Measuring dose Flashcards
Why do we measure dose?
patient dosimetry, environmental, contamination, checking machine outputs, imaging
what are the two main categories of measuring devices?
gas-filled chambers and solid state detectors
what are examples of gas-filled chambers?
ionisation chamber, proportional counter, GM tube
what are examples of solid-state detectors?
scintillation detectors, semi-conductor detectors and thermoluminescent detectors.
what are the properties we look for in detectors?
Energy resolution: differentiation of spectrum of energies allows us to determine where the peaks occur within the spectrum.
Efficiency: how much of the radiation entering the detector will be counted.
Sensitivity: response of detector per unit energy deposited
Dead time: time between a signal being received and processed, before it is live and ready to measure again.
Angular dependance: ability of detector to detect incident radiation photons from different angles.
how do gas-filled detectors work?
ion pairs created –> ionisation –> apply an electrical field –> ions attracted to anode and cathode –> collect the charge –> measure the current.
what does KERMA mean?
kinetic energy released per unit mass
what are solid state detectors?
provide a signal by collecting the charge liberated in the passage of the particle through a semiconductor.
what is an example of a solid state detector?
TLD’s (thermoluminescence dosimetry). they have small volume, good resolution, no instantaneous dose read out, in-vivo patient dosimetry.
what are scintillation detectors?
used to determine the high energy part of the x-ray spectrum. very efficient, good energy resolution
what are GM tubes?
Amplifies the signal, no energy resolution, has dead time
what are semiconductor diodes?
quick check of the machine, in-vivo patient dosimetry, tissue equivalent, relatively noisy, smaller measuring volume, temperature dependant, higher signal
what re ionisation chambers?
accurate measurement of absorbed dose, not tissue equivalent, stable, large measuring volume.
what measuring tool would we use for each type of monitoring?
environmental monitoring: GM, TLD
contamination monitoring: GM, scintillation
Personal monitoring: TLD
output measurement in diagnostic/ radiotherapy: ionisation chamber
gamma camera: scintillation
