The absolute dosimetry of negative pions

An absolute dose determination has been made for the negative pion beam at TRIUMF using an ionization chamber. The relationship required to convert the ionization per unit mass, J/M, measured by the chamber to dose in tissue is D = J/M WrF where W, r and F are calculated quantities: W is the average energy expended in the gas per ion pair produced; r is the ratio of dose in the wall material to dose in the gas; F is the ratio of dose in tissue to dose in wall material. Experimentally, the ionization per unit mass was measured in a parallel plate chamber as a function of pressure for various gases with carbon, aluminum and TE-A150 electrodes. The pressure dependence of J/M measured for methane and carbon dioxide with carbon electrodes was compared to the behaviour predicted by the calculation. Qualitatively, the prediction was confirmed: the ionization per unit mass for CO2 decreases more dramatically with increased pressure than for CH4. Therefore pion capture in the gas is significant and the energy released to charged secondaries per pion capture on oxygen is less than for carbon. Quantitatively, the percentage change is larger than predicted. The value of J/M extrapolated to zero pressure and the appropriate values calculated for W and r enabled a determination of the absolute dose in carbon with an estimated accuracy of ±5%. The ionization created with aluminum electrodes was compared to that for carbon in order to estimate the dose in aluminum