nuclear med detectors Flashcards
scintillation crystal
radiation given to electrons in scintillation crystal via photo. effect or compton,
some electrons move to high energy state
de-excitation of electrons to ground state produces light
all light is summed
scintillator detector
scintillator converts incident radiation to light
photodetector converts light to electric signal
electronics amplify, shape, and process
scintillator crystal properties
density:
higher, more sensitive
light output:
higher, higher sensitivity, lower noise
decay constant:
time needed for crystal to reset for new photon detection
refractive index:
ease at which scintillator can couple to PMT
wavelength of light
match to detector
eg. Nal, Csl, BGO, LSO
sodium iodide crystal
good absorber of x rays or gamma
photoelectric absorption
efficient
inexpensive
fragile to mechanical and thermal stress
need good temperature regulation and moisture control
photomultiplier tubes
photodetectors incorporated into a photomultiplier tube in the form of a photocathode
when struck by light, photocathode ejects electrons
electrons are attracted through an amplification process to the anode of the photomultiplier tube
amplification in PM tube
occurs through a series of dynodes in the PM tube
electron leaves photocathode
focused and accelerated towards dynode at higher potential and gains Ek
Ek absorbed in dynode and released as multiple electrons
electrons accelerated to successive dynodes at higher potentials, multiplies in each stage
collected at anode as pulse of charge
determination of photon energy
scintillator:
light produced is proportional to energy absorbed
pulse height energy spectra
pulse height proportional to energy absorbed
pulse height = Ein - Eout
energy absorbed due to photo effect or compton
relatively flat then pulse at certain photon energy
counts vs photon energy graph
energy absorbed by the photoelectric effect
full energy of incident photon absorbed
characteristic radiation may or may not be absorbed
semi conductor detectors adv
scintillator detectors
bulky
relatively poor energy resolution
semi conductor detectors
superior energy resolution
slim
costly
direct conversion to electrical signal
more efficient absorber than gas chambers
eg. CZT
semi conductor detectors disadv
cooling required
sensitive to temperature
impurities
limits electrical signal
expensive to prevent
semiconductor detectors
CZT cadmium zinc telluride
interaction of gamma photon (with czt) is measured directly as electrical pulse
hole electron pair collected across a potential difference
generation of electron hole pairs which are proportion to the energy of the incoming and absorbed radiation
avalanche photodiodes (APDs)
semiconductor detector sensitive to light photons
incident light penetrates
then it gets absorbed
then electron-hole pairs are generated.
avalanche photo diodes adv and disadv
compact
high quantum efficiency
low bias voltage
low background signal
disadv
lower gain than PMT
noise due to lower signal
low timing resolution
more temperature sensitive
silicon photomultipliers SIPM
array of APDs operated in Geiger mode
(same charge regardless of no. of incident photons)
large gain
faster than APD
insensitive for magnetic field
