SensL Silicon Photomultipliers Research Articles

Coincidence time resolution of radiation detector based on 6×6 mm2 ToF SiPM detectors with different readout schemes

Introduction: Nowadays, attention is growing on the Silicon Photomultipliers (SiPMs) detector for many applications, especially in nuclear medicine. In Positron Emission Tomography (PET) scanner, timing performance of a PET detector plays a significant role in image reconstruction.Methods: This work mainly aims at the processing of timing signal for the purpose of achieving a good timing performance. We applied a timing detector made up of a 3 × 3 × 10 mm3 LYSO crystal directly coupled with a large-size SensL SiPM with a sensitive area of 6 × 6 mm2. The standard output of the SiPM was used for energy calculation while the fast output was for timing pickoff. Three different readout configurations for fast timing signals were used for timing performance evaluation: 1) the recommended RF transformer-based readout, 2) the cascaded Common Emitter Amplifier (CEA), 3) the commercial RF amplifier.Results: Experiment results show that the best FWHM CTR values for the three were 228.3 ± 1.4 ps, 235.4 ± 1.1 ps and 231.1 ± 1.5 ps for the RF transformer-based, the CEA-based and the RF amplifier-based readout configurations respectively. The schemes based on the CEA-based and the RF amplifier-based configurations have a good uniformity at different trigger thresholds.Discussion: For practical application, the amplified timing signal based on the CEA circuit is more desirable because it is more feasible for trigger threshold selection in multichannel readout electronics system.

Radiation damage assessment of SiPMs for scintillation detectors

Silicon Photomultipliers (SiPMs) are quickly replacing traditional photomultiplier tubes (PMTs) as the readout of choice for gamma-ray scintillation detectors in space. While they offer substantial size, weight and power saving, they have been shown to be susceptible to radiation damage. SiPM models from various manufacturers were irradiated with 64 MeV protons to simulate the radiation effects seen during previous space missions. The work here expands upon a previous efforts with SensL SiPMs and focuses on characterizing the increase in current observed with increasing radiation damage. This work was performed to better quantify this effect as a function of dose and find commercially available options that would prove more robust in the harsh radiation environment of space. In general, results show larger cell sizes are more susceptible to radiation damage with the largest showing the greatest increase in current as a function of dose for similarly designed SiPMs. Within the models tested here, two were found to be roughly an order of magnitude less susceptible to radiation damage compared to the majority of commercially available SiPMs.

Radiation damage assessment of SensL SiPMs

Silicon Photomultipliers (SiPMs) are quickly replacing traditional photomultiplier tubes (PMTs) as the readout of choice for gamma-ray scintillation detectors in space. While they offer substantial size, weight and power saving, they have shown to be susceptible to radiation damage. SensL SiPMs with different cell sizes were irradiated with 64 MeV protons and 8 MeV electrons. In general, results show larger cell sizes were more susceptible to radiation damage with the largest 50μm SiPMs showing the greatest increase in current as a function of dose. Current increases were observed for doses as low at ∼2 rad(Si) for protons and ∼20 rad(Si) for electrons. The U.S. Naval Research Laboratory’s (NRL) Strontium Iodide Radiation Instrument (SIRI-1) experienced a 528μA increase in the bias current of the onboard 2x2 SensL J-series 60035 SiPM over its one-year mission in sun-synchronous orbit. The work here focuses on the increase in bulk current observed with increasing radiation damage and was performed to better quantify this effect as a function of dose for future mission. These include the future NRL mission SIRI-2, the follow on to SIRI-1, Glowbug and the GAGG Radiation Instrument (GARI).

