The Use of an Ethylene-filled Polythene Chamber for Dosimetry of Megavoltage X-rays

An ionization chamber, made from polythene and filled with ethylene has been used as an energy-independent substandard to extend the 2 MV calibration factor for a Baldwin-Farmer substandard dosemeter. The methods used to intercompare the chambers at the various qualities are described. Using these intercomparisons and measured absorption coefficients, multiplying factors have been derived to convert the readings given by the Baldwin-Farmer dosemeter to rads in water. The factors obtained were 0·941 N , 0·936 N , 0·950 N , for 60 Co, 4 MV and 20 MV radiations respectively, where N is the N.P.L. calibration factor to convert the dosemeter readings to roentgens at 2 MV.