Monte Carlo Study of the 60Co Calibration Field of the Dosimetry Laboratory Seibersdorf

Beschreibung

The 137Cs and 60Co gamma radiation field of the reference irradiation facility of the Dosimetry Laboratory Seibersdorf with collimated beam geometry according to ISO standard 4037-1 is used for calibrating personal and environmental radiation protection dosemeters. A close to reality simulation model of the facility was set-up using the Monte Carlo N-Particle (MCNP-4C) code. Due to the complex source geometry including the active volume, a steel and aluminium capsule, and the aluminium holder, only one 60Co source out of six available sources of different activities was investigated. The lead and concrete shielding structures around the source, the concrete and earth ground, the concrete and wooden walls and ceilings, and the air environment are included in the final model. The goal of this study is to characterise the radionuclide gamma calibration field inside the irradiation hall with a total of 20 m in length to provide information about the spatial variation of the photon fluence and the resulting air kerma distribution. For the whole range of possible source-detector-distances along the central beam axis, the computer simulation results are compared to ionisation chamber measurements carried out by the Federal Office for Metrology and Surveying (BEV). All deviations between simulated and experimental relative air kerma rates are within ±0.7%. The study is also focused on getting information which is not or not easily achievable by experimental means, i.e. changes in the gamma spectrum and the influence of scattered radiation from various facility components. In addition, the contribution of photons with different numbers of scattering events and their resulting spectra are calculated. The total scatter contributions along the central beam axis relative to the total air kerma values are compared to the ISO 4037-1 recommendations for calibration fields. For distances below 1 m and above 7 m, the scatter contribution from the collimator, the shielding, the ground, and the walls is below 1%. For distances above about 7 m, the scattered radiation contribution from the ground becomes dominant and increases the total scatter contribution to about 5% at the maximum possible source-detector-distance. An important part of this study is the investigation of a number of influences on the accuracy of the simulation results, i.e. source mass density, position uncertainty of the source, size dependence of the detector volume, and lower photon transport energy cut-off. The optimisation of those influencing parameters resulted in a mean deviation between simulated and experimental kerma rates along the beam axis of 0.2%.

Zeitraum	11 Apr. 2005 → 15 Apr. 2005
Ereignistitel	IM2005
Veranstaltungstyp	Sonstiges
Bekanntheitsgrad	International