Cadmium telluride β-ray detector

Abstract

The fabrication and characteristics of a prototype CdTe (cadmium telluride) detector for a β-ray thickness gauge are described. The detector consists of a pn-junction diode, operating in photovoltaic mode with no applied voltage. It needs no cooling because its dark current is less than 7 × 10⁻¹⁴ A/mm²at 0.01 V and 30 °C. An output current of 5 × 10⁻⁸ A with a statistical noise of 1.5 × 10⁻¹² A/(Hz)^{$\text{1}\text{2}$} has been achieved with ⁸⁵Kr radiation on an active area 16 mm in diameter. The (111) CdTe wafer used in the detector is 22 mm × 22 mm × 1.5 mm.

Single crystals with no twins and a low dislocation density (less than 1 × 10⁶ cm⁻²) are used because it is found that the generation-recombination centres of carriers related to crystallographic defects (such as twins and dislocations) increase the dark current and decrease the output current.

As the temperature coefficient of the output current for an ⁸⁵Kr radiation source is approximately 0.23%/°C, a thermostatistically-controlled module is used to keep the detector's operating temperature constant to within 0.1 °C, thus solving the problem of drift of output current with temperature.