Ionising Photon Interactions (II) and Detection

Question 1

What is the reaction cross section?

Answer 1

probability of an interaction interpreted as an effective area of interaction per particle

Question 2

What is the differential cross section?

Answer 2

A refined version of the reaction cross section to account for dependence on other parameters (eg. directional effects such as scatter)

Question 3

What is the attenuation?

Answer 3

Photoelectric; Compton; Pair production

Question 4

What is the photoelectric absorption?

Answer 4

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

Question 5

What is Compton?

Answer 5

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

Question 6

What is pair production?

Answer 6

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)

Question 7

What do all detectors rely on?

Answer 7

ability of radiation to ionise matter

Question 8

What are some different types of detectors?

Answer 8

Gas ionisation
Scintillation media
Semiconductors
Photographic emulsion
Thermal luminescence
Chemical deposition media
Radiophoto luminescence and optical density measurement
Calorimetry

Question 9

What is gas ionisation detection?

Answer 9

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.

Question 10

What are the GM tube modes?

Answer 10

Low voltage region (A)
Ionisation chamber region (B)
Proportional counter region (C)
Geiger counter region (D)

Question 11

What happens in the low voltage region in a GM tube?

Answer 11

Gamma ray ionises gas
Ions drift toward cathode/anode
Ions recombine before reaching anode/cathode.

Question 12

What happens in the ionisation chamber region of a GM tube?

Answer 12

Gamma ray ionises gas
No time for ion recombination
Ions attracted to anode/cathode.

Question 13

What happens in the proportional counter region of a GM tube?

Answer 13

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

Question 14

What happens in the Geiger counter region of a GM tube?

Answer 14

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

Question 15

What is Scintillation detection?

Answer 15

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

Question 16

What is photographic emulsion?

Answer 16

Film is sensitive to light and ionising radiation.

Used for imaging and personnel monitoring.

Question 17

How does photographic emulsion work?

Answer 17

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.

Question 18

What is thermal luminescence dosimetry?

Answer 18

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).