Radiation detectors Flashcards
What are films used for?
Brachy source position measurements
Some patient specific QA
What are the advantages of gaphachromic film?
Characteristic response curve Energy Independence Dose rate linearity Excellent spatial resolution Gold standard of assessing radiation shape
What are the disadvantages of graphachromic film?
Expensive
Planar measurement
What are the advantages of radiographic film?
Excellent spatial resolution
Gold standard of assessing radiation shape
What are the disadvantages of radiographic film?
Poor non-linear energy response
Messy/time consuming
Planar measurement
How does radiographic film work?
Radiation produces electrons which collect in impurities creating negative charge
Positive aluminium ions collect on other impurities making a positive charge
Developing the film amplifies the signal
Grain in the positive aluminiom environment acts as an electron gate
Fixing plate removes aluminium not involved in amplification
washing removes fixer and developer
How do radiosensitive gels work?
Fe2+ molecules turn into Fe3+ molecules when exposed with radiation.
Absorption spectroscopy performed to see dose.
What is used to stop the diffusion of Fe3+ molecules in gels?
Polymer gels
What is the advantage of using a gel?
3D resolution
What are the disadvantages of using a gel?
Toxic
Messy
Need a CT/MR scanner
Polymer gels oxidise
What do properties do scintillation crystals need?
Transparent Sensitive Fast Response Efficiently convert KE to light Light proportional to KE Density and composition similar to water
What are the disadvantages currently of using scintillation crystals?
Cerenkov radiation makes SNR too low without processing - need to use subtraction and spectral discrimination
Need a special photosensitive electrometer
How does alanine dosimetry work?
When exposed it produces a stable ammonium radical (SAR)
The abundance of SAR can be found using electron paramagnetic resonance - the same process as MR spectroscopy
The energy difference between the low and high energy states must be known
What is the NPL alanine service?
Provide traceable alanine which can be used in stacks to find the dose distribution for small fields. Precision is greater than +/-1% at 1sigma for 5-10Gy
How do TLDs work?
Radiation excited electrons, which either relax to their ground state or fall into a trap.
Expose to heat to relax the electron from the trap to the ground state, which releases light
Need to do low temperature annealing before to remove low temperature peaks
anneal during readout to induce light
High temperature anneal post readout to remove residual signal