Electric charge estimation using a SensL SiPM

The Silicon Photomultipliers (SiPMs) are commonly used in the construction of radiation detectors such as those used in high energy experiments and its applications where an excellent time resolution is required for triggering. In most of this cases, the trigger systems electric charge information is discarded due to limitations in data acquisition. In this work we propose a method, using a simple radiation detector based on an organic plastic scintillator 2×2×0.3 cm3 size, to estimate the electric charge obtained from the acquisition of the fast output signal of a SensL SiPM model C-60035-4P-EVB. Our results suggest a linear relation between the reconstructed electric charge from the fast output of the SiPM used with respect to the one reconstructed with its standard signal output. Using our electric charge reconstruction method, we compared the sensitivity of two plastic scintillators, BC404 and BC422Q, under the presence of 90Sr, 137Cs, 60Co and 22Na radiation sources.

Single photon detection with SiPMs irradiated up to 1014 cm−2 1-MeV-equivalent neutron fluence

Silicon photomultipliers (SiPM) are solid state light detectors with sensitivity to single photons. Their use in high energy physics experiments, and in particular in ring imaging Cherenkov (RICH) detectors, is hindered by their poor tolerance to radiation. At room temperature the large increase in dark count rate makes single photon detection practically impossible already at 1011 cm−2 1-MeV-equivalent neutron fluence. The neutron fluences foreseen by many subdetectors to be operated at the high luminosity LHC range up to 1014 cm−2 1-MeV-equivalent. In this paper we present the effects of such high neutron fluences on Hamamatsu and SensL SiPMs of different cell size. The advantage of annealing at high temperature (up to 175 ∘C) is discussed. We demonstrate that, after annealing, operation at the single photon level with a SiPM irradiated up to 1014 cm−2 1-MeV-equivalent neutron fluence is possible at cryogenic temperature (77 K) with a dark count rate below 1 kHz.

Timing performance of organic scintillators coupled to silicon photomultipliers

This work compares the time resolution obtained from coincidence measurements using combinations of four types of silicon photomultipliers (SiPMs), five types of organic scintillators, and two timing algorithms. Organic scintillators in combination with SiPMs have shown to have equivalent or better time resolution and pulse shape discrimination (PSD) capability than with photo-multiplier tubes (PMTs). In addition, SiPMs are more compact and require low operating voltage. These factors have made them suitable for the development of compact neutron scatter cameras (NSCs) and potentially low-cost positron emission tomography (PET) systems. The following measurements analyze combinations of organic scintillators and SiPMs that could be used for these applications. Measurements and analyses assessed the time resolution of standard-output and fast-output pulses of the various scintillator and SiPM combinations. The coincidence measurements were performed using a Na-22 source to characterize time resolution where all coincident events above about a 40 keVee threshold were analyzed. Uncertainties were determined by applying bootstrapping to the acquired data sets. The full width at half maximum (FWHM) of the time difference distribution for each combination is reported as the time resolution of the system. For standard-output pulses, we found that using a digital implementation of analogue constant fraction discrimination (DIACFD) improved timing resolution greater than a single standard deviation for six of the fourteen tested combinations compared to using digital constant fraction discrimination (DCFD). Conversely, for fast-output pulses, DCFD with a moving average filter produced the best timing resolution. Among the tested non-PSD capable organic scintillators, EJ-228 coupled to a MicroFJ-SMA-60035 SensL SiPM yielded the best time resolution of 303 ± 4 ps for standard-output pulses and 227 ± 4 ps for fast-output pulses. The best time resolution found for PSD capable organic scintillators was obtained using Inrad Optics stilbene coupled to a MicroFJ-SMA-60035 SensL SiPM which yielded a time resolution of 428 ± 6 ps for standard-output pulses and 293 ± 4 ps for fast-output pulses.

