Systematic Characterisation and Non-Linear Response Correction of SiPMs Using the Single-Step Method for High-Precision Calorimetry

The detection of low-level light is a key technology in various experimental scientific studies. As a photon detector, the silicon photomultiplier (SiPM) has gradually become an alternative to the photomultiplier tube (PMT) in many applications in high-energy physics, astroparticle physics, and medical imaging because of its high photon detection efficiency (PDE), good resolution for single-photon detection, insensitivity to magnetic field, low operating voltage, compactness, and low cost. However, primarily because of the geometric fill factor, the PDE of most SiPMs is not very high; in particular, for those SiPMs with a high density of micro cells, the effective area is small, and the bandwidth of the light response is narrow. As a building block of the SiPM, the concept of the backside-illuminated avalanche drift detector (ADD) was first proposed by the Max Planck Institute of Germany eight years ago; the ADD is promising to have high PDE over the full energy range of optical photons, even ultraviolet light and X-ray light, and because the avalanche multiplication region is very small, the ADD is beneficial for the fabrication of large-area SiPMs. However, because of difficulties in design and fabrication, no significant progress had been made, and the concept had not yet been verified. In this paper, preliminary results in the design, fabrication, and performance of a backside-illuminated ADD are reported; the difficulties in and limitations to the backside-illuminated ADD are analyzed.

Optimization of the SiPM Pixel Size for a Monolithic PET Detector

The Silicon Photomultiplier (SiPM) is a novel photodetector being developed for high energy physics applications. SiPM is attractive for PET imaging because it is compact; provides high gain at low voltage; is insensitive to magnetic fields; has a fast timing response; and is potentially inexpensive and CMOS-technology compatible. Researchers have characterized SiPM performance mostly in areas relevant to high-energy physics and astrophysics applications, with some findings demonstrating the potential usefulness of SiPM in positron emission tomography (PET) imaging. Using our photodetector characterization test-stand equipped with a 20 GSps sample rate and 6 GHz bandwidth oscilloscope, we have measured the gains in the range of 1-3 times 105 room temperature for SiPM samples obtained from different producers. We have measured the rising times and their standard deviations. Based on the measured dark-count rates at room temperature, we have also estimated the achievable energy resolution of SiPM in PET imaging when used with LSO and BGO scintillators.

