MV Calibration Chain Flashcards
How is a graphite calorimeter used to find the absorbed dose of a beam?
The ionising radiation will increase the temperature in the graphite calorimeter core. This can either be measured directly or more modernly a thermisotor circuit measures the current required to keep the graphite at a constant temperature.
E = m.c.detlaT, so D = c.deltaT
Why use graphite rather than water?
Graphite has a lower c
Is easier to thermally isolate
Has fewer impurities
Why is the graphite core thermally isolated?
Graphite has high thermal diffusivity so the energy would dissipate very quickly.
How is dose to water found from dose to graphite?
Use published interaction data and knowledge of how the chambers are constructed or calculate the ratio of absorbed dose in graphite and water phantoms consecutively in the same photon beam using the photon fluence scaling theorem.
How is the calibration factor to water found from the calibration factor to graphite?
Use ratios of dose to collision KERMA and mass energy absorption coefficients
MC simulations
Measurement in phantoms
Ndw/Ndg = (Dw/Qw)/(Dg/Qg) = (Qg/Qw).(Dw/Dg)
What are the mathematical options for converting dose to graphite to dose to water?
Dw = Dg.Kcoll,wbetaw/Kcoll,gbetag = Dg. ratio of energy fluences. ratio of average mass attenuation coefficients. ratio of the quotient of dose to collision KERMA.
OR
Dw = Dg. Measurement in water/measurement in graphite, with chamber correction factors in water and graphite at the points of measurement and the restricted collision mass stopping power ratio.
Is the graphite calorimeter used for every calibration?
No, the NPL has 3 reference chambers which are calibrated against the graphite calorimeter yearly as it is very time consuming to use the calorimeter. There is also an external monitor on the linac so variations in beam output between measurements in the replace method are considered.
How are secondary standards calibrated?
Substitution method in a water phantom for Co-60 and a range of MV energies. Calibrated in air for kV energies.
Calibration factors given in ka factors in reference to Co-60 as that is stable
What is the equation for the dose to water from a chamber measurement?
Dw = M.Ndw.kTP.kion.kcharge.klin.krange
How does the MV CoP suggest cross calibrations be performed?
Simultaneous method: Measure 3 times, swap chambers over, measure 6 times, swap chambers over, measure 3 more times.
Replace method: same but replace chambers in beam.
What equation should be used for the calibration factor?
CF = Mss.kion/Mf . ND
Is the geometric or arithmetic mean used in cross calibrations and why?
Geometric mean as it doesn’t introduce errors due to difference in dose at each position.