Improved single photon time resolution for analog SiPMs with front end readout that reduces influence of electronic noise

A key step to improve the coincidence time resolution of positron emission tomography detectors that exploit small populations of promptly emitted photons is improving the single photon time resolution (SPTR) of silicon photomultipliers (SiPMs). The influence of electronic noise has previously been identified as the dominant factor affecting SPTR for large area, analog SiPMs. In this work, we measure the achievable SPTR with front end electronic readout that minimizes the influence of electronic noise. With this readout circuit, the SPTR measured for one FBK NUV single avalanche photodiode (SPAD) was also achieved with a mm2 FBK NUV SiPM. SPTR for large area devices was also significantly improved. The measured SPTRs for mm2 Hamamatsu and SensL SiPMs were 150 ps FWHM, and SPTR 100 ps FWHM was measured for mm2 and mm2 FBK NUV and NUV-HD SiPMs. We also explore additional factors affecting the achievable SPTR for large area, analog SiPMs when the contribution of electronic noise is minimized and pinpoint potential areas of improvement to further reduce the SPTR of large area sensors towards that achievable for a single SPAD.

Pulse shape discrimination based on fast signals from silicon photomultipliers

Recent developments in organic plastic scintillators capable of pulse shape discrimination (PSD) enable a breakthrough in discrimination between neutrons and gammas. Plastic scintillator detectors coupled with silicon photomultipliers (SiPMs) offer many advantages, such as lower power consumption, smaller volume, and especially insensitivity to magnetic fields, compared with conventional photomultiplier tubes (PMTs). A SensL SiPM has two outputs: a standard output and a fast output. It is known that the charge injected into the fast output electrode is typically approximately 2% of the total charge generated during the avalanche, whereas the charge injected into the standard output electrode is nearly 98% of the total. Fast signals from SiPMs exhibit better performance in terms of timing and time-correlated measurements compared with standard signals. The pulse duration of a standard signal is on the order of hundreds of nanoseconds, whereas the pulse duration of the main monopole waveform of a fast signal is a few tens of nanoseconds. Fast signals are traditionally thought to be suitable for photon counting at very high speeds but unsuitable for PSD due to the partial charge collection. Meanwhile, the standard outputs of SiPMs coupled with discriminating scintillators have yielded nice PSD performances, but there have been no reports on PSD using fast signals. Our analysis shows that fast signals can also provide discrimination if the rate of charge injection into the fast output electrode is fixed for each event, even though only a portion of the charge is collected. In this work, we achieved successful PSD using fast signals; meanwhile, using a coincidence timing window of less 3 nanoseconds between the readouts from both ends of the detector reduced the influence of the high SiPM dark current. We experimentally achieved good timing performance and PSD capability simultaneously.

Comparison of SensL and Hamamatsu 4×4 channel SiPM arrays in gamma spectrometry with scintillators

The market of Silicon Photomultipliers (SiPMs) consists of many manufacturers that produce their detectors in different technology. Hamamatsu (Japan) and SensL (Ireland) seems to be the most popular companies that produce large SiPM arrays. The aim of this work is characterization and comparison of 4×4 channel SiPM arrays produced by these two producers. Both of the tested SiPMs are made in through-silicon via (TSV) technology, consist of 16, 3×3mm avalanche photodiode (APD) cells and have fill factor slightly above 60%. The largest difference is a single APD cell size and hence total number of APD cells (55,424 for Hamamatsu, 76,640 for SensL). In the case of SensL SiPM, its spectral response characteristics is shifted slightly toward shorter wavelengths with maximum at 420nm (450nm for Hamamatsu). The presented measurements cover selection of the SiPM optimum operating voltage (in respect to energy resolution), verification of the excess noise factor and check of the linearity characteristics. Moreover, the gamma spectrometry with LSO, BGO and CsI:Tl scintillators together with pulse characteristics for these crystals (rise time and fall time) is reported, as well as temperature dependence. The presented measurements show better performance of the SensL array comparing to the Hamamatsu detector.