The main topic of this work is the study of detector systems composed of a scintillator, a photodetector and readout electronics, for Positron Emission Tomography (PET). In particular, the timing properties of such detector systems are studied. The first idea is to take advantage of the good timing properties of the NINO chip, which is a fast preamplifier-discriminator developed for the ALICE Time of flight detector at CERN. This chip uses a time over threshold technique that is to be applied for the first time in medical imaging applications. A unique feature of this technique is that it delivers both timing and energy information with a single digital pulse, the time stamp with the rising edge and the energy from the pulse width. This entails substantial simplification of the entire readout architecture of a tomograph. The scintillator chosen in the detector system is LSO. Crystals of 2x2x10mm<sup>3</sup> were used. For the photodetector, APDs were first used, and were then replaced by SiPMs to make use of their higher gain. These different elements that constitute the whole detector system are presented, and their functioning is explained. Within the European FP6 BioCare project, a test setup comprising 2 identical detector systems in coincidence was developed. Each one is composed of a LSO scintillator, an APD, a preamplifier and the NINO readout electronics. The energy resolution was measured to 16% for 511keV y-rays. This is comparable to the resolution obtainable with PMT based systems. The same APD based system was also studied with 122keV X-rays, to assess its potential for combined PET-CT imaging. The energy resolution in this case was measured to 70% as compared to 50% with PMTs. This is explained by a lack of sensitivity of the readout electronics to low charges. The time resolution for 2 such detectors in coincidence was demonstrated to be of 1.6ns FWHM. This is 3 times worse than what one could obtain with PMT based systems under the same conditions. The contributions of the different elements of the detector system to the time precision were identified. The relative contributions of the electronics, the APD and the LSO were found to be 20%, 30% and 50%, respectively. However, the fact that the LSO crystal dominates the time resolution is partly attributed to the readout mechanism of the APD. The relatively low gain of the APD prevents the readout electronics from detecting fewer than 20 photoelectrons coming from the LSO whereas the PMT is sensitive to a single photon. Therefore, a new photodetector was chosen and characterized: the Silicon PhotoMultiplier (SiPM). This photodetector finds increasing interest in the scientific community, offering better characteristics than APD in terms of gain and of single photon sensitivity. They could also be used for TOF applications in PET. The SiPM used were supplied by ST microelectronics and tested at CERN in the context of a scientific collaboration. SiPM from Hamamatsu were also tested for comparison. The study of a 1x1mm<sup>2</sup> SiPM has demonstrated that the time resolution of the SiPM coupled to the NINO chip is of 180ps rms from the detection of single 405nm laser photon. This means that this combination of SiPM+NINO can also be used as a detector system for the detection of single photons such as in Cerenkov light detection or in fluorescence spectroscopy, where good time precision is required. In the case of PET, the response of the SiPM to LSO photons following the interaction of a 511keV y-ray was modeled. Since in this case the output current from the SiPM is too high to be directly read out by the NINO circuit, an interface consisting of a differentiating circuit was developed. Furthermore, the typical size of LSO crystals (2x2mm<sup>2</sup>) together with the high number of SPAD cells required to detect the photons emitted by the LSO implies that larger size SiPM (3x3mm<sup>2</sup>) have to be used. The work done during the thesis has shown a crucial influence of the SiPM terminal capacitance, which may be as high as 320pF for the Hamamatsu SiPM of 3x3mm<sup>2</sup>. In contrast with SiPMs of smaller size, this capacitance in parallel with a load resistance (e.g. scope or NINO) is large enough to significantly increase the rise time of the SPAD signals to the extent that the timing performance of the ensemble is severely degraded. An improved electronics interface is currently being studied to overcome this limitation. Another novel photodetector has also been studied in the context of this thesis: the microchannel plate (MCP) that is made of hydrogenated amorphous silicon (a-Si:H). The first samples were developed at the Institute of MicroTechnology (IMT) of Neuchatel and tested at CERN in a scientific collaboration. The advantage of this detector is the possibility to deposit on top of ASIC in a direct and vertical integration. Our preliminary investigation indicate that a current increase takes place along the borders of the MCP pores, possibly indicating the generation of a cascade of secondary electrons.

Silicon photomultipliers (SiPM) have a wide range of applications in high energy and medical detector physics. Their excellent timing properties and compactness make them particularly interesting for time of flight positron emission tomography (TOF-PET). This study aims at determining the optimum detector conditions for highest time resolution in a TOF-PET system. The measurements are based on the time over threshold method in a coincidence setup using the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. We compare commercial SiPMs of different SPAD types (Hamamatsu MPPC S10931-025P, S10931-050P and S10931-100P) and production batches. To explore the intrinsic timing capabilities of the SiPM, we coupled short, i.e. 2x2x5mm 3 , Ca-co-doped LSO:Ce crystals to the different MPPCs and obtained best results for the 50mm type of 142 4ps FWHM CTR. This performance makes SiPMs ideal candidates for photodetectors in high resolution TOF-PET systems. The results will be discussed in terms of SiPM dark noise and photon detection efficiency (PDE).

Silicon Photo-Multipliers (SiPMs) are increasingly becoming popular for discrete photon counting applications due to the wealth of advantages they offer over conventional photo-detectors such as photo-multiplier tubes and hybrid photo-diodes. SiPMs are used in variety of applications ranging from high energy physics and nuclear physics experiments to medical diagnostics. The gain of a SiPM is directly proportional to the difference between applied and breakdown voltage of the device. However, the breakdown voltage depends critically on the ambient temperature and has a large temperature co-efficient in the range of 40-60 mV/°C resulting in a typical gain variation of 3%-5%/°C [Dinu et al., in IEEE Nuclear Science Symposium, Medical Imaging Conference and 17th Room Temperature Semiconductor Detector Workshop (IEEE, 2010), p. 215]. We plan to use the SiPM as a replacement for PMT in the cosmic ray experiment (GRAPES-3) at Ooty [Gupta et al., Nucl. Instrum. Methods Phys. Res., Sect. A 540, 311 (2005)]. There the SiPMs will be operated in an outdoor environment subjected to temperature variation of about 15 °C over a day. A gain variation of more than 50% was observed for such large variations in the temperature. To stabilize the gain of the SiPM under such operating conditions, a low-cost, multi-channel programmable power supply (0-90 V) was designed that simultaneously provides the bias voltage to 16 SiPMs. The programmable power supply (PPS) was designed to automatically adjust the operating voltage for each channel with a built-in closed loop temperature feedback mechanism. The PPS provides bias voltage with a precision of 6 mV and measures the load current with a precision of 1 nA. Using this PPS, a gain stability of 0.5% for SiPM (Hamamatsu, S10931-050P) has been demonstrated over a wide temperature range of 15 °C. The design methodology of the PPS system, its validation, and the results of the tests carried out on the SiPM is presented in this article. The proposed design also has the capability of gain stabilization of devices with non-linear thermal response.

