Intercomparison of the national exposure standards of New Zealand and Australia.

An intercomparison of medium energy x-ray and 60Co gamma radiation exposure standards held by the Australian Radiation Laboratory and the National Radiation Laboratory (New Zealand) is reported. The standards agree within 0.4%.

Interface effects in the Monte Carlo simulation of electron tracks.

An analog of Fano's theorem for ionization in cavities is shown to hold for the stepwise representation of electron paths used in Monte Carlo computer models of electron transport. This brings to light an error in the distribution of electron paths and hence energy deposition which is induced by interrupting steps which cross the interface between media of different densities. The magnitude of the error depends on the shape of the cavity and its size relative to the electron path length in the cavity gas. In a typical calculation of a cylindrical chamber exposed to 60Co radiation, if the electron step size is taken as 10% of the remaining path, then a 3% energy deficit in the cavity results. An algorithm for crossing an interface is described which does not produce this error.

The effect of scattered radiation in 60Co beams on wall correction factors for ionization chambers.

The Awall correction factors used in dosimetry protocols are obtained by Monte Carlo methods that assume 60Co beams equivalent to 1.25-MeV radiation. Real 60Co beams contain lower energy components; in addition, Awall is defined differently by different authors. In the present work, Awall as defined by Rogers, Bielajew, and Nahum (1985) has been calculated by Monte Carlo methods at several energies, for a graphite cavity chamber, using the revised stopping-power data of Seltzer and Berger (1982). It is concluded that the true value of Awall for chambers designed for use in 60Co beams will not move more than 0.1% below the value calculated for 1.25-MeV photons.