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 dose Dose 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.