Reach-Through Avalanche Photodiodes in Soft X-ray Detection

Reach-through avalanche photodiodes (RT-APDs) from Hamamatsu Photonics, with different active areas, 5 × 5 and 3 × 3 mm², were investigated to evaluate their suitability for the muonic helium Lamb shift experiment. The gain has been determined as a function of voltage and temperature for both prototypes. As expected, it increases with increasing bias voltage and with decreasing temperature. The gain variation with temperature is smaller than per -2% °C even for higher bias voltages applied to the RT-APD. The non-linearity between gain obtained for X-rays and visible light pulses has been investigated for different temperatures. The non-linearity was found to increase with decreasing temperature. For example at 350 V, it is as high as 25% at -20 °C and about 10% at 20 °C. The RT-APDs performance for the detection of 8 keV X-rays has been investigated. The best energy resolution was achieved for the larger prototype for a temperature of 0 °C. The minimum energy resolution values, between 9.2 and 9.9%, were obtained for gains between 55 and 80. The minimum detectable energy observed is about 0.2 keV for the higher gain region, for both RT-APDs, investigated, with no significant improvement at lower temperatures.