Comparative Measurements of 60Co Hot Particle Depth-Dose Distributions and Monte Carlo Calculations (invited paper)

The present study attempts to compare several possible measurement techniques and MCNP Monte Carlo calculations of beta/gamma doses from test 'bench-mark' 60Co hot particle sources. Depth-dose distributions have been obtained by ionometric measurements using ionisation extrapolation chamber (ECH) and densitometric measurements using radiation induced colour changes in radiochromic dye films (RDF). The ECH provides absorbed dose information averaged over specific areas dictated by the measuring electrode diameter (1 mm, 3 mm, 1 cm and 3 cm). The RDF displays full two-dimensional dose distributions in a single measurement with a high spatial resolution down to 25 μm. In general there was good agreement between the two measurement techniques for the doses measured close to the surface after correction due to the non-linear response of the ionisation response relative to electrode separation (mainly due to the spatially non-uniform radiation field). However the agreement is markedly worse at greater depths where the dose is dominated by the gamma ray component. The measurements and MCNP4B calculations agree within a few per cent for the surface dose rate and to within γ30% at 30 mg.cm-2. The reasons for these differences are discussed. (author)