Prospects to Use Silicon Photomultipliers for the Astroparticle Physics Experiments EUSO and MAGIC A. Nepomuk Otte Max-Planck-Institut für Physik München

A. Nepomuk Otte MPI für Physik2 Outline EUSO & MAGIC Why new photon detectors? Photon detector requirements The SiPM principle MEPhI and Pulsar HLL in Munich

A. Nepomuk Otte MPI für Physik3 Extreme Universe Space Observatory

A. Nepomuk Otte MPI für Physik4 Major Atmospheric Gamma Imaging Cherenkov Telescope ~ 10 km Particle shower ~ 1 o Cherenkov light ~ 120 m Gamma ray

A. Nepomuk Otte MPI für Physik5 Motivation for new Photon Sensors Photon detection efficiency (PDE) of state of the art photomultiplier tubes ≈20% A higher PDE results in a better signal to noise ratio (SNR) ≈ 80% PDE improves SNR by a factor 2…3 Same effect as increasing the MAGIC mirror from 17m diameter to 70m Both experiments can lower their energy threshold with more sensitive sensors

A. Nepomuk Otte MPI für Physik6 What is gained by a lower Threshold? access to lower γ-energies → deeper look into the universe (higher redshifts) → new sources → Egret 280 sources with 0.1m² active detector area (<10GeV) → ACT‘s 15 sources with m² active detector area (>300GeV) extend accessible energy range –overlap with existing experiments AUGER, AGASA, HIRES detailed study of GZK cutoff improved energy resolution MAGICEUSO

A. Nepomuk Otte MPI für Physik7 Photon Detector Requirements sensitive range [nm] sensor size [mm²] single photon counting dynamic range per sensor [phe] max. dark noise per pixel [1/s] rate capability per pixel [1/s] detection efficiency radiation hardness EUSO 330…4004 x 4yes >50%yes MAGIC 300…60030 x 30yes >20%no most requirements are similar large differences in sensitive range and pixel size challenging: detection efficiency

A. Nepomuk Otte MPI für Physik8 The Silicon Photomultiplier An avalanche photodiode (APD) in Geiger mode is a high efficient single photon counting device

A. Nepomuk Otte MPI für Physik9 The Silicon Photomultiplier BUT: Output signal of a single Geiger APD is independent of number of photoelectrons An avalanche photodiode (APD) in Geiger mode is a high efficient single photon counting device

A. Nepomuk Otte MPI für Physik10 The Silicon Photomultiplier An avalanche photodiode (APD) in Geiger mode is a high efficient single photon counting device BUT: Output signal of a single Geiger APD is independent of number of photoelectrons Solution: Combine an array of small Geiger APDs onto the same substrate (less then 1 photon per cell) …

A. Nepomuk Otte MPI für Physik11 MEPhI and Pulsar Enterprize 1 mm P. Buzhan et al. about 20% active area limits photon detection efficiency

A. Nepomuk Otte MPI für Physik12 Characteristics characteristics of current prototypes: geometry: 24 x 24 pixels = 576 pixels within 1mm 2 available up to 1024 pixels / mm² Operating voltage: 50 V to 58 V Gain: 10 5 up to ~ single pixel time resolution: 570 ps FWHM single pixel recovery time: 1μs dark count rate: 10 6 counts per second at room temperature

A. Nepomuk Otte MPI für Physik13 R&D Goals to improve existing MEPhI-Pulsar Prototypes P. Buzhan et al. NIM A 504 (2003) Luminescence of hot avalanche electrons gives rise to crosstalk with neighboring APD cells Gain 10 6 ) Counter measures: grooves between pixels to absorb photons reduce gain (4% Gain 10 5 ) Photon detection efficiency determined by: Intrinsic QE packing density of pixels Geiger breakdown probability transmittance of entrance window work on: reduction of dead area improve blue sensitivity optimization of entrance window

A. Nepomuk Otte MPI für Physik14 MPI Semiconductor Laboratory in Munich Different approach to increase photon detection efficiency use of back illumination principle → no dead area Si photon depleted bulk avalanche regions path of the photo electron output 50µm … 450µm Blow up of one “micro pixel”

A. Nepomuk Otte MPI für Physik15 MPI Semiconductor Laboratory in Munich Simulations are in final stage: Operating voltage of avalanche region 50V Geiger breakdown probability 60%...90% average drift time differences < 1ns drift rings p+ shallow p+ avalanche region photondrift path of a photo electron quench resistor output line deep n n bulk

A. Nepomuk Otte MPI für Physik16 Summary and Outlook We investigate the SiPM as photon detector in MAGIC and EUSO First SiPM prototypes are very promising SiPM prototypes already usable for some applications (e.g. PET, TileCal for Tesla) The development is pursued in two different ways -front MEPhI and Pulsar -back HLL in Munich A lot of R&D ahead: increase effective QE up to 70% increase UV sensitivity reduce crosstalk increase SiPM size