Spectral Resolution Enhancement by Multi-Gain Adjustment of an SiPM Array

Introduction: Currently, silicon photomultiplier (SiPM) technology is used in many new radiation detection designs as an optical converter instead of the classical photomultiplier tube (PMT). SiPM consists of up to a few tens of thousands of pixels, each pixel is implemented by an avalanche photodiode. When a reverse voltage biased diode is triggered by an incoming photon or by intrinsic thermal noise, a quantity of charge is produced through avalanche breakdown. Accordingly, SiPM produces a semi-analogue output, proportional to the number of triggered diodes. SiPM technology introduces advantages such as compactness, low bias voltage and immunity to electromagnetic interference. However, as common to any semiconductor biased in a reversed voltage, the performance severely degrades by gain variations and stability issues, due to temperature changes and breakdown voltage variations [1], [2].

Methods: Fig. 1 demonstrates a system based on a 30 mm X 30 mm X 15 mm scintillator, a large area Quad-SiPM and dedicated electronics for the biasing, amplification and analysis.

Fig. 1: Detector schematics: scintillator, Quad-SiPM, electronics and MCA.

Since SiPMs are manufactured in small sizes (1mm X 1 mm to 6mm X 6 mm), a Quad 6mm X 6 mm SiPM is implemented to cover the surface of 15 mm X 15 mm scintillator. As mentioned above, practical SiPM based system must use gain over temperature stabilization. The gain is a function of the overvoltage V_OV, which is a function of the bias voltage V_B and the temperature dependent breakdown voltage V_BR(T) [3]. The total gain is also avalanche initiation (AIP) dependent, which is itself a function of V_OV.

However, the breakdown voltage is also V_BR0 dependent, which is the initial V_BR offset introduced due to manufacture limitation. Selected SiPM specifications are V_BR = 24.5 V, with ±250 mV variations due to V_BR0 and V_OV range of 2.5 ÷ 6 V [4]. For small V_OV variations, the gain of individual SiPM can be modeled as parabolic function of V_OV (Fig. 2A). For normal distribution of V_BR ~ N(µ = 24.5 V, σ = 40 mV) we can expect significant gain dissimilarity of the 4 sub-SiPM in the Quad-SiPM. By randomly generating parameters of the 4 SiPMs, the following V_BR were obtained 24.516, 24.523, 24.495 and 24.472 V.

Fig. 2: (A) Gain (V_OV) for the sub-SiPM; (B) Resolution (V_OV) for the total Quad-SiPM.

The measured resolution, (Fig. 2B), is best at V_OV of 2.7 ÷ 4.7 V. By using individual gain corrections, implemented as individual bias voltage adjustment, for matching the V_OV, resolution can be improved.

Results and Conclusions:

Fig. 3: (A) Net. Peak ¹³⁷Cs spectra for 4 sub-SiPMs, the total spectrum for Quad-SiPMs (black) and the enhanced spectrum (red); (B) Measured resolution (blue) and enhanced resolution (green) for Quad-SiPM.

For V_OV = 4 V, each of the 4 sub-SiPM’s peak centers have different offset relative to the required 662 keV (Fig. 3A). The sum of the 4 sub-SiPMs (Quad-SiPM) is plotted as a black line. By adjusting individual bias voltage for each of the 4 sub-SiPMs, before summing, the spectra peaks are aligned so that an enhanced (red) peak is obtained. The enhanced peak (Fig. 3A) is characterized by resolution improvement from 5.5% to 5% (Fig. 3B). Overall, for small V_OV, resolution enhancement of about 1.5% is achievable.