TG51

Question 1

What are the 4 correction factors for photon calculations with TG51?

Answer 1

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

Question 2

How do you calculate Ptp?

Answer 2

Ptp = (273.2+T)/(273.2+22)*(760/P) - for a pressure measured in mmHg

Question 3

How do you convert between kPa and mmHg?

Answer 3

1 kPa = 7.5 mmHg

Question 4

How do you calculate Ppol?

Answer 4

Ppol = abs(Mraw+ - Mraw-)/2Mraw

Question 5

What is a reasonable range for Ppol?

Answer 5

within 0.3% - 0.997 to 1.003

Question 6

At what point should you be concerned about Pion and what should you do?

Answer 6

if Pion > 1.05, use a different chamber

Question 7

At what point should you be concerned about Pion and what should you do?

Answer 7

if Pion > 1.05, use a different chamber

Question 8

How do VH and VL relate when measuring Pion?

Answer 8

VL must be at least a factor of 2 less than VH.

Question 9

How do you calculate Pion for pulsed/swept beams?

Answer 9

Pion = (1-VH/VL)/(MH/HM - VH/VL)

Question 10

How do you calculate Pgrad for electron calculations?

Answer 10

Pgrad = M(dref + 0.5 rcav)/M(dref)

Question 11

How big does your water tank need to be for a TG51 measurement?

Answer 11

at least 30 x 30 x 30 cm^3

Question 12

What is dref in an electron beam?

Answer 12

dref = 0.6*R50 - 0.1 where dref is in cm.

Question 13

How thick of a lead foil should be used for a TG51 measurement in a high-energy beam?

Answer 13

1 mm (+/- 20%)

Question 14

Which energy beams should use a lead foil for a TG51 measurement?

Answer 14

all FFF beams (and 10 MV+ but addendum says no)

Question 15

At what distance should the lead foil be from the phantom surface for a TG51 measurement?

Answer 15

~50 cm (+/- 5 cm), or 30cm +/- 1 cm if 50cm is not possible

Question 16

What is Pgrad for a parallel plate chamber?

Answer 16

1

Question 17

What is Qecal for a TG51 measurement?

Answer 17

arbitrary electron beam quality of R50 = 7.5 cm

Question 18

What is the reference field size for electron TG51 measurements?

Answer 18

> = 10x10 unless R50 > 8.5 cm (E > 20 MeV), then >=20x20

Question 19

What is the TG51 equation for photons beams?

Answer 19

D = MKqND,w,Co-60

Question 20

What is the TG51 equation for electron beams?

Answer 20

D = MPQgradk’R50kecalND,W,Co-60

Question 21

How long should you wait for your chamber to reach thermal equilibrium with water in a water tank?

Answer 21

5-10 min

Question 22

What is the assumed range of humidity and what uncertainty does that introduce into a TG51 calculation?

Answer 22

20-80%, 0.15%

Question 23

What SSD must your PDD be taken in when determining your beam quality for TG51?

Answer 23

100

Question 24

Where is your effective point of measurement in a cylindrical chamber in a photon beam?

Answer 24

0.6*rcav upstream

Question 25

Where is your effective point of measurement in a cylindrical chamber in an electron beam?

Answer 25

0.5*rcav upstream

Question 26

What is the reference depth for TG51 photon beam measurements?

Answer 26

10 cm

Question 27

How do you find Kq if your chamber isn't on the table?

Answer 27

closest match by: 1) wall material 2) radius of air cavity 3) electrode material 4) wall thickness

Question 28

What is the volume of a farmer chamber?

Answer 28

0.6 cc

Question 29

What is a typical farmer chamber radius?

Answer 29

3 mm

Question 30

What is a typical farmer chamber length?

Answer 30

~2 cm

Question 31

What is the limit on the magnitude of Pleak in TG51 addendum?

Answer 31

Should be <= 0.1%, anything above 0.5% must be investigated

Question 32

What is the purpose of Prp in TG51 addendum?

Answer 32

To take into account off-axis variation in the intensity profile across the active volume of the chamber.

Question 33

What energies does TG51 apply to?

Answer 33

Co60 - 50 MV photon beams and 4 to 50 MeV electron beams

Question 34

What is kQ for electron beams in TG51?

Answer 34

kQ = PQgrad * kR50

Question 35

What is kR50 for electron beams in TG51?

Answer 35

kR50 = kR50' * kecal

Question 36

At what wall thickness do you need to correct measurements through the wall for the effective depth of measurement in a TG51 measurement??

Answer 36

thickness > 0.2 cm

Question 37

How do you calculate the effective thickness of PMMA?

Answer 37

physical thickness * 1.12

Question 38

What theory is kq dependent on in TG51?

Answer 38

Spencer-Attix cavity theory

Question 39

How sensitive is kQ to changes in PDD10x?

Answer 39

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

Question 40

How much uncertainty is introduced by measuring without a lead foil in higher energy beams?

Answer 40

0.2%

Question 41

How much can your calibration coefficient change over the 2-year calibration span?

Answer 41

<0.3%

Question 42

How much can your chamber reading change from first irradiation to stabilization?

Answer 42

<0.5%/MU