spectrum components due to scattered photons (bumps). These tests were completed by Monte Carlo simulations. Finally, a peak-shape calibration was performed using several radionuclides ((109)Cd, (241)Am, (133)Ba, (137)Cs and (152)Eu). This allowed in identifying the main contributions to the bumps and improving the spectra processing.

Measurement of (241)Am L X-ray emission probabilities.

Measurements of (241)Am L X-ray emission probabilities were conducted using both HPGe and Si(Li) detectors. The efficiency calibrations of these detectors were performed by means of a tunable, monochromatic photon beam and the determination of the thickness of absorbing materials inside the detector. These efficiency calibrations were obtained without any reference to radionuclide decay data, and with 0.8% relative standard uncertainty. The complex L X-ray region was processed using Voigt functions and by taking account of the detector response function established with the monochromatic beam. Twenty-two components of the L X-ray group were identified and quantified. The present results are compared with previously published data.

Standardization of (67)Ga using a 4pi(LS)beta-gamma anti-coincidence system.

(67)Ga is an interesting radionuclide as it is widely used in nuclear medicine. The meta-stable level related to the 93.3keV gamma-transition represents the main difficulty when using the coincidence method to standardize this radionuclide. The 4pi(LS)beta-gamma anti-coincidence system implemented at LNHB is based on the use of electronic modules specifically designed for radioactivity metrology. On the contrary to classical coincidence systems, activity measurements of (67)Ga are carried out as for prompt beta-gamma emitters; indeed, when using a live-timed anti-coincidence system with extendable dead times, the problem due to the excess of counting generated by the meta-stable level is avoided. Considering that the standardization of (67)Ga does not depend on the decay scheme parameters (except for the half-life), the measurement of the gamma-emission intensities has been performed. The standardization of this radionuclide was also a good opportunity for a new participation of our laboratory in the SIR of (67)Ga (International Reference System); the result obtained with the 4pi(LS)beta-gamma anti-coincidence system is compared with those submitted by other National Metrology Institutes (NMIs). The non-extendable dead times used in most of the participations could be one of the causes responsible for the abnormal dispersion of the results. The optimization of the standard solution of (67)Ga for the radioactive source preparation is also discussed.

High-energy resolution alpha spectrometry using cryogenic detectors.

Applications such as environment monitoring implying alpha emitters activity measurement associated with isotope identification, require high-energy resolution detectors. Conventional silicon detectors are inexpensive therefore widely used, although intrinsically limited in energy resolution. Thermal detection principle of cryogenic detectors introduces a breakthrough in alpha particle measurement. For the first time, spectra with 5.5 keV FWHM energy resolution have been obtained for several external alpha emitting sources using a copper-germanium bolometer specially developed for alpha spectrometry.

Metrological characterization of the ADONIS system used in gamma-ray spectrometry.

In gamma-ray spectrometry, new acquisition systems based on digital signal processing are now commercially available. In order to determine their performance at high count rates, the Commissariat à l'Energie Atomique (CEA), Laboratoire National Henri Becquerel has tested several of these systems. These tests have clearly shown that the performance levels announced by the manufacturers were generally not met. It was therefore a logical step to include the Atelier de DévelOppement Numérique pour l'Instrumentation en Spectrométrie (ADONIS) system in these tests. ADONIS is the new numerical system for gamma-ray spectrometry, developed by the CEA Service d'Instrumentation et d'Application des Rayonnements.

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.

Cryogenic detectors below 100 mK for X-ray measurements in metrology

Due to the intrinsic performances of cryogenic detectors such as energy resolution, LPRI has decided to use these devices to improve the quality of the radioactive measurements usually obtained with classical semiconductor detectors. A bolometer with a 10 microg tin absorber has been developed at IAS (Institut d'Astrophysique Spatiale) and has been tested in the cryogenic installation of LPRI; an energy resolution (full width half maximum, FWHM) of 39 eV has been obtained on the Kalpha line of Mn. Besides these good spectrometry results, an absolute activity measurement using bolometers is proposed by adapting an absorber geometry for 4 pi counting.

Results from the EUROMET action 410 concerning the decay data of 169Yb

An international comparison EUROMET, action No. 410, was organized with the objective of improving the knowledge of nuclear data for 169Yb decay. To determine the photon emission probabilities, the participants were asked to measure at least one of the quantities, activity per unit mass and/or photon emission rate per unit mass. In addition, the participants were requested to report the count rates observed with point source samples for an eventual coaxial-type germanium detector characterization. Eleven laboratories participated, giving one or more sets of results. In all, 34 sets of results were received, 17 for the activity measurement, 11 for the photon emission rate measurement and six for the detector characterization. Using the accurate activity value obtained from this exercise, it was possible to determine the emission probabilities of the main X- and gamma-rays with an uncertainty of 1-2% for the LX-rays, 1% for the KX-rays and < or =0.5% for the main gamma-rays. These data can be very useful for the calibration of so-called gamma-X ray detectors'. The measurement of 169Yb with type P or N coaxial structure detectors has also made possible an estimation of their diameters and volumes.

Efficiency transfer and coincidence summing corrections for gamma-ray spectrometry

ETNA (Efficiency Transfer for Nuclide Activity measurements) is a new computer program providing both detector efficiency transfer for various types of sources for coaxial source-detector geometries and coincidence summing corrections. This program, available for use on PCs, has a direct link to the computerised database NUCLEIDE in order to automatically import the necessary radionuclide decay data. The specific features and facilities are outlined and examples are given.