Signal and noise properties of position-sensitive avalanche photodiodes

Abstract

After many years of development, position-sensitive avalanche photodiodes (PSAPDs) are now being incorporated into a range of scintillation detector systems, including those used in high-resolution small-animal PET and PET/MR scanners. In this work, the signal, noise, signal-to-noise ratio (SNR), flood histogram and timing resolution were measured for lutetium oxyorthosilicate (LSO) scintillator arrays coupled to PSAPDs ranging in size from 10 to 20 mm, and the optimum bias voltage and working temperature were determined. Variations in the SNR performance of PSAPDs with the same dimensions were small, but the SNR decreased significantly with increasing PSAPD size and increasing temperature. Smaller PSAPDs (10 mm and 15 mm in width) produced acceptable flood histograms at 24 °C, and cooling lower than 16 °C produced little improvement. The optimum bias voltage was about 25 V below the break down voltage. The larger 20 mm PSAPDs have lower SNR and require cooling to 0-7 °C for acceptable performance. The optimum bias voltage is also lower (35 V or more below the break down voltage depending on the temperature). Significant changes in the timing resolution were observed as the bias voltage and temperature varied. Higher bias voltages provided better timing resolution. The best timing resolution obtained for individual crystals was 2.8 ns and 3.3 ns for the 10 mm and 15 mm PSAPDs, respectively. The results of this work provide useful guidance for selecting the bias voltage and working temperature for scintillation detectors that incorporate PSAPDs as the photodetector.

Figure 1

The four central crystals and coordinate system used to calculate the flood histogram quality parameter, k.

Figure 2

Signal, noise and SNR vs. bias voltage curves for three different size PSAPDs at four different temperatures.

Figure 3

SNR vs. bias voltage curves (temperature 0°C) for all PSAPDs measured in this study.

Figure 4

Flood histograms from three PSAPDs (one of each size) measured at different bias voltages. Bias voltage is expressed in terms of voltage below breakdown voltage for each PSAPD. Measurements were acquired at 7°C.

Figure 5

Optimal flood histograms obtained by changing bias voltage to minimize quality parameter for different PSAPD sizes and temperatures.

Figure 6

Flood histogram quality parameter (lower value corresponds to better quality) vs. bias voltage curves for three different size PSAPDs measured at four temperatures.

Figure 7

Flood histogram quality parameter vs. bias voltage curves for all seven PSAPD samples measured at 0 °C.

Figure 8

Average timing resolution of individual crystals vs. bias voltage curves for 10 mm (left) and 15 mm (right) PSAPDs at four different temperatures.