Principles of Radiation Detection

1
Chapter 6

• Principles of Radiation Detection

2
Measurement of Radiation

• X-rays and electrons produced by radiation
  therapy treatment machines are measured using
  ionization detectors.
• Mounted within the machine assembly (monitor
  chambers)
• Used for radiation protection purposes
• To calibrate machine output at the depth of
  maximum dose.
• Detectors of ionizing radiation make use of
  ionization and excitation processes.

3
Gas Ionization Detectors

• Ionization Chambers
• Thimble chamber
• Cutie-pie portable ionization chamber
• Geiger-Mueller (G-M) counters
• Proportional counters

4
Gas Ionization Detectors

• Chamber (probe) isolates the gas between the two
  electrodes.
• Two electrodes (charged plates of capacitor) act
  as the collectors of ions created in the
  container when ionizing radiation strikes it.
• Container with a fixed volume of gas (air,
  methane)

5
Gases

• Gases chosen to minimize the energy dependence of
  the ionization chambers to ensure that the
  reading per roentgen is about the same,
  independent of the photon energy.
• Ionization chamber air, methane
• G-M counters inert gases (argon, neon)

6
Gas Ionization Detectors

• Gas molecules are ionized by incoming particulate
  or photon beams and produce ion pairs
• Positive ions travel to negative electrode
• Negative ions travel to positive electrode
• Ionization current indicates the ionization rate
  in the ionization chamber
• Dependant on voltage

7
Polarization Voltage

• Polarization voltage collects charges of
  opposite sign at opposite electrodes
• The higher the voltage, the faster the ions move
  
• Ion recombination
• ion pairs recombine after they are created (low
  voltage)
• Ionization chamber region
• efficiency close to 100- nearly all liberated
  electrons are collected (above 300 volts)

8
Polarization Voltage

• Proportional counter region
• voltage of the electrodes high enough (600-800
  volts)
• ions liberated by the incoming radiation are
  energetic enough to ionize additional gas
  molecules in the chamber (secondary ionization
  events)
• efficiency greater than 1 (sometimes 1000s)
• Geiger Mueller (GM) region
• electrons reach an energy high enough to produce
  excitation of the chamber gas,
• releases ultraviolet (UV) radiation
• cause the entire volume of gas to ionize at once
• creates a discharge or pulse (measured in counts
  per minute) of current across the chamber volume

9
Collection Efficiency

• Collection Efficiency of the ion chamber (f) is
  the fraction of charges collected (those that do
  not recombine), over the charges liberated by
  initial ionization.

10
Wall materials

• Wall materials have significant effect on
  performance
• Ionization chambers atomic numbers close to
  those of air or water (plastic, carbon)
• Thimble chamber condensed air- solid material,
  same effective atomic number as air but 1000
  times more dense
• Allows a reduced size
• G-M higher Z materials (metal), difference in Z
  produces energy dependence in the detector
• Under-respond at very low energies (lt30 keV)
  because of beam attenuation in the walls
• Over-respond at moderate energies (about
  30-100keV) because of the P.E. effect in the
  electrodes due to high Z material in walls

11
Caps

• Cap designed to be as thin as possible but still
  thick enough to establish electron equilibrium
• Electron equilibrium as many electrons are
  captured as are released in interactions.
• Buildup caps used for high energy photons beams
  materials with atomic numbers similar to those of
  air or tissue
• Thickness dependant of the photon energy of the
  beam
• Must be thick enough to supply electron
  equilibrium for that energy

12
Ionization Chambers

• For accurate measurement of high-radiation fields
  such as clinical therapy electron and photon
  beams.
• Amount of current produced in an ionization
  chamber is directly related to the HVL of the
  beam.
• Used to
• Calibrate linear accelerators or 60Co units
• Measure treatment beam characteristics (flatness,
  symmetry)
• Use in a linear accelerator monitor chamber

13
Cutie Pie

• Very large collection volume so that it can
  measure relatively low-intensity radiation levels
  and give accurate measures of radiation exposure
  rates
• Much less sensitive than G-M detectors
• Survey meter used to
• Measure dose rate around an implanted patient
  (137Cs, 192Ir) and patient room
• Survey in and around the storage area in which
  radioactive materials are kept
• Survey areas around radiation producing machines
  such as 60Co units (leakage- always on)

14
Proportional counters

• Proportional counters
• Measure low intensity radiation
• they can discriminate between alpha and beta
  particles.
• Count radioactive spills
• Use as a detector in some CT scanners

15
Geiger-Mueller counters

• Useful for measuring low-intensity radiation
  because of their ability to produce a large
  electrical signal from a single ionization event.
• Sensitive produce a very large signal even after
  a small event by discharging the polarization
  voltage to provide that signal? have a dead time?
  must recharge after every event
• Quenching agents (alcohol, chlorine)
• suppress the electrical discharge caused by UV
  light
• Allow the chamber to be reset quickly before the
  next discharge
• Above about 4R/hr detector can read zero

16
Geiger-Mueller Counters

• Survey of operating room, personnel, and
  instruments after implant procedures
• Find lost radioactive seeds or ribbons (125I,
  192Ir)
• Monitor incoming radioactive source material
  packages
• Search for holes in the walls of the linear
  accelerator room
• Use as an in-room radiation monitor for treatment
  room (not in beam)

17
Scintillation Detectors

• De-excitation electrons returning to their
  ground state after being excited.
• Made visible by the emission of characteristic
  radiation
• Fluorescence- if de-excitation time is short
• Phosphorescence- if de-excitation time longer
  (e.g. glow in the dark)
• Scintillation crystals absorbs a photon, the
  interaction produces ionization, which in turn
  produces light.
• The amount of light produced is proportional to
  the energy of the absorbed photon

18
Scintillation Detectors

• More sensitive than G-M detectors
• Includes photomultiplier tube detects light
  pulse and produces an electrical pulse with a
  strength dependent on the amount of light
  detected
• The energy of the photon can be determined by
  measuring the strength of pulse.
• Used to
• Measure activity of nuclides
• Discriminate one isotope from another by
  evaluating the differences in pulse strength
  (energy)
• Measure surface contamination and brachytherapy
  source leakage

19
Neutron Dosimeters

• Low Z moderating detectors slow down neutrons
  and detect their presence.

20
Thermoluminescent Dosimeters

• In the form of rods (cylinders) or chips,
  contains Lithium fluoride (LiF)- has an effective
  Z similar to tissue and air
• X-ray exposure raises electrons that normally
  reside in a lower energy state, the valence band
  of the crystal, to the conduction band, a region
  in which the electrons have a higher energy
  state.
• The electrons drop back toward the valence band
  as they de-excite however, they are often caught
  in traps between the two bands. May stay here for
  many years.
• Heating the crystal empties the traps by pushing
  out the electrons (thermoluminescence). The final
  de-excitation of the electrons emits visible
  light. The total amount of emitted light (TL) is
  related to the original radiation dose absorbed
  by the crystal.

21
Thermoluminescent Dosimeters

• Small, reusable, wide dynamic range, dose rate
  independent.
• Measurement of dose at radiation therapy field
  abutments.
• Used almost exclusively for treatment field dose
  determinations and personnel monitoring
• Measurement of skin dose
• Dose to patient patient reading
• Calibration dose calibration reading

22
Diode Detectors

• Solid state detectors that measure dose and/or
  dose rate
• Capable of reading dose immediately
• Can be used in megavoltage equipment to measure
  flatness and symmetry of the beam, dose, and dose
  rate
• When used at different depths, can measure beam
  energy.