Photomultipliers
Photomultiplier tube (PMT) diagram
Click on the diagram to show an interaction
Gamma ray interacts with scintillating crystals and generates visible light.
The crystal is in light-tight housing with reflective material on surfaces to reflect light back to the PMT.
Light pipe is a thin layer of transparent optical gel to couple crystal to the PMT entrance window.
Photocathode emits electrons when the light photons hit it.
Focusing grid is just beyond the photocathode and is a low voltage just to direct electrons to first dynode
Each dynode is held at an additional ~100 V relative to the previous. The dynodes attract the electrons, and each ejects several (~3) additional electrons from the incident electrons.
PMTs typically have 10-12 dynodes (stages) with a total amplification of 310 to 312. The last element is the anode that then sends the signal to a preamplifier and is shaped into a logic pulse.
The amplitude of the pulse is proportional to number of electrons produced at photocathode ∝ energy of incident radiation. Energy discrimination is then typically performed with a pulse height analyzer. Pulses within a preset window, based on a lower-level discriminator (LLD) and upper level discriminator (ULD), are counted.
PMTs must be encases in a thin magnetic shield to protect against the effects of the earth's magnetic field. Orientation relative to a magnetic field changes in the gains. This susceptibility is why PMTs cannot be used in hybrid PET/MR machines.
Avalanche Photodiodes (APD), Silicon photomultipliers (SiPM)
Solid state photo multipliers can be much more compact and are much less susceptible to magnetic fields.
Has similar photo detection efficiency and gain as a traditional PMT. Can use a lower voltage and get faster response.
Semiconductors (CZT, CdTe)
Can be used to replace crystal+PMT system. Pixelated. Costly, so typically used in small systems (cardiac). Because of high efficiency (density and low energy to create ion pairs), can be made small (2 mm × 2mm). Can be used at room temperature.
Energy resolution is independent of energy, depends on charge collection. Generally has better energy resolution (~6%) than scintillators (~10%) -- more electron-hole pairs per keV than light photons.
Have been used as hand probes to look for injected radioactivity intra-operatively.