Photosensor Characterization for the Cherenkov Telescope Array: Silicon Photomultiplier versus Multi-Anode Photomultiplier Tube

Photomultiplier tube technology has been the photodetector of choice for the technique of imaging atmospheric Cherenkov telescopes since its birth more than 50 years ago. Recently, new types of photosensors are being contemplated for the next generation Cherenkov Telescope Array. It is envisioned that the array will be partly composed of telescopes using a Schwarzschild-Couder two mirror design never built before which has significantly improved optics. The camera of this novel optical design has a small plate scale which enables the use of compact photosensors. We present an extensive and detailed study of the two most promising devices being considered for this telescope design: the silicon photomultiplier and the multi-anode photomultiplier tube. We evaluated their most critical performance characteristics for imaging gamma-ray showers, and we present our results in a cohesive manner to clearly evaluate the advantages and disadvantages that both types of device have to offer in the context of GeV-TeV gamma-ray astronomy.Comment: submitted to SPIE Optics+Photonics proceeding

Pixel Crosstalk and Correlation with Modulation Transfer Function of CMOS Image Sensor

The Modulation Transfer Function is a common metric used to quantify image quality but inter-pixel crosstalk analysis is also of interest. Because of an important number of parameters influencing MTF, its analytical calculation and crosstalk predetermination are not an easy task for a CMOS image sensor. A dedicated test chip (using a technology optimized for imaging applications) has been developed in order to get both MTF data and influence of the various areas of the pixel to its own response and the one of its neighbors. In order to evaluate the contribution of pixel elementary patterns (particularly the in-pixel readout circuitry), several kernels of shielded pixels have been implemented with the central pixel locally unmasked. The results obtained with pixel kernels and direct MTF measurements, performed on the same chip at different wavelengths, are analyzed and compared in order to correlate them and draw conclusions that can be applied at the design level. Additional data resulting from spotscan measurements allow us to verify our hypothesis on different pixels

SPICE Electrical Models and Simulations of Silicon Photomultipliers

We present and discuss a comprehensive electrical model for Silicon Photomultipliers (SiPMs) based on a microcell able to accurately simulate the avalanche current build-up and the self-quenching of its Single-Photon Avalanche Diode (SPAD) “pixel” with series-connected quenching resistor. The entire SiPM is modeled either as an array of microcells, each one individually triggered by independent incoming photons, or as two macrocells, one with microcells all firing concurrently while the other one with all quiescent microcells; the most suitable approach depends on the light excitation conditions and on the dimension (i.e. number of microcells) of the overall SiPM. We validated both models by studying the behavior of SiPMs in different operating conditions, in order to study the effect of photons pile-up, the deterministic and statistical mismatches between microcells, the impact of the number of firing microcells vs. the total one, and the role of different microcell parameters on the overall SiPM performance. The electrical models were developed in SPICE and can simulate both custom-process and CMOS-compatible SiPMs, with either vertical or horizontal current-flow. The proposed simulation tools can benefit both SiPM users, e.g. for designing the best readout electronics, and SiPM designers, for assessing the impact of each parameter on the overall detection performance and electrical behavior

CMOS pixels crosstalk mapping and its influence on measurements accuracy for space applications

Due to different local intra-pixel sensitivity and crosstalk between neighboring pixels, the Pixel Response Function of detectors (PRF - signal of the pixel as a function of a point source position) is generally non-uniform. This may causes problems in space application such as aperture photometry and astrometry (centroiding). For imaging applications, an important crosstalk yields to a loss of resolution, i.e. a poor image quality, commonly quantified by the Modulation Transfer Function (MTF). So, crosstalk study is of primary importance for our applications. A dedicated test chip (using a technology optimized for imaging applications) has been developed in order to get both MTF data and influence of the various areas of the pixel to its own response and the one of its neighbors. The results obtained with pixel kernels and direct MTF measurements, performed on the same chip at different wavelengths, are analyzed and compared in order to correlate them. So it is possible to draw conclusions -that can be applied at the design level - allowing to get a better MTF and to minimize errors on aperture photometry and centroiding computation

Radiation effects studies for the high-resolution spectrograph

The generation and collection of charge carriers created during the passage of energetic protons through a silicon photodiode array are modeled. Pulse height distributions of noise charge collected during exposure of a digicon type diode array to 21 and 75 MeV protons were obtained. The magnitude of charge collected by a diode from each proton event is determined not only by diffusion, but by statistical considerations involving the ionization process itself. Utilizing analytical solutions to the diffusion equation for transport of minority carriers, together with the Vavilov theory of energy loss fluctuations in thin absorbers, simulations of the pulse height spectra which follow the experimental distributions fairly well are presented and an estimate for the minority carrier diffusion length L sub d is provided

A Compact Neutron Scatter Camera Using Optical Coded-Aperture Imaging

The detection and localization of fast neutron resources is an important capability for a number of nuclear security areas such as emergency response and arms control treaty verification. Neutron scatter cameras are one technology that can be used to accomplish this task, but current instruments tend to be large (meter scale) and not portable. Using optical coded-aperture imaging, fast plastic scintillator, and fast photodetectors that were sensitive to single photons, a portable neutron scatter camera was designed and simulated. The design was optimized, an experimental prototype was constructed, and neutron imaging was demonstrated with a tagged 252Cf source in the lab

A position sensitive photon detector for the CLAS12 ring imaging Čerenkov application

The upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) to CLAS12 will offer unique possibilities to study the strong interaction and the internal nucleon dynamics. For this, excellent hadron identification over its full kinematic range is essential and a Ring Imaging CHerenkov detector (RICH) has been proposed for installation into CLAS12 to achieve this. This thesis describes studies performed towards the selection of a photon detector for use in the CLAS12 RICH, which will be a crucial component for the success of the RICH performance. MultiAnode PhotoMultiplier Tubes (MAPMTs) have been selected as the most promising photon detectors for the CLAS12 RICH. A high-precision laser test-stand was developed to characterise candidate MAPMTs and the results were used to determine the Hamamatsu H8500 MAPMT as the optimal device. Throughout the laser tests the H8500 MAPMT proved to be single photon sensitive as demanded, the spatial resolution was found to satisfy the required value of less than 1cm by 1cm and the device exhibited sufficiently low crosstalk levels of less than 3%. The response and crosstalk of the device showed a dependency upon the MAPMT construction, the magnitude of which was shown to be negligible and overall the H8500 provides a plane of suitable uniformity to satisfy the imaging functionality of the CLAS12 RICH. To further confirm the H8500 MAPMT choice, its performance in response to Cherenkov light within two prototype setups was evaluated. First a small-scale prototype was designed and constructed, incorporating one H8500 MAPMT to image Cherenkov rings created by cosmic muons traversing aerogel radiators. Extraction of the results required full understanding of the device based upon calibrations made with the laser tests. The prototype was also simulated in detail, allowing for a model description of the MAPMT to be validated. Secondly, 28 H8500 MAPMTs were used in a large-scale prototype to image Cherenkov rings produced by mixed hadrons traversing aerogel radiators, to evaluate their performance in a geometry and an environment similar to that expected in the CLAS12 RICH. Both prototypes revealed that the H8500 MAPMT can be used to successfully detect the required 7 photons per Cherenkov event to achieve pion/kaon separation in the CLAS12 RICH. Furthermore, a pion/kaon separation of more than 3 sigma at 6GeV/c was observed through a preliminary analysis of data extracted with the large-scale prototype. The prototype studies also confirmed the low-noise behaviour of the H8500 MAPMT. As a result of the laser test-stand and prototype tests, the choice of the H8500 MAPMT as the photon detector for the CLAS12 RICH was validated