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The absolute dosimetry of negative pions

University of British Columbia

Soon three centres will be treating cancer with beams of negative pions: LAMPF, Los Alamos, U.S.A.; SIN, Villigen, Switzerland; and TRIUMF, Vancouver, Canada. In order to understand this new modality, it will be necessary to compare the results of pion therapy to those achieved with conventional means and to compare results among the three centres. Absolute dosimetry is the basis of this comparison. 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 W r F where W, r and F are calculated quantities. W is the average energy expended in the gas per ion pair produced. Since the W-value for a secondary liberated during pion capture depends upon its energy, it was necessary to average over the energy spectra for the various secondaries. r is the ratio of dose in the wall material, carbon, to dose in the gas, either methane or carbon dioxide. Since the pion secondaries have ranges which are of the same size as the cavity of the ion chamber, it was necessary to explicitly consider those secondaries which emerge from the wall with insufficient energy to cross the cavity and those pions which form stars inside the cavity. Thus, r was found to be pressure dependent. The pressure dependence for carbon dioxide arises as follows:- as the pressure is increased, there are more pion captures in the gas, but since there is less energy released to charged secondaries per pion capture on oxygen than carbon, the dose in the gas decreases. Therefore, the calculation predicted that, as pressure is increased, J/M would remain unchanged for methane, but would decrease rather dramatically for carbon dioxide. F is the ratio of dose in tissue to dose in wall material, carbon in this case. F is the product of two factors: P, which accounts for the difference in the stopping pion density for tissue compared to carbon, and K, which accounts for the difference in energy released per pion capture in tissue compared to carbon. 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 behavior 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. After considering the dose contribution from beam contaminants, the value of F required to convert dose in carbon to dose in tissue was estimated to be 0.93 ± 0.05. Therefore, the absolute dose in tissue was determined to an accuracy of ± 7%. In order to increase the accuracy, it is necessary to improve the data available as input to the calculation.

Text

2010-03-18

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http://hdl.handle.net/2429/22081

Physics

University of British Columbia

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