Codes of Practice

Question 1

How does the response of an unsealed ionisation chamber vary with temperature? Why is this?

Answer 1

Ion chamber response is proportional to number of air molecules in chamber
Chambers are usually vented (open to local atmosphere)
PV=nRT
R, V constant
P/T prop to n prop to chamber response
=> T/P prop to correction

ftp= T/T0 . P0/p

Question 2

What is the pressure and temperature correction factor?

Answer 2

T/293.15(K) * 1013.25/P(mbar)

Question 3

What factors may influence the ionisation chamber readings?

Answer 3

Temperature
Pressure
Recombination
Electrometer
Polarisation

Question 4

How is recombination corrected for in an ionisation chamber?

Answer 4

Two voltage method, linear correction, Jaffe plots

Question 5

What factors affect ion recombination in an ion chamber?

Answer 5

Voltage
Number of molecules
Dose rate
Chamber geometry
Pulsed vs continuos radiation

Question 7

What systems are in place to ensure that all UK hospitals are using the same value for the Gray?

Answer 7

Calibration Chains
All UK calibrations are traceable to National Physical Laboratory (NPL) through inter-comparisons.
All calibrations and inter-comparisons must follow a code of practice.
Secondary standard will be sent to NPL every 2/3 years for calibration at a range of energies.
Users interpolate between energies to obtain factors for all their energies.
Field instruments will be calibrated against secondary standards every year.

𝐷𝑜𝑠𝑒 = 𝐶ℎ𝑎𝑟𝑔𝑒×𝑁𝑑𝑤×𝑓_𝑖𝑐

NPL compares their graphite calorimeter to our reference chambers in a variety of beams using a Co-60 source and an Elekta linac.
Secondary standards sent for calibration then get a calibration value for a range of TPR20,10s
Hospitals can then work out energy-specific calibration coefficients for their individual equipment.