Development of CdTe/CZT Drift Ring Detectors for X and γ Ray Spectroscopy.

The applications of radiation detectors in astronomy and space science require such detectors that provide high quantum efficiency and good energy resolution. CdTe and CZT material is an excellent candidate for the fabrication of high energy X-ray spectroscopic detectors due to their good quantum efficiency and room temperature operation. The main material limitation is associated with the poor charge transport properties of holes. The motivation of this project is to develop new CdTe and CZT detectors fabricated with a drift ring geometry that is insensitive to the transport of holes. The performance of a prototype (8 x 8) mm2 Ohmic CdTe drift ring detector fabricated by Acrorad Ltd and a CZT fabricated at University of Surrey with 3 drift rings are investigated in which their thicknesses are 1 mm of CdTe and 2.3 mm of CZT. For both detectors, the inner anode has a diameter of 0. 5 mm as are the widths of all three rings which are separated by gaps of 0. 5 mm which are quite large in comparison to drift ring geometries which are published in literature. The energy resolution was studied by using different biasing schemes in terms of biasing the cathode (bulk field) and the rings (lateral field) and the data shows that the energy resolution of the CdTe at room temperature is limited by leakage current which is a combination of bulk and surface leakage current which was improved significantly by cooling the detector due to reducing the leakage current. For example, the CdTe detector shows energy resoltion of (5.1 ± 0.3) keV at 59. 5 keV at room temperature which was improved to (3.6 ± 0.1) keV at -15°C as a result of reducing the leakage current from 23 to 0. 4 nA using the same bias scheme. The CZT detector shows low leakage current at room temperature. Under an optimised biasing scheme, the FWHM of a 59. 5 keV photo-peak was found to be (2.8 ± 0.2) keV at -15°C and (3.3 ± 0.1) keV at room temperature using CdTe and CZT respectively with a trade off between energy resolution and quantum efficiency. In addition, the performance was studied as a function of incident X-ray position with an X-ray microbeam at the Diamond Light Source and the results show that the lateral fields i. e. the bias gradient across the rings) has a significant effect on increasing the active area, evaluated by the detected count rate