1. 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
Spring, 2009
Phys 521A

2. 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

3. 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 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

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

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

6. 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

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

8. 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

9. 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

10. Spring, 2009
Phys 521A

11. 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

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

13. Scintillators
Spring, 2009
Phys 521A

14. 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

15. Spring, 2009
Phys 521A