Download Chapter 6 - RadTherapy

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 =
Calibration dose
patient reading
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.