Photodetectors Flashcards
What is a photodetector?
A way of recording the optical light output from scintillator
What are two types of photodetectors?
Photomultiplier Tubes:
very sensitive
well understood
fragile
Solid state:
mass produced
scalable
robust
What is a photomultiplier tube (PMT)?
Sensitive optical photon measurement device
High voltage between anode and cathode, divided between dynodes
What is the process of the PMT?
Electrons created at photocathode
Accelerated and guided towards first dynode
Impacts dynode and creates more electrons
New electrons accelerated to next dynode
Number of electrons multiplied at each stage
Electron bunch reaches anode as measurable signal
What must be achieved for maximise the efficiency of optical photon transfer from scintillator to photocathode?
The changes in refractive index along the photon path must be minimised
This is achieved using a coupling compound to mount the scintillator
What is the gain of a photodetector?
The ratio of its output current to the current produced by the photons that hit it
(amplifies the S/N ratio)
What is the amount of multiplication controlled by?
the high voltage (HV)
which typically goes from a few 100 to 1000V
What is the divider in a photodetector?
Where the HV is split between the dynodes to from a multiplication chain
Can change resistor values to alter behaviour (e.g. can assist charge collection at specific terminal)
What happens to the electron after it is absorbed by the photocathode?
The electron’s energy is equal to the photon energy (hv)
What happens when the electron migrates through the photocathode?
Energy lost due to electron-electron interactions
What happens when the electron escapes the surface of the photocathode?
It must have energy at least equal to work function of photocathode
It travels into vacuum towards dynode chain
What is the photocathode?
Thin layer on inside of PMT window
materials: Bialkali, multialkali
What are solid state photodetectors?
Semiconductors pn junctions
What are the two types of photodiodes in the solid state photodetector?
Conventional photodiode: no internal gain, simple electron-hole pair collection
Avalanche photodiode: internal gain due to applied electric field
What must the readout sensor be matched to?
The scintillator output
