Influence of the active area size and read-out method on the timing performance of SiPMs coupled to LYSO scintillators

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Abstract

In this work we report on the coincidence resolving time performance of SiPMs with different sizes, produced at FBK, coupled to the same LYSO scintillators. The measurements are performed both with and without the differential leading edge discriminator at three different temperatures, 20 °C, 0 °C and −20 °C. The photo-detectors feature an active area of 2×2 mm2 and 4×4 mm2. The scintillators have a cross-section of 1.8×1.8 mm2 and height of 10 mm. The measurements show that, once we eliminate the effect of noise on the timing measurements, we obtain similar coincidence resolving times for the two SiPM sizes considered. This means that the SiPM capacitance, at least up to 4×4 mm2, is not a limiting factor.

Introduction

Two critical aspects when measuring the coincidence resolving time (CRT) of large area Silicon Photomultipliers with the leading edge discriminator (LED) algorithm are the high level of dark counts and the rise time of the SiPM pulse.

The first aspect, combined with the long tail of the pulse, causes baseline fluctuations. As a consequence, the LED threshold must be set at higher levels where the photons arrival time statistics is worse, thus degrading the performances. We showed in Ref. [1] that a considerable improvement in the timing precision can be obtained using a baseline correction algorithm that preserves the first part of the signal. We called this method differential leading edge discriminator (DLED). We also developed an analog circuit implementation based on this method. It is called Pole-Zero [2] and is characterized by a simple circuit, well suited for an ASIC implementation.

The second critical aspect for the CRT is related to the parasitic capacitance of the devices that limits the slope of the rising edge of the SiPMs signal. Having a finite value, it contributes in generating a time jitter that affects the timing precision, especially in large area devices.

In this work we show that, with the good noise suppression of the DLED method, we get similar timing performance for SiPMs with small and big active area.

Section snippets

Test setup description

The setup used for measurements is shown in Fig. 1. Each SiPM is coupled to a teflon wrapped LYSO crystal and is connected to an amplifier board. The board is designed in order to minimize the parasitic capacitance of the connection. The 22Na source is placed in between the sensors. Both outputs of the amplifiers are connected, through a T-connector, to a channel with a 1MΩ input impedance of an oscilloscope and then terminated on another channel with a 50Ω impedance.

Energy spectra are

Comparison of timing measurements

To evaluate the influence of the dark rate and time pick-off method on the timing performance, we measured the CRT of 2×2 mm2 and 4×4 mm2 SiPMs coupled to the same 1.8×1.8×10 mm3 LYSO scintillators with the two methods described. The devices have a 50μm2 cell size. All the measurements are performed in a thermostatic chamber at 20 °C, 0 °C and −20 °C. To find the optimal working point, we performed a voltage sweep for every measurement. The time delay used for the DLED is 500 ps.

Fig. 2 shows the CRTs

Conclusion

In this work we showed a comparison between the timing resolutions obtained with the LED and DLED methods for SiPMs of different size, 2×2 mm2 and 4×4 mm2, coupled to scintillator crystals with 1.8×1.8×10 mm3 size. As already shown, with the noise compensation, the effect of the dark rate on the timing performance is significantly reduced. As a consequence, it is possible to measure similar timing resolutions for every temperature considered (20 °C, 0 °C and −20 °C) and also for different device

Acknowledgments

This work is partially supported by the EU FP7 project SUBLIMA, Grant Agreement No. 241711.

References (2)

  • A. Gola et al.

    IEEE Transactions on Nuclear Science

    (2012)
  • A. Gola, C. Piemonte, A. Tarolli, Analog circuit for timing measurements with large area SiPMs coupled to LYSO...
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