Silicon photomultipliers (SiPMs) attract extensive attentions for detecting optical photons in high energy physics and medical imaging due to its high gain, high photon detection efficiency (PDE), low operation voltage and fast timing response. We use a binary optical element (BOE) made of the silicon flexible optical material to transform the incident light intensity expressed as Gaussian distribution into uniform in space, making the incident photons be detected by the SiPM evenly. In this way, we can make full use of all cells of the SiPM, and more cells operating means more photons being detected for a certain pulse, which can increase the count rate of the incident photons and therefore improve the detection efficiency of the SiPM. Also, by comparing the outputs of the SiPM in different light intensity inputs, we can obtain the best light intensity fit for the SiPM along with the suitable scintillator and surface treatment for the positron emission tomography (PET) imaging based on the SiPM. In our experiments, we use the laser pulse as the SiPM input, whose light intensity can be expressed as Gaussian distribution in space, and then compare the readouts of the SiPM with and without the BOE. As expected, by using the BOE, the timing resolution and energy resolution of the SiPM are better than those without using it for some certain light intensity.

There is a need for low cost, miniature, integrated optical systems for bioassay monitoring to meet the growing in vitro and point-of-care diagnostics markets. To this end, we are investigating the use of silicon photomultipliers (SiPM) as device upon which to base our technology development. SiPMs have been used successfully in many high-energy physics applications, but their application as a fully integrated biological detection platform has not been shown. In this paper we will present a new detection platform for the measurement of fluorescent biomolecules at much lower concentrations than commercially available systems. Our results show approaches that demonstrate the use of SiPM for the detection of fluorescent proteins and fluorescent-labelled DNA sequences. The SiPM and sample platforms are integrated so that the minimum distance separating the detector from the sample is realised. In addition, direct immobilisation of the DNA sequences onto the SiPM surface is achieved. This combined approach shows improved sensitivities for both the fluorescent proteins and fluorescent-labelled DNA. We are presenting results that show the use of SiPM as a successful technology for the measurement of fluorescent biomolecules at improved lower concentrations.

GAGG:Ce (Gadolinium Aluminum Gallium Garnet doped with Cerium) crystals coupled with SiPMs (Silicon Photomultipliers) form a scintillation detector system commonly used in various fields such as medical imaging, high-energy physics, and radiation detection. The purpose of this study is to compare the energy resolution performance of GAGG:Ce crystals coupled to various SiPMs. Comparisons were conducted among crystals and photomultipliers from various companies, all featuring a pixel size of 3 × 3 mm2. The detector system consisted of a crystal optically coupled to a SiPM array, along with the photomultiplier electronics. The entire system was housed within a black box, positioned adjacent to a radiation source—a closed-type 137Cs isotope, intended for laboratory use—with a radioactivity value of 0.03 MBq. All crystals underwent surface polishing and were subsequently wrapped with multiple layers of Teflon. Additionally, an optical grease was applied between the crystal and the photomultiplier to enhance their optical properties. The size of the crystals and their coupling with the SiPMs were found to significantly impact the energy resolution. Excellent energy resolution values, up to 4.9%, were achieved for the majority of the crystals and SiPMs. Utilizing the 30035 C-Series SiPM from Onsemi, GAGG-HR single crystals exhibited the most favorable energy resolution values, achieving 5% for both 3 × 3 × 5 mm3 and 3 × 3 × 8 mm3 sizes. Furukawa GAGG:Ce crystals demonstrated consistent energy resolution ranging from 6.6% to 7.2%. Employing the PM3350 Trench type SiPM by Ketek, GAGG-HR single crystals achieved the best energy resolution values of 5.5% for 3 × 3 × 5 mm3 and 5.7% for 3 × 3 × 8 mm3 sizes. Furukawa GAGG:Ce samples of 3 × 3 × 5 mm3 crystals yielded an energy resolution value of 6.8%. When using the S10362-33 Series SiPM from Hamamatsu, the best energy resolution values were obtained with the S10362-33-050 SiPM coupled with 3 × 3 × 8 mm3 GAGG:HR crystals, resulting in an energy resolution of 8%.

