Calculations have been made of the dE/dx distribution (LET spectrum) and central-axis depth-dose curves (Bragg curves) of stopping negative pion beams in water. Such beams are of interest because of the large deposition of energy at depth in the stopping pion region relative to that deposited at the surface. Nuclear interactions occurring when the pions come to rest cause low-energy highly ionizing particles to be emitted as the capturing nucleus breaks up, thus increasing the dose deposited in the stopping pion region. The calculations show that, for beams similar to those presently available experimentally, peak depth-to-entrance ratios of 3.4 and 2.9 can be expected in the absorbed dose in water for pure and contaminated beams, respectively, with a width of around 3.5 cm. The contaminated beam was assumed to contain 65% pions, 10% muons, and 25% electrons. The pions in these beams have a range of 25 cm of water. Comparison with experimental results taken with a lithium-drifted silicon detector shows that this detector gives a reasonable picture of the relative doses in water, and so can be used to measure isodose distributions and Bragg curves in water phantoms.

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