The detectors

All the detectors (except for two described in section 6) were fabricated with SI U GaAs substrates [5], with thicknesses varying from 115$\mu$m to 500$\mu$m. The substrates were lapped and polished at Glasgow University to obtain the desired thickness. Various thicknesses were used so that the dependence of the leakage current and charge collection efficiency on this parameter could be studied. All the detectors had 3mm diameter circular pad contacts with a 200$\mu$m wide guard ring separated from the pad by 10$\mu$m. The Schottky contact on one face of the wafer was metallized with Ti/Pd/Au and the ohmic contact on the other face was Pd/Ge annealed at 300^oC. The leakage current characteristics before and after the irradiations were obtained with a Keithley 237 voltage source measurement unit. A ⁹⁰Sr source was used to determine the response of the detectors to minimum ionising particles with an amplifier shaping time of 500ns. The charge collection efficiencies of either electrons or holes created in the first 20$\mu$m of the detector thickness were obtained with the use of an ²⁴¹Am alpha particle source placed on the Schottky or ohmic contact, respectively. An amplifier with a 500ns shaping time was used. The current characteristics and charge collection efficiencies from the ⁹⁰Sr source were obtained at both 20^oC and -10^oC. The -10^oC temperature was chosen as this is the proposed operating temperature of the ATLAS semiconductor tracker.