Stopping power and ranges of low-energy protons in tissue-equivalent gas

Stopping power data for protons in tissue-equivalent gas (64.4% CH4 + 32.4% CO2 + 3.2% N2, partial pressures) were derived from experimentally determined ranges and from differential ionisation distributions using the differential energy required per ion pair formed to convert ionisation to energy loss. The proton energy covered the region from 1 to 100 keV. The results are compared with other data available. In general the present experimental stopping power data tend to higher values than calculated data for energies lower than 20 keV; between 20 and 100 keV they agree well within the stated uncertainties.