1990 MV Code of Practice

Question 1

Equation for NPL absorbed dose calibration service

Answer 1

D = RNd

Question 2

Standard ambient conditions

Answer 2

T = 20 degrees celcius
P = 760 mmHg/1013.25 mbar
H = 50%

Question 3

Depth of chamber used in calibration service & intercomparison

Answer 3

4-10 MV = 5 cm

12-19 MV = 7 cm

Question 4

Field size used in calibration service & intercomparison

Answer 4

10x10

Question 5

Water phantom size

Answer 5

Full scatter phantom, extending at least 5 cm outside the beam edges

Question 6

Quality index for a MV beam

Answer 6

Tissue phantom ratio 20/10: the ratio of the corrected instrument readings with the chamber at 20 cm and 10 cm depths in a water phantom with a 10x10 field and constant source-chamber distance.

Question 7

Why do we rarely correct for relative humidity?

Answer 7

Only 0.1% difference when relative humidity is at 10%-90%

Question 8

How do you determine ion recombination for pulsed radiation?

Answer 8

Half-voltage technique. If chamber reading decreases by x% when the voltage is half its usual value, then the correction to be applied to the chamber reading is x%.

For NE 2561 chambers, if you don’t want to measure it then you can use, k = 1.0014 + 0.23p where p is the dose per pulse in the chamber in cGy.

Question 9

How do you determine ion recombination for continuous radiation?

Answer 9

Ion recombination losses are often small here. For NE 2561 chambers, just use standard value of 1.0014.

Question 10

What phantom can you use for intercomparison and what set-up method?

Answer 10

Water OR perspex phantom, sequential OR side-by-side

Question 11

Merging all this information, what are the steps to performing an intercomparison between a field instrument and the SS?

Answer 11

1. Make sure you have/measure the beam quality using TPR 20/10 to identify Nd for the SS
2. Make sure you have/measure the ion recombination factor.
3. Set up the two chambers sequentially or side-by-side, in a water or perspex phantom that extends at least 5 cm beyond the beam edges and has a field size of 10x10, with the chambers at the appropriate reference depth for the energy.
4. Allow equipment to stabilise and phantom to come to room temperature. Take T and P.
5. Obtain measurements, correct for FTP and ion recombination.
6. Using D = RNd(SS) = RNd(F) to deduce the calibration factor for the field instrument.

Question 12

What is the primary standard for MV dosimetry?

Answer 12

Graphite calorimeter