The effects of 14 MeV neutron and 2 MeV electron irradiations on surface barrier counters have been related to the permanent damage effects of identical irradiations on the parameters carrier lifetime, Hall coefficient and mobility of bulk silicon taken from the same n-type ingots. A tentative discussion of mechanisms causing the observed counter behaviour is given, and the suggestion is made that irradiation damage followed by cooling may provide useful thick counters of constant capacity. This is made possible by the fact that for this special high purity silicon, irradiation can raise the resistivity well before degradation of carrier lifetime destroys the resolution.