ARPANSA - Australian Radiation Protection and Nuclear Safety Agency

This Page:

• Primary Standards
• Air kerma calibration in ⁶⁰Co
• Absorbed dose to water calibration in ⁶⁰Co
• Polarity and recombination corrections
• Medium-energy kilovoltage x-rays
• Low-energy kilovoltage x-rays
• Air kerma calibrations for ¹³⁷Cs and ¹⁹²Ir
• Electrometer calibrations
• Additional notes about ARPANSA therapy calibrations

Primary standards

ARPANSA maintains primary standards for ionizing radiation (Table 1) and perform calibrations of therapy dosimetry equipment that are traceable to these standards. Calibrations at ⁶⁰Co are used as the basis for megavoltage dosimetry, and are suitable for use with protocols TRS-277, TRS-381, TRS-398 and the “ACPSEM Protocol (2nd Edition)”.

Contact our Ionizing Radiation Standards Section for assistance.

Table 1: Australian primary standards maintained at ARPANSA

Quantity	Beam quality	Primary standard
Air kerma / exposure	60Co	Graphite cavity chamber
Absorbed dose to graphite (1)	60Co	Graphite calorimeter
Air kerma / exposure	Medium-energy x-rays (50-300 kVp)	Medium-energy free air chamber (MEFAC)
Air kerma / exposure	Low-energy x-rays (20-50 kVp)	Low-energy free air chamber (currently operating as a secondary standard traceable to the MEFAC)

Note: absorbed dose to water is calculated from the absorbed dose to graphite using the photon fluence scaling theorem.

Air kerma calibration in ⁶⁰Co

• Ionization chamber is calibrated with the build-up cap on
• The reference point is the geometrical centre of the chamber
• The reference point is placed at 99.3 cm from the source where the air kerma rate is known
• The beam is square with approximate size 10 cm x 10 cm
• The air kerma rates varies as the ⁶⁰Co decays. Over the 10 year life of the source the rate varies from about 7 mGy/s down to about 1 mGy/s.
• The calibration coefficient is N_K

Absorbed dose to water calibration in ⁶⁰Co

• Ionization chamber is calibrated with the build-up cap removed and (if the chamber is not waterproof) placed in a 0.5 mm thick Perspex sleeve in a water phantom. The reference point is the geometrical centre of the chamber.
• The reference point is placed 5 cm beyond the surface of the water phantom where the absorbed dose to water rate is known
• The source-surface distance (SDD) is 100 cm
• The beam is square with approximate size 10 cm x 10 cm
• The phantom is a custom built Perspex tank of dimensions 40 cm x 40 cm x 40 cm. The beam enters the side through a 2 mm thick Perspex window. The absorbed dose at 5 cm depth is corrected for the non-water equivalence of this window.
• The absorbed dose to water rates varies as the ⁶⁰Co decays. Over the 10 year life of the source the rate varies from about 7 mGy/s down to about 1 mGy/s.
• The calibration coefficient is N_D,w

Polarity and recombination corrections

• Polarity correction measured at ⁶⁰Co by reversing the polarizing voltage of the ionization chamber.
• Recombination correction at ⁶⁰Co is checked using the two-voltage method (for therapy ionization chambers, this correction is usually less than 0.1% at ⁶⁰Co).

Medium-energy kilovoltage x-rays

• The ionization chamber is calibrated in a horizontal beam
• The source-detector distance is 1331 mm and the beam diameter approximately 11 cm
• The build-up cap is removed (except in certain cases for ST17)

Table 2: Medium energy therapy radiation qualities available from the Seifert x-ray generator for a source-detector distance of 1331 mm, nominal beam diameter 11 cm

Beam ID	kVp	Tube current (mA)	Additional filtration			Air kerma rate (mGy/s)	First HVL (mm Al)	First HVL (mm Cu)	Average energy (keV)
			Al(mm)	Cu (mm)	Sn(mm)
ST1	30	10	0.32			0.4	0.35		19
ST2	50	10	0.32			0.8	0.56		26
ST3	75	20	0.32			2.5	0.77		34
ST4	100	20	0.32			3.1	1.01		42
ST5	75	20	1.05			1.4	1.50		37
ST6	100	20	1.05			2.0	2.06		45
ST7	135	20	1.05			2.7	2.93		54
ST8	100	20	3.45			1.0	3.94	0.14	50
ST9	150	20	3.45			1.8	5.81	0.27	62
ST10	135	20	1.05	0.25		1.0	8.42	0.47	66
ST11	135	20	2.02	0.27		0.8	8.86	0.51	68
ST12	150	20	2.02	0.27		1.0		0.60	71
ST13	180	20	2.02	0.54		1.1		0.99	84
ST14	250	15	2.02	0.27		2.2		1.17	89
ST15	250	15	2.02	0.54		1.7		1.53	97
ST16	250	15	0.99	1.86		0.9		2.52	123
ST17	300	12	1.95	3.94	0.25	0.6		3.98	165

Low-energy kilovoltage x-rays

• The ionization chamber is calibrated in a horizontal beam
• The source-detector distance is 782 mm and the beam diameter approximately 11 cm
• The build-up cap is removed (if relevant)
• Suitable for parallel plate chambers

Table 3: Low/medium energy therapy radiation qualities available from the RT100 x-ray generator for a source-detector distance of 782 mm, nominal beam diameter 10 cm

Beam ID	Tube kVp (kV)	Tube current (mA)	Additional filtration		Air kerma rate (mGy/s)	First HVL (mm Al)	Average energy (keV)
			(mm Al)	(mm Cu)
RT1	20	10	0.15		2.4	0.125	12
RT2	30	10	0.30		2.3	0.23	18
RT3	37	10	0.40		2.5	0.38	21
RT4	45	10	0.55		2.5	0.58	24
RT5	55	10	0.78		2.6	0.87	29
RT6	70	10	1.25		2.6	1.40	35
RT7	100	8	1.70		2.9	2.34	44
RT8	100	8	1.02	0.25	0.7	6.67	57

Air kerma calibrations for ¹³⁷Cs and ¹⁹²Ir

ARPANSA does not maintain primary standards at these qualities. Instead, ionization chambers can be calibrated at ⁶⁰Co and at the highest energy X-ray beam (ST17) with the build-up cap on. The calibration coefficient at the energy of interest can then be found by interpolating the results at 1,250 keV (⁶⁰Co) and 164 keV (ST17).

Note: ANSTO provide a service for the calibration of well-chambers for selected radionuclides.

Electrometer calibrations

• Charge sensitivity, traceable to Australian electrical standards
• Linearity
• Ratio of ranges
• Ratio of integral and differential measurement modes

Additional notes about ARPANSA therapy calibrations

1. During the calibration of a therapy dosemeter, the ionization chamber is calibrated separately. This allows the ionization chamber to be used with a different electrometer, and enables drifts etc related to the chamber to be separated from electrometer effects. During at least on of the 60Co measurements a combined calibration is performed as a check.
2. A ⁹⁰Sr check source measurement with the client’s source if supplied. ARPANSA has its own ⁹⁰Sr check sources which may also be used.
3. Electrometer calibrations are made using the client’s ionization chamber. The current from the ionization chamber in a ⁶⁰Co beam is measured using a custom-built current integrator, which is traceable to national standards of electrical measurement. The electrometer is calibrated using the known current delivered over a time interval either with the ARPANSA source timer, or using the internal timer of the electrometer.

A series of fact sheets concerning radiotherapy calibrations made at ARPANSA can be found on the equipment calibration page.

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