Ionising Photon Interactions (II) and Detection Flashcards
What is the reaction cross section?
probability of an interaction interpreted as an effective area of interaction per particle
What is the differential cross section?
A refined version of the reaction cross section to account for dependence on other parameters (eg. directional effects such as scatter)
What is the attenuation?
Photoelectric; Compton; Pair production
What is the photoelectric absorption?
Resonant coupling between radiation and inner electron shell
Electron ejected with excess kinetic energy
The absorption process in diagnostic imaging
Produces contrast in an X-ray image.
Responsible for dose.
Proportional to Z4/E3
Note high Z dependence
What is Compton?
Billiard ball type interaction (relativistic) between photon and free electron
Incident photon deflected with energy transfer to electron
Energy lost in interaction responsible for change in deflected photon wavelength
A scattering process.
Produces scatter/noise in diagnostic image.
Responsible for dose.
Proportional to Z/E2
What is pair production?
Photon in excess of 1.02MeV passes close to nucleus of atom
Spontaneous conversion into electron and positron pair
Relevant to megavoltage radiotherapy
Not relevant to diagnostic X-ray imaging (typical energy is 40-100keV)
What do all detectors rely on?
ability of radiation to ionise matter
What are some different types of detectors?
Gas ionisation Scintillation media Semiconductors Photographic emulsion Thermal luminescence Chemical deposition media Radiophoto luminescence and optical density measurement Calorimetry
What is gas ionisation detection?
Geiger Muller (GM) tube
Several modes of operation
HT dependent
Efficiency of detection is poor: 2% at few MeV
Glass and gas are low Z materials: Glass walls are thin, Gas density is low.
What are the GM tube modes?
Low voltage region (A)
Ionisation chamber region (B)
Proportional counter region (C)
Geiger counter region (D)
What happens in the low voltage region in a GM tube?
Gamma ray ionises gas
Ions drift toward cathode/anode
Ions recombine before reaching anode/cathode.
What happens in the ionisation chamber region of a GM tube?
Gamma ray ionises gas
No time for ion recombination
Ions attracted to anode/cathode.
What happens in the proportional counter region of a GM tube?
Gamma ray ionises gas
Ions accelerated to cathode/anode
Collisions along their path produces further ionisation
Single gamma ionisation produces a many-ion current pulse
Gas amplification factor up to 10^6
What happens in the Geiger counter region of a GM tube?
HT set to place tube just below the threshold of spontaneous discharge
Gamma ray ionises gas
Ions greatly accelerated to cathode/anode
Collisions produce ion avalanche.
Gas amplification factor 10^10
What is Scintillation detection?
NaI(Tl) crystal scintillator
Sealed to prevent ingress of moisture and housed in a light-tight can.
A gamma interaction within the crystal generates a photon shower.
Number of photons proportional to gamma energy
What is photographic emulsion?
Film is sensitive to light and ionising radiation.
Used for imaging and personnel monitoring.
How does photographic emulsion work?
Relies on reduction of AgBr to Ag+ and Br- ions as a result of ionisation.
Incident radiation reduces (a few) silver bromide molecules in the grains to silver.
When the film is placed in developer these silver atoms act as a catalyst and accelerates the grain’s reduction to silver. The result is a distribution of silver grains across the plastic sheet.
It is necessary to remove any remaining silver bromide since this slowly blackens when exposed to light. This is done by a dissolving process (sodium thiosulphite - the fixer) which also hardens the gelatin of the emulsion.
The film is subsequently washed to remove all residual chemicals, in addition to which they might form streaks across the film when dry.
The optical density of the film is a direct record of past radiation exposure and apart from X-ray imaging is used in personnel monitoring.
What is thermal luminescence dosimetry?
A tool for personnel monitoring
Energetic structure of crystal includes energy traps (LiFl)
Ionisation causes some electrons to be caught by the traps
It has a regular crystalline structure, but when impurities are added imperfections arise in the lattice which act as energy traps.
Following an interaction with radiation, most excited electrons rapidly return to the ground state, but a few remain trapped in the impurity levels.
Upon heating, the trapped electrons are released from the traps and de-excite with subsequent emission of light.
These de-excitations occur in a well defined manner - ‘glow curve’.
Photon counting requires the use of a PMT.
The lithium fluoride used in TLD can take several forms, from small discs of material to cylindrical rods to powder.
The advantages of TLD includes the ability to measure accurately (2%) a dose over a very wide range (10-5 to 103 Gray).
