RT2 - Dosimetry equipment

Question 1

3 ways to measure energy deposited from ionising radiation:

Answer 1

1. Ionisation - collect ion pairs produced in air
2. Calorimetry - ionising particle shares energy with many others, ions recombine, energy ends up as heat
3. Chemical effects - free radicals produced by ionising particals can cause chemical effects

Question 2

Define exposure

Answer 2

• Charge liberated per unit mass [C/Kg]
• total chartge of ions in dry air when all electrons liberated in mass dm of air are completely utilised

1. Need to collect all ions produced by electrons originating in mass dm
2. Electrons are produced both inside and outside of dm

Question 3

Define Electronic Equilibrium

Answer 3

• Ions produced by electrons travelling into dm is equal to ions leaving dm.
• I.e. collecting as many electrons from outside as you are not collecting the ones that leave
• Exposure is only measurable under electronic equilibrium
• Minmum dimension between boundaries of dm and surrounding interacting homegenous medium > range of electrons

Question 4

Operation of Free-Air ion chamber

Answer 4

• Photon beam passes between paralell plates (high voltage)
• X-rays produce electrons which cause ionisation
• Ions are collected and measured electronically
• V needs to be high enough to separate as many =ve ans -ve ions before they recombine

Question 5

What is a primary standard?

Answer 5

• Absolute measurement against which secondary standards and reference instruments are calibrated
• Measurement from first principles
• NPL
  
  • Free-air ion chamber (kV photons)
  • Graphite calorimeters (MV photons & electrons)

Question 6

How does NPLs free air ion chamber calibrate a KV beam?

Answer 6

• Free air ion chamber
• Kerma = Dose (to air) = energy required to create ion pair * number of ion pairs produced/unit mass
• Collect all the ions, know the energy needed to create a pair, work back to dose
• User puts thimble ion chamber in same beam conditions, and you get a calibration factor

Question 7

How do you convert air kerma to absorbed dose in water?

Answer 7

• Dose to water = dose to air * ration of mass energy absorption coefficents (water/air) averaged over photon spectrum in air
• Dose on surface of water phantom requires backscatter factor, which you measure with TLD (fixed SSD one on surface of water, other in air)

Question 8

Thimble Ion chamber

Answer 8

• Needed for high energies where free air chambers would be too large
• Used for clinical measurements, measure exposure (proportional to dose)
• Free-air chamber - small air cavity dm + surrounding air volume (dimensions > R)
• Thimble chamber - small air cavity dm + surrounding air-equivelent wall of graphite (dimensions > R)

Question 9

Farmer chamber

Answer 9

• Thimble-type ion chambe enclosing small volume of air in conducting graphite cap with insulated axial collecting electrode
• Walls have z similar to air
• Wall thickness > R in wall (1mm wall = 1m air)
• Dose is proportional to exposure, calibrated against primary dosimetry standards

Question 10

Operation of thimble chamber

Answer 10

• Hiigh polarising voltage between wall and central electrode (200Vcm-1)
• Ions produced in cavity move to one wall or other
• Collected charge is measured with electrometer
• Volume typically 0.6ml

Question 11

Ion chamber sizes

Answer 11

Large

• Sensitive & low spat res
• used for environmental monitoring

Small

• good spat res, small singal
• fine resolution scanning

Question 12

Paralell plate chamber

Answer 12

• Used to measure high dose gradient fields (e.g. electrons)
• Need a small chamber so that you son’t get contribution from other parts of high gradient field
• Thin = better res
• Detecting volume still large enough for good signal

Question 13

Principle of Calorimetry

Answer 13

• Absorbed dose = energy/mass
• E = mc dT
• Dose = specific heat capacity * temp rise

Direct measurement of absorbed dose. Difficult to sue water due to effects of purity, NPL uses graphite and converts to water by comparing ratio of electron densities.

Question 14

NPL Graphite Calorimeter method

Why do you use graphite?

How do you measure temp rise?

Answer 14

• Inner graphite core surrounded by two jackets
• Core thermally isolated from outer jackets by vacuum
• Temperature rise in core measured with sensitive thermistors
• Use graphite because 1Gy produces temp rise of 1.5mK in graphite, but only 0.24mK in water. Lower specific heat = more temp to measure, better precision
• More accurate to measure by keeping the temperature constant (peltier element) and recording the power in the heating circuit, Dose to graphite = change in power * time beam is on / mass of core

Question 15

Secondary standard

Answer 15

• High quality dose meter owned by hospital
• sent to NPL to be calibrated against primary reference (calibrated against primary)
• Correction factor
• Transferred to field instruments by cross-calibration

Question 16

Photon secondary standard

Answer 16

• Thimble chamber used for MV & KV photons
• Callibrated agaainst NPL reference
• Calibrated at NPL in air for KV, in water for MV
• Calibrated at users hospitals against field instruments in air (KV), water (300kV), Perspex (MV)

Question 17

Calibration for MV photons

What does CF depend on?

Answer 17

• Code of practice recommends
• Field and secondary placed side by side in large perspex phantom, irradiated at fixed depth (same dose)
• Ratio of readings = factor
• Repeat 12 times with position swapped after each 3 times

Chamber CF is a quality dependent factor, Nd. Chamber construction means that response varies with beam spectrum (quantified with beam quality)

Question 18

How do you quantify beam quality?

What is it analgous to?

Answer 18

• Quality index = TPR20/10 ratio
• Ratio of dose in water (FSD = 100CM, 10x10 field) with 20cm depth and 10cm depth
• Like adding more water on top.
• Analogous to HVL, measuring penetration reveals beam hardness or quality.

Question 19

List all the correction factors (x5) to get from reading on secondary standard (M) to dose to water (D_W)

Answer 19

Dw = M * Nd(Q) * f_TP*f_ion*N_elec*f_nonlin

• Nd(Q) - Quality specific secondary standard correction factor from NPL
• f_TP - Pressure & Temp factor (PoT/ToP) chamber is unsealed
• f_ion - Ion recombination factor: Not all charge liberated is collected, some recombine (low V, high dose rate), quantify by measuring charge as function of voltage
• N_elec - Does the charge displaed equal the actual charge
• *f_nonlin - Non-linearity correction

Question 20

Electron secondary standard

Answer 20

• Calibrated in water at NPL against 3 reference NACP chambers
• NACP chambers calibrated in graphite against electron primary standard calorimeter