SI LEC GaAs Schottky barrier detectors have been irradiated with high energy protons (24 GeV/c, fluence up to 16.45´ 10¹³ p/cm²). The detectors have been characterised in terms of I/V curves, charge collection efficiency (cce) for incident 5.48 MeV a-, 2 MeV proton and minimum ionising b-particles and of cce maps by microprobe technique IBIC (Ion Beam Induced Charge).

At the highest fluence a significant degradation of the electron and hole collection efficiencies and a remarkable improvement of the FWHM energy resolution have been measured with a- and proton particles. Furthermore the reduction in the cce is greater than the one measured with b-particles and the energy resolution worsens with increasing the applied bias, V_a, above the voltage V_d necessary to extend the electric field all the way to the ohmic contact.

On the contrary, in the unirradiated detectors the charge collection efficiencies with a-, b- and proton particles are quite similar and the energy resolution improves with increasing V_a > V_d.

IBIC spectra and IBIC space maps obtained by scanning a focused (8 mm²) 2 MeV proton microbeam on front (Schottky) and back (ohmic) contacts, support the observed electric field dependence of the energy resolution both in unirradiated and most irradiated detectors.

The results obtained let us explain the effect of the electric field strength and the plasma on the collection of the charge carriers and the FWHM energy resolution.