The plasma particle source resulting from ionization by microwaves in the electron-cyclotron range of frequencies is investigated in the simple magnetized torus TORPEX ( Fasoli A et al 2006 Phys. Plasmas 13 055902), together with its dependence upon experimental parameters, such as the absorbed microwave power, the magnetic field configuration and the neutral gas pressure. The relative contributions to the ionization rate from thermal electrons and from suprathermal electrons accelerated at the plasma resonances are quantified through a global particle balance. An expression for the spatial profile of the particle source is derived from the experimental results and a simple numerical code.