Cobalt-60 calibration

Calibrations are performed with the ionisation chamber in a water phantom using ARPANSA’s Eldorado teletherapy head.

The absorbed dose to water rate from this source is measured annually with the Australian primary standard graphite calorimeter. ARPANSA ionisation chambers are measured at the same time as the chamber under test, and are used to make sure that the Cobalt-60 source decay has been properly accounted for, and that the measurement geometry is correct.

Excerpt from a calibration certificate for a Farmer-type chamber. To find kQ for a user beam, a simple quadratic equation is used
Figure 1: Farmer chamber in waterproof sleeve in front of the Cobalt-60 source.

 

The ionisation 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, which is placed 5 cm beyond the surface of the water phantom where the absorbed dose to water rate is known. 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 source-surface distance (SDD) is 100 cm and the beam is square with approximate size 10 cm x 10 cm.

Over the 10 year life of the source the absorbed dose to water rate varies from about 10 mGy/s down to about 2.5 mGy/s. The calibration coefficient is denoted ND,w in the IAEA TRS-398 Code of Practice.

If the ionisation chamber is sent for calibration with an electrometer, they will be calibrated together in Cobalt-60. However this measurement is only used to confirm that the combined calibration coefficient is equal to the product of the separate calibration coefficients of the chamber and electrometer. For the remainder of the measurements the chamber is connected to an ARPANSA electrometer.

Polarity and recombination corrections

Polarity correction measured at Cobalt-60 by reversing the polarising voltage of the ionisation chamber. The recombination correction at Cobalt-60 is measured using the two-voltage method (for therapy ionisation chambers, this correction is usually less than 0.1% at Cobalt-60). The calibration report contains both the corrected and uncorrected ND,w.