TIMING RESPONSE OF CDTE DETECTORS

Semiconductor CdTe detectors are gaining wide acceptance in many applications where X- and gamma-ray measurements are necessary, such as in astrophysical research, medical imaging and industrial radiography. Good timing response is critical both in applications like positron emission tomography, where fast coincidence capabilities are required, and in single photon counting when a high counting rate is needed. The typical configuration employed, where the direction of the impinging radiation beam is parallel to the collecting electric field, has one well known drawback: an increase in active layer, necessary in order to reach a satisfactory absorption efficiency for the detection of high energy photons, leads to a longer transport path for the charge carriers generated. As a consequence, there is a degradation in energy resolution and a broadening in time response. In the present paper, measurements of the timing response for an unusual configuration of CdTe detectors are presented. In this configuration, which we call a PTF (planar transverse field) detector, the collecting electric field and hence the transport direction of carriers is transverse to the direction of the incoming photons and so detection thickness and transport length are independent. In this way the absorption layer can be increased without impairing the timing performance. The measurements described herein have been performed using a PTF detector having dimensions of 2.5 x 2.5 x 20 mm3, in order to have a good efficiency for annihilation gamma-ray photons.