Radiation Protection Dosimetry Advance Access originally published online on October 23, 2007
Radiation Protection Dosimetry 2008 128(3):279-288; doi:10.1093/rpd/ncm426
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Optimised geometry to calculate dose rate conversion coefficient for external exposure to photons
1 Unité de Physique Nucléaire et des Hautes énergies, Faculté des sciences de Tunis, 1080 Tunis, Tunisia
2 Centre National de Sciences et de Technologies Nucléaires, Sidi Thabet, Tunisia
* Corresponding author: adel.trabelsi{at}cern.chadel.trabelsi{at}cnstn.rnrt.tn
Received January 18, 2007, amended August 2, 2007, accepted August 9, 2007
A single-parameter geometry to describe soil is achieved for Monte Carlo calculation of absorbed dose rate in air for photon emitters from natural radionuclides. This optimised geometry based on physical assumptions consists of the soil part whose emitted radiation has a given minimum probability to reach the detector. This geometry was implemented in Geant4 toolkit and a significant reduction in computation time was achieved. Simulation tests have shown that for soil represented by a cylinder of 40 m radius and 1 m deep, >98% of the calculated dose rate conversion coefficients in air at 1 m above the ground is generated by only 6% of the soil volume in the case of uniform distribution of radioactivity, and >99.2% of the calculated dose rate for an exponential distribution. When the soil is represented by the entire optimised geometry, 99% of the conversion coefficients values are reached for a soil depth of 1 m and 100% for that of
2 m.