A large area, silicon photomultiplier-based PET detector module

The introduction of silicon photomultipliers (SiPM) has facilitated construction of compact, efficient and magnetic field-hardened positron emission tomography (PET) scanners. To take full advantage of these devices, methods for using them to produce large field-of-view PET scanners are needed. In this investigation, we explored techniques to combine two SiPM arrays to form the building block for a small animal PET scanner. The module consists of a 26×58 array of 1.5×1.5mm2 LYSO elements (spanning 41×91mm2) coupled to two SensL SiPM arrays. The SiPMs were read out with new multiplexing electronics developed for this project. To facilitate calculation of event position with multiple SiPM arrays it was necessary to spread scintillation light amongst a number of elements with a small light guide. This method was successful in permitting identification of all detector elements, even at the seam between two SiPM arrays. Since the performance of SiPMs is enhanced by cooling, the detector module was fitted with a cooling jacket, which allowed the temperature of the device and electronics to be controlled. Testing demonstrated that the peak-to-valley contrast ratio of the light detected from the scintillation array was increased by ~45% when the temperature was reduced from 28°C to 16°C. Energy resolution for 511keV photons improved slightly from 18.8% at 28°C to 17.8% at 16°C. Finally, the coincidence timing resolution of the module was found to be insufficient for time-of-flight applications (~2100ps at 14°C). The first use of these new modules will be in the construction of a small animal PET scanner to be integrated with a 3T clinical magnetic resonance imaging scanner.

FPGA-Only MVT Digitizer for TOF PET

The multi-voltage threshold (MVT) method, proposed by us, is a method for digitizing a PET event pulse by sampling with respect to certain reference voltages. Previously, we have implemented this method by using discrete voltage comparators together with discrete time-to-digital converters (TDCs) or TDCs implemented on a field-programmable-gate-array (FPGA), and shown that the method is useful for developing PET detectors capable of time-of-flight (TOF) measurement. In this paper, we investigate an FPGA-only implementation in which the differential I/Os of the FPGA operating in the low-voltage-differential-signaling (LVDS) receiver mode are configured to provide the function of the voltage comparators. Using an Altera Cyclone II Family FPGA (EP2C70F896C7), we built two MVT digitizing channels, each of which contains four comparators to yield eight samples for each event pulse. We describe the design, implementation and calibration of this FPGA-only MVT digitizer and report its performance properties. By connecting the channels to SensL's silicon photomultipliers (SiPMs) FM30035 coupled to 2 ×2×10 mm3 LYSO crystals, we measured an energy resolution of ~16% (at 511 keV) and a coincidence resolving time (CRT) of ~ 500 ps (FWHM). These resolutions are promising for developing PET and TOF PET detectors. On the other hand, based on digitized waveforms of pulses generated by the same SiPMs and LYSOs acquired by using a 50 giga-sample-per-second (Gsps) digital oscilloscope (Tektronics DPO71604B), we measured an energy resolution of ~9% and a CRT of ~390 ps. The comparison suggests that our current implementation of the FPGA-only MVT digitizer contributes an ~13% energy resolution and ~310 ps CRT.

Evaluation of High Density Pixellated Crystal Blocks With SiPM Readout as Candidates for PET/MR Detectors in a Small Animal PET Insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxyorthosilicate (LYSO) crystals their energy resolution, and coincidence response function. Two SensL SPMarray-4 were used as light sensors. Two L YSO crystal blocks consisting of a arrays of 8 × 8 1.2 mm × 1.2 mm × 6.0 mm crystals on the lower layer and 7 × 7 1.2 mm × 1.2 mm × 4.0 mm crystals on the upper layer (which is offset by Yz the crystal width) were mounted on the SensL arrays. A second study was performed with single layer arrays of 8 × 8 1.2 mm × 1.2 mm × 10.0 mm crystals. All of the crystals in the dual layer block were easily identified with a peak to valley ratio of 7.2 while the single layer blocks had a peak to valley ratio of 11.2. The crystal dots were more uniformly spaced in the dual layer arrays suggesting that the inter-layer connection affords some useful light sharing. The average energy resolution for all 113 crystals was 16.3 ± 2.3%. The coincidence response to a 0.25 mm Na-22 source in plastic was 0.97 ± 0.12 mm for normally incident gamma rays with the detectors 120 mm apart. These results indicate that it should be possible to achieve a spatial resolution of about one millimetre near the center of the field of view.