Silicon Drift Detectors (SDDs) and Silicon PhotoMultipliers (SiPMs) are two silicon based photo-detector technologies for scintillator readout with applications in medical imaging, nuclear physics, space research and others. These devices provide compact scintillator readout solutions for spectroscopy, imaging and timing measurements. Goal of this work, is to make a comparison between these two photo-detector technologies, in particular for gamma-ray spectroscopy measurements. SDDs are characterized by very high quantum efficiency (> 80%) with no multiplication which helps to keep the statistical contribution to the energy resolution close to Poisson limit but, at the same time, makes the system very sensitive to readout electronics noise. SiPM devices on the other hand, have a very high multiplication gain (105–106) which makes the electronics noise contribution almost negligible, with only the dark count rate contribution which can be reduced with moderate cooling. However, they are characterized by lower Photo Detection Efficiency (PDE) ∼30–40% and statistical spread of the multiplication gain. In order to compare the spectroscopy performances of the two photo-detectors, we evaluate the energy resolution achievable with them using a large Lanthanum Bromide scintillator. Expected energy resolution with 1″ LaBr 3 :Ce and its sensitivity to various parameters of SDDs and SiPMs are evaluated to analyze the photo-detector performances. With SDD arrays of 2 nA/cm2 leakage current technology, an energy resolution of 3.0% has already been measured at −20 °C with Cs-137 source at a shaper peaking time of 6 μs. In this work, we measure an energy resolution of 4.9% with a SiPM tile readout by an RC filter (300 ns time constant) and a temperature of around 36 °C. These results are then used to validate the theoretical energy resolution estimates and to generalize the comparison of these two readout technologies.

Silicon photo-multipliers, often called SiPM, are semiconductor photon detectors built from a square matrix of avalanche photo-diodes on common silicon substrate. SiPM have been proposed for several different applications in High Energy Physics, in particular where a large detection granularity is needed. In this presentation the results of a radiation hardness test performed at the Frascati Neutron Generator are presented. Several SiPM of different manufacturers have been irradiated integrating up to 7 1010 1-MeV-equivalent neutrons per cm2. For the first time, their performance have been recorded during the neutron irradiation and a gradual deterioration of their properties was found to happen already after an integrated dose of the order of 108 1-MeV-equivalent neutrons per cm2. The Frascati Neutron Generator (FNG) FNG uses a deuteron beam accelerated up to 300 keV impinging on a deuteron target to produce a nearly isotropic 2.5 MeV neutron output via the D(d,n)3He fusion reaction. The beam current at the target can be regulated up to 1 mA resulting in a maximum neutron production rate of 5 108 neutrons on the whole solid angle per second. Through the monitoring of the rate of associated emitted particles, protons or alpha, the neutron emission rate can be monitored on-line. This gives the unique possibility of measuring the effect of neutrons as long as the irradiation takes place. On-Line Measurements Six devices produced by the IRST and four produced by the Hamamatsu have been tested with neutrons. Depending on the distance from the production point, in four days of test, the SiPM integrated between 0.18 and 7.32 1-MeV-equivalent neutron per cm2. The current drawn by each device and its dark counting rate were continuously monitored and recorded while being irradiated. Fig. 1 shows that the current drawn by the SiPM starts to increase soon after the beginning of the irradiation. No differences between the current behavior of tested devices were found. The effects of the different neutron fluences are not visible at the level we operated. The neutron flux was kept off for a whole night while the currents were recorded. No significant recovery effects appeared. The absolute value of the current and the increasing rate, once the flux was back on, didn't change. The neutron beam has been paused several times in order to perform low voltage scans during the irradiation runs and to measure the effects on the dark currents and on the dark counting rates for different bias values. In the low voltage scans the current behavior changed rapidly with the integrated dose as it is shown in Fig.2. Off-Line Measurements The SiPM have been tested with cosmic rays before and after the neutron irradiation and the charge spectra obtained are shown in Fig 3. After the neutron irradiation, the gain was found to be about the half of the initial one (Fig.3 Bottom) and the noise pedestals (Fig. 3 Top) are much broader. The main effect is an important reduction of the detection efficiency from more than 95% to about 70%. Fig2: Measured currents as a function of the low voltage supply after different integrated doses Fig3: SiPM charge spectra with cosmic rays before (top) and after (bottom) the neutron irradiation. Fig1: Increasing factor of the current drawn by the SiPM as a function of the integrated neutron dose.

