M. Allab

Monte-Carlo investigation of radiation beam quality of the CRNA neutron irradiator for calibration purposes

An irradiation system has been acquired by the Nuclear Research Center of Algiers (CRNA) to provide neutron references for metrology and dosimetry purposes. It consists of an (241)Am-Be radionuclide source of 185 GBq (5Ci) activity inside a cylindrical steel-enveloped polyethylene container with radially positioned beam channel. Because of its composition, filled with hydrogenous material, which is not recommended by ISO standards, we expect large changes in the physical quantities of primary importance of the source compared to a free-field situation. Thus, the main goal of the present work is to fully characterize neutron field of such special delivered set-up. This was conducted by both extensive Monte-Carlo calculations and experimental measurements obtained by using BF(3) and (3)He based neutron area dosimeters. Effects of each component present in the bunker facility of the Algerian Secondary Standard Dosimetry Laboratory (SSDL) on the energy neutron spectrum have been investigated by simulating four irradiation configurations and comparison to the ISO spectrum has been performed. The ambient dose equivalent rate was determined based upon a correct estimate of the mean fluence to ambient dose equivalent conversion factors at different irradiations positions by means of a 3-D transport code MCNP5. Finally, according to practical requirements established for calibration purposes an optimal irradiation position has been suggested to the SSDL staff to perform, in appropriate manner, their routine calibrations.

Neutron field characterisation of the OB26 CRNA irradiator in view of its use for calibration purposes.

The main goal of the present work is to characterise the neutron field of an OB26 irradiation system acquired by the Nuclear Research Center of Algiers for radiation protection purposes. Extensive Monte-Carlo (MC) calculations and measurements using BF(3)- and (3)He-based neutron area dosemeters were performed to estimate the contribution, on the energy neutron spectrum, of each component present in the bunker facility of the Algerian Secondary Standard Dosimetry Laboratory (SSDL) where the irradiator has been installed. For this purpose, new irradiation configurations based on the (241)Am-Be source placed in the OB 26/2 biological shielding inside its environment have been investigated by MC simulations, and comparison with the ISO spectrum has been performed. During MC simulations, sensitivity analysis has been considered to estimate the effect of several physical parameters on the neutron fluence and dose equivalent rates. In addition, the contribution of the gamma dose equivalent rates to the total neutron dose equivalent rates was estimated for both selected source-detector distances (SDDs) 150 and 200 cm. Finally, a theoretical approach has been adopted, using MCNP5 fluence rates, to estimate the readings of the instruments taking into account their response functions. A low mean difference (12 %) between measured and predicted dose equivalent rates for two selected SDDs has been observed. Overall, the obtained MCNP5 results regarding the actual SSDL irradiation facility are particularly encouraging, but need to be supported by further experimental data.

Accurate characterization of weak neutron fields by using a Bayesian approach

A Bayesian analysis of data derived from neutron spectrometric measurements provides the advantage of determining rigorously integral physical quantities characterizing the neutron field and their respective related uncertainties. The first and essential step in a Bayesian approach is the parameterization of the investigated neutron spectrum. The aim of this paper is to investigate the sensitivity of the Bayesian results, mainly the neutron dose H*(10) required for radiation protection purposes and its correlated uncertainty, to the selected neutron spectrum parameterization.

A comparative evaluation of luminescence detectors: RPL-GD-301, TLD-100 and OSL-AL2O3:C, using Monte Carlo simulations

The luminescent dosimeters are widely used in clinical practice, for the monitoring of patient dose in external radiation therapy. Three of the most common dosimeter categories are the thermoluminescence (TLDs), the radiophotoluminescence (RPLs) and the optically stimulated luminescence (OSLs), with similar physical processes on their properties. The aim of the present study is to compare and evaluate the dosimetric properties of three specific luminescent detectors namely: a) RPL glass dosimeter, commercially known as GD-301, b) lithium fluoride TLD-100 (LiF:Mg,Ti) and c) carbon-doped aluminum oxide (Al2O3:C). For this purpose, Monte Carlo simulations were applied, using the MCNP5 code to estimate the responses of these dosimeters in terms of absorbed dose, output factor, the angular and energy dependence. In the present study, we found that the differences between the output factors were less than ± 4.2% for all detector materials RPLGD, TLD and OSLD. The variations in sensitivity for angles up to ± 80 degrees from the central axis of the beam were approximately 0.5%, 0.8% and 1.5% for the TLD-100, GD-301 and Al2O3:C, respectively. The energy dependence of the RPL and OSL dosimeters are stated as less than a 2.2%, and within 5.8% for TLD.