The use of a calorimeter for neutron dosimetry (for radiotherapy)

The dose given to a polythene energy absorber in the radiation field from a 14 MeV neutron generator has been measured calorimetrically. The calorimeter was calibrated by giving the energy absorber a known dose of gamma-rays. The dose measured in this way was compared with that determined with a polythene-ethylene ionization chamber, and the two results agreed to better than 1%, though the estimated uncertainties on the calorimeter and ionization chamber results were each about 4%. This result can be used to determine a value of the ratio Wn/We, for 14 MeV neutrons in ethylene, where Wn is the W value for the charged particles generated in the neutron field in ethylene, and We is the W value for electrons in the same gas. The measured value of Wn/We was found to be 1.07 plus or minus 3.9%. The significance of the 'thermal defect' in polythene is discussed.