Analysis and characterisation of the confining mechanism of the controlled-drift detector

The controlled-drift detector is a recently designed position-sensing X-ray detector. It operates by switching between integration and drift mode. The basic idea is to generate potential wells within the otherwise linear drift potential. During the integration mode signal electrons are being accumulated within these wells. The removal of the barriers in an externally controlled way restores a uniform drift field that sweeps the electrons towards the readout anodes. The height of the confining barrier depends on the drift field and on the amplitude of the potential perturbation at the surface. An experimental study of the charge handling capability has been carried out as a function of the amplitude of the surface perturbation (up to 2.1 V) and as a function of the drift field (250-400 V/cm). The study provides a sensitive measurement of the actual potential barrier of the integration well in different operating conditions. A detailed analysis and characterisation of the mechanisms to confine the signal electrons in the 3D well is presented. For a drift field of 300 V/cm, storage of more than 10/sup 5/ electrons in 180/spl times/180 /spl mu/m/sup 2/ pixels has been achieved with 1.67 V surface perturbation.