Spring, 2009 Phys 521A 1

Color Wavelength (nm)

Red 625 – 740Orange 590 – 625Yellow 565 – 590Green 520 – 565Cyan 500 – 520Blue 435 – 500Violet 380 - 435

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

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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?

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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, thermionicand field emission);fn of operating voltage

• Need special windowsfor input in UV

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More on PMTs

• Light collection area can be large (50cm diameter in Super-K)

• Spectral response (photocathode):

Lake Super-K

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Large range of PMT choice

• Hamamatsu tubes (part of catalog of >400 models)

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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)

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Photodiode Specifications

• Hamamatsu specs (of ~80)

• Absorption strong fn of wavelength

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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

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PMT/APD comparison

• PMT and avalanche-photodiode response must be matched to the output spectrum from the scintillator used; some common examples shown here

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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

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PPD used by T2K

• Hamamatsu MPPC – array of APD operated in Geiger mode

• 50x50μm pixels; 667/device• Operating voltage ~70V; quantum

efficiency ~15% @ 550nm

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Scintillators

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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

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