Bryan Muir and Trevor Stocki

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Date: 
Thursday, March 21, 2013
Time: 
3:30 - 5:00 pm - March 21, 2013
Location: 
Conference room A&B, Room C2362, 2nd floor, Cancer Centre South, General Campus, 501 Smyth Road

“Measurements and Monte Carlo simulations for reference dosimetry of electron beams”

Bryan Muir – Carleton University

Abstract: Clinical medical physicists follow protocols, such as the AAPM's TG-51, to calibrate high-energy radiation therapy sources.  A working group of the AAPM is currently engaged in updating the TG-51 protocol for high-energy reference dosimetry.  This work investigates current recommendations for electron beam reference dosimetry using measurements and Monte Carlo simulations of ion chamber response.  Depth-ionization measurements with parallel-plate and cylindrical ion chamber types are performed at NRC in clinical electron beams with energies ranging from 4 to 18 MeV.  Variable results are observed in terms of the short- and long-term stability of ratios of chamber readings to reference ion chambers. Monte Carlo simulations of the absorbed dose to the gas in an ion chamber and the absorbed dose to water are performed as a function of depth using the EGSnrc egs_chamber user-code.  A variety of realistic clinical accelerator models as well as less realistic electron beam sources are used to study the dependence of dosimetric data on the incident source model.  Using these simulations, beam quality conversion factors and gradient effects are investigated for several different cylindrical and parallel-plate chamber types.

“Environmental Transfer Modelling to Determine Radiation Dose to Humans”

Trevor Stocki – Health Canada

Abstract: In January 2009, the IAEA EMRAS II (Environmental Modelling for Radiation Safety II) program was launched. The goal of the program is to develop, compare and test models for the assessment of radiological impacts to the public and the environment due to radionuclides being released or already existing in the environment; help countries build and harmonize their capabilities; and to model the movement of radionuclides in the environment. Within EMRAS II, nine working groups are active; this presentation will focus on the activities of Working Group 1: Reference Methodologies for Controlling Discharges of Routine Releases. Within this working group environmental transfer and dose assessment models are tested under different scenarios by participating countries and the results are compared. This process allows each participating country to identify characteristics of their models in order to refine their methods of estimating the impact of radionuclide releases into the environment. The goal of this working group is to identify reference methodologies for the assessment of exposures to the public due to routine discharges of radionuclides to the terrestrial and aquatic environments.  In the framework of this working group, several different models are being applied to estimate the transfer of radionuclides in the environment for various scenarios. In the first phase of the project, the group has been working on a scenario where a nuclear power reactor with a coastal location routinely (continuously) discharges 60Co, 85Kr, 131I, and 137Cs to the atmosphere and 60Co, 137Cs, and 90Sr to the marine environment.  In this scenario many of the parameters and characteristics of the representative group were given to the modellers and cannot be altered by the users.  Various models have been used by the different participants in this inter-comparison (PC-CREAM, CROM, IMPACT, CLRP POSEIDON, and others).  The first scenario is to enable a comparison of the radionuclide transport and dose modelling. These scenarios will facilitate the development of reference methodologies for controlled discharges.  A review of the Canadian standard on how to perform these calculations will also be reviewed during this presentation.