Inorganic scintillators are widely used for fast timing applications in high-energy physics (HEP) experiments, time-of-flight positron emission tomography and time tagging of soft and hard x-ray photons at advanced light sources. As the best coincidence time resolution (CTR) achievable is proportional to the square root of the scintillation decay time it is worth studying fast cross-luminescence, for example in BaF2 which has an intrinsic yield of about 1400 photons/MeV. However, emission bands in BaF2 are located in the deep-UV at 195 nm and 220 nm, which sets severe constraints on photodetector selection. Recent developments in dark matter and neutrinoless double beta decay searches have led to silicon photomultipliers (SiPMs) with photon detection efficiencies of 20%–25% at wavelengths of 200 nm. We tested state-of-the-art devices from Fondazione Bruno Kessler and measured a best CTR of 51 ± 5 ps full width at half maximum when coupling 2 mm × 2 mm × 3 mm BaF2 crystals excited by 511 keV electron–positron annihilation gammas. Using these vacuum ultraviolet SiPMs we recorded the scintillation kinetics of samples from Epic Crystal under 511 keV excitation, confirming a fast decay time of 855 ps with 12.2% relative light yield and 805 ns with 84.0% abundance, together with a smaller rise time of 4 ps beyond the resolution of our setup. The total intrinsic light yield was determined to be 8500 photons/MeV. We also revealed a faster component with 136 ps decay time and 3.7% light yield contribution, which is extremely interesting for the fastest timing applications. Timing characteristics and CTR results on BaF2 samples from different producers and with different dopants (yttrium, cadmium and lanthanum) are given, and clearly show that the the slow 800 ns emission can be effectively suppressed. Such results ultimately pave the way for high-rate ultrafast timing applications in medical diagnosis, range monitoring in proton or heavy ion therapy and HEP.

Comparison of new SiPM devices for applications in High-Energy physics

The silicon photomultiplier (Si-PM) is a promising photodetector for a high-resolution PET scanner due to its small size, high gain and lower sensitivity to magnetic fields. There are several commercially available Si-PM arrays with different pixel sizes and fill factors, and these parameters can affect the performance of a PET block detector read out by these devices. We compared the performance of block detectors using 4 × 4 Si-PM arrays with 25 µm (Hamamatsu S11064-025P) and 50 µm (S11064-050P) pixels combined with the same 15 × 15 matrix LGSO block made of 0.7 × 0.7 × 6 mm3 scintillator pixels. Evaluated characteristics include photopeak linearity, energy resolution and positioning performance. Although the photopeak linearity and energy resolution are slightly better for the Si-PM with 25 µm pixels, the position performance measured by the separation of the position histogram is significantly better for the Si-PM with 50 µm pixels. We conclude that using the Si-PM with 50 µm pixels will provide a better solution for the development of ultrahigh-resolution PET systems.

A novel digital imaging system using silicon-photomultiplier (SiPM) sensors is currently under development at DESY, Hamburg, for applications in high energy physics and photon science. The system is a hybrid based on a thinned SiPM-sensor chip from MPG-HLL, Munich. The final readout chip will comprise a 32-by-32 pixel matrix with 50-μm pitch and will be realized in IBMs 130-nm CMOS technology. It provides active quenching and recharging, fast and combinatorial trigger, time-to-digital converter, and fast readout of the pixel pattern at MHz-frame rates. A first prototype ASIC with 16-by-16 pixel matrix was designed including corresponding pixel electronics, fast trigger and single-row combinatorial trigger. Measurements on two samples of prototype ASIC give information about timing requirements. A TDC-bin width of ≤90 ps and a veto-time window of eight clock cycles are demanded. The maximal power consumption amounts to 15 μW per pixel at 1.2-V and 4 μW per pixel at 3.3-V supply voltage.