Abstract
The total cross sections of methane, ethane, ethene, propane, propene, cyclopropane, n-butane, isobutane and 1-butene were measured in a transmission experiment utilising positrons from a 22Na source and a tungsten moderator and electrons from a thermionic cathode. As expected the cross sections for positrons are smaller than those for electrons and both grow with the size of the molecules. Above 30 eV the positron and electron cross sections decrease with increasing energy and approach each other. All measured positron and electron cross sections between 100 and 400 eV can be described by a formula containing the energy, the number of molecular electrons and only three fit parameters.