TG51 Flashcards
What are the 4 correction factors for photon calculations with TG51?
Pelec - electrometer correction factor for the electrometer calibration
Ptp - temperature and pressure correction factor correcting for the amount of mass actually in the chamber at the time of measurement
Pion - ion recombination correction factor to correct for ion pairs that were created but not collected
Ppol - polarity correction factor to correct for differences in collection based on the polarity of the chamber
How do you calculate Ptp?
Ptp = (273.2+T)/(273.2+22)*(760/P) - for a pressure measured in mmHg
How do you convert between kPa and mmHg?
1 kPa = 7.5 mmHg
How do you calculate Ppol?
Ppol = abs(Mraw+ - Mraw-)/2Mraw
What is a reasonable range for Ppol?
within 0.3% - 0.997 to 1.003
At what point should you be concerned about Pion and what should you do?
if Pion > 1.05, use a different chamber
At what point should you be concerned about Pion and what should you do?
if Pion > 1.05, use a different chamber
How do VH and VL relate when measuring Pion?
VL must be at least a factor of 2 less than VH.
How do you calculate Pion for pulsed/swept beams?
Pion = (1-VH/VL)/(MH/HM - VH/VL)
How do you calculate Pgrad for electron calculations?
Pgrad = M(dref + 0.5 rcav)/M(dref)
How big does your water tank need to be for a TG51 measurement?
at least 30 x 30 x 30 cm^3
What is dref in an electron beam?
dref = 0.6*R50 - 0.1 where dref is in cm.
How thick of a lead foil should be used for a TG51 measurement in a high-energy beam?
1 mm (+/- 20%)
Which energy beams should use a lead foil for a TG51 measurement?
all FFF beams (and 10 MV+ but addendum says no)
At what distance should the lead foil be from the phantom surface for a TG51 measurement?
~50 cm (+/- 5 cm), or 30cm +/- 1 cm if 50cm is not possible
What is Pgrad for a parallel plate chamber?
1
What is Qecal for a TG51 measurement?
arbitrary electron beam quality of R50 = 7.5 cm
What is the reference field size for electron TG51 measurements?
> = 10x10 unless R50 > 8.5 cm (E > 20 MeV), then >=20x20
What is the TG51 equation for photons beams?
D = MKqND,w,Co-60
What is the TG51 equation for electron beams?
D = MPQgradk’R50kecalND,W,Co-60
How long should you wait for your chamber to reach thermal equilibrium with water in a water tank?
5-10 min
What is the assumed range of humidity and what uncertainty does that introduce into a TG51 calculation?
20-80%, 0.15%
What SSD must your PDD be taken in when determining your beam quality for TG51?
100
Where is your effective point of measurement in a cylindrical chamber in a photon beam?
0.6*rcav upstream
Where is your effective point of measurement in a cylindrical chamber in an electron beam?
0.5*rcav upstream
What is the reference depth for TG51 photon beam measurements?
10 cm
How do you find Kq if your chamber isn’t on the table?
closest match by:
1) wall material 2) radius of air cavity 3) electrode material 4) wall thickness
What is the volume of a farmer chamber?
0.6 cc
What is a typical farmer chamber radius?
3 mm
What is a typical farmer chamber length?
~2 cm
What is the limit on the magnitude of Pleak in TG51 addendum?
Should be <= 0.1%, anything above 0.5% must be investigated
What is the purpose of Prp in TG51 addendum?
To take into account off-axis variation in the intensity profile across the active volume of the chamber.
What energies does TG51 apply to?
Co60 - 50 MV photon beams and 4 to 50 MeV electron beams
What is kQ for electron beams in TG51?
kQ = PQgrad * kR50
What is kR50 for electron beams in TG51?
kR50 = kR50’ * kecal
At what wall thickness do you need to correct measurements through the wall for the effective depth of measurement in a TG51 measurement??
thickness > 0.2 cm
How do you calculate the effective thickness of PMMA?
physical thickness * 1.12
What theory is kq dependent on in TG51?
Spencer-Attix cavity theory
How sensitive is kQ to changes in PDD10x?
1% change in PDD10x is a 0.1% change in kQ - therefore, the largest uncertainty in kQ is likely in picking the right curve and interpolating to your quality
How much uncertainty is introduced by measuring without a lead foil in higher energy beams?
0.2%
How much can your calibration coefficient change over the 2-year calibration span?
<0.3%
How much can your chamber reading change from first irradiation to stabilization?
<0.5%/MU
