Some properties of a linear Boltzmann collision operator acting in the L1 space of absolutely integrable functions of the velocity are derived. The system considered consists of particles moving in a dilute equilibrium gas. The case of a constant accelerating force acting upon the particles (as encountered in electron swarm experiments) is also studied. It is found that the collision operator is dissipative operator which generates a strongly continuous contraction semi-group. It is also shown that the time evolution leaves the positivity and normalisation of the distribution function invariant.