Chapter 6

Principles of Radiation Detection

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.

Gas Ionization Detectors

• Ionization Chambers– Thimble chamber– Cutie-pie: portable ionization chamber

• Geiger-Mueller (G-M) counters

• Proportional counters

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)

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)

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

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)

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 1000’s)

• 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

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.

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 (<30 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

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

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

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)

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

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

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)

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

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

Neutron Dosimeters

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

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.

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 doseDose to patient = patient reading

Calibration dose calibration reading

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.