Design of a low dose rate calibration facility for environmental radiation protection dosimetry

Clementine Chardeur (1) , jean-marc bordy (1) , Kaoutar Lahal (1) , Johann Plagnard (1)

1 LNHB (CEA, LIST) - Laboratoire National Henri Becquerel (CEA, LIST)

Introduction In normal situations, the natural H*(10) rates measured in France in the environment represent on average a few tens of nSv/h. The calibration or verification of the devices used for these measurements must be performed with H*(10) rates close to those to be measured. In order to reach a level of uncertainty compatible with the metrological needs. Low background facilities are therefore necessary, i.e. lower by a factor of about 10 compared to the natural background. Methods and Results Inspired by an existing Japanese facility and following the main specifications of the ISO FDIS 20956 standard, the LNHB decided to set up a photon beam within a shielded box (1mx1mx1.2m) in which the natural radiative level is reduced by at least a factor of 10. The steps of the installation and the characterization of the photon beams within this shielded box are described. The identification and the spectrometric characterization up to 2 MeV of the components of the natural radiative background allow choosing 25 mm thick lead walls, by means of Monte Carlo calculations. The calculations show a theoretical decrease of almost 2 decades of the background inside the box. The calculation of the theoretical H*(10) rate led to install two 60Co photon sources of 50 and 300 kBq. The collimator opening gives a beam radius of 15 cm at the reference plane located at 40 cm with a scattered radiation component of about 15% at the measurement point and a profile varying by about 18% between the edge and the center of the beam in terms of dose equivalent. This collimator would be optimized to lower these values. The traceability of the dose equivalent measurements to the French primary reference in terms of air kerma is achieved by calibrating a BERTIN type 6150AD survey meter in a collimated photon beam with higher dose rates. Conclusions The results of the modelling are compared to the measurements and the uncertainty budget is evaluated following the ISO 13005 guide method.

Mots clés: instrumentation; nuclear instrumentation; dosimetry; natural H*(10) rate; metrology; natural radiative background; ISO FDIS 20956 standard; photon beam; radioactivity; ionizing radiation; shielded box; spectrometry; Monte Carlo; simulation; particle transport; collimator; radiation scattering; dose equivalent; dose measurement; French primary reference; air kerma; calibration; modelling

Domaines: Instrumentations et Détecteurs [physics.ins-det]; Physique Médicale [physics.med-ph]

Clementine Chardeur, jean-marc bordy, Kaoutar Lahal, Johann Plagnard. Design of a low dose rate calibration facility for environmental radiation protection dosimetry. ICDA-4 - 4th International Conference on Dosimetry and its Applications, Oct 2023, Valence, Spain. ⟨cea-04554442⟩