Photon detection Visible or near-visible wavelengths Need photosensitive element and transparency (long λabs length) Generate photoelectron (or e-h pair), amplify and collect signal Photomultiplier tubes Workhorse; sensitive, relatively cheap, operating issues Si photodiodes Cheap, reliable, widely used Pixellated photon detectors High efficiency and good spatial resolution (e.g. CCD) Issues around data readout speed Developing area Color Wavelength (nm) Red 625 – 740 Orange 590 – 625 Yellow 565 – 590 Green 520 – 565 Cyan 500 – 520 Blue 435 – 500 Violet 380 - 435 Spring, 2009 Phys 521A

Photodetector device characteristics Quantum efficiency (photoelectrons/incident photon) Collection efficiency (geometrical acceptance, etc) Gain: electrons collected per photoelectron Dark current: signal in absence of light (noise) Energy resolution: function of signal statistics and noise level Dynamic range: difference between single photon and input optical power at which signal saturates Time response: delay and width of electrical signal relative to incident photon time Rate capability: How quickly can subsequent photons be registered? Spring, 2009 Phys 521A

Photomultiplier tubes Evacuated tube supplied with high voltage (many 100s of volts) Photocathode ejects electrons (PE effect) E-field accelerates them toward surface (dynode) with low work function, liberating additional electrons Amplification factor of 3-5 per dynode; many stages lead to large 104-107 amplification factors (resistive voltage divider network) that can be tuned via operating voltage Cannot operate in strong B fields (ev x B force) Dark current (leakage current, thermionic and field emission); fn of operating voltage Need special windows for input in UV Spring, 2009 Phys 521A

More on PMTs Light collection area can be large (50cm diameter in Super-K) Spectral response (photocathode): Lake Super-K Spring, 2009 Phys 521A

Large range of PMT choice Hamamatsu tubes (part of catalog of >400 models) Spring, 2009 Phys 521A

Silicon Photodiodes P-N junction; input photon creates e-h pair, pushing e into conduction band P-layer collects holes, N-layer collects electrons Features: High quantum efficiency Linear flux response Spectral response peaked toward “red” Insensitive to B fields Low noise (dark current) Spring, 2009 Phys 521A

Photodiode Specifications Hamamatsu specs (of ~80) Absorption strong fn of wavelength Spring, 2009 Phys 521A

Avalanche Photodiodes Photodiodes with large reverse bias (>100 V) applied Large bias accelerates liberated electrons, causing them to create additional e-h pairs (avalanche) Signal amplification is strong function of bias for moderate bias the signal remains proportional to the input, but bias and temperature must be controlled Large bias generates large, saturated signal (“Geiger”mode, output signal independent of input signal size) Large quantum efficiencies possible, along with sub-ns time response Spring, 2009 Phys 521A

PMT/APD comparison PMT and avalanche-photodiode response must be matched to the output spectrum from the scintillator used; some common examples shown here Spring, 2009 Phys 521A

Spring, 2009 Phys 521A

Pixellated photon detectors (PPD) Recent development – solid state devices based on arrays of avalanche photodiodes Also known as “SiPM, or silicon photomultipliers” Create large array (~103 APDs) packed into small (~1mm2) area Each APD operates in limited Geiger mode (binary signal) Count photons by digitally summing cell outputs Goal is to obtain CCD-like efficiency and spatial resolution with fast, integrated readout (combined manufacture of PPD and ASIC) ASIC = application specific integrated circuit, i.e. custom electronic chip Spring, 2009 Phys 521A

PPD used by T2K Hamamatsu MPPC – array of APD operated in Geiger mode 50x50μm pixels; 667/device Operating voltage ~70V; quantum efficiency ~15% @ 550nm Spring, 2009 Phys 521A

Scintillators Spring, 2009 Phys 521A

Scintillation counters Workhorse of particle detectors Ionization from charged particles excites molecules; de-excitation results in scintillation light Two main types: organic (e.g., hydrocarbons) and inorganic (crystals, like NaI) Important co-process is fluorescence, where photon excites a molecule (fluor) which subsequently de-excites via a longer wavelength photon Fluors are needed both to avoid self-absorption and to enable better spectral match to photon detectors Only few % of deposited energy converted to scintillation light Spring, 2009 Phys 521A

Spring, 2009 Phys 521A