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Secondary standard calibrations in terms of kerma in air in Co-60 beam
The kerma in air measured with a calibrated ionization chamber in a 60Co beam is given by the following equation:
Kair = Mcorr NK
where Mcorr is the reading of the electrometer under the reference conditions used in the standard laboratory and is the calibration coefficient in terms of air kerma. The reading of the electrometer has to be corrected for the influence quantities: atmospheric pressure, temperature, humidity, electrometer calibration, polarity effect and ion recombination. Therefore the reading of the electrometer Μ can be written as:
Mcorr = M∏ki
where Μ is the uncorrected reading from the electrometer expressed in nC or rdg (readings) and ∏ki is a product of correction factors for the influence quantities.
The kerma in air at the reference position can be written as:
Kair=NK M ∏ki , where
∏ki=kTP kh kelec kpol ks and
- kTP is the correction factor for temperature and atmospheric pressure,
- kh is the correction factor for the atmospheric humidity
- kelec is the calibration coefficient of the electrometer in nC/nC or in rdg/nC (readings/nC)
- kpol is the correction factor for the polarity effect . Fixed polarity is typically used for each chamber and therefore no correction is done at LNK Gent for the polarity effect (kpol=1). The polarity is mentioned in the calibration certificate.
- ks is the correction factor for the ion recombination. For most of the ionization chambers to be calibrated the ion recombination effect is smaller than 0.1% in 60Co beam. No correction is done at LNK Gent for the ion recombination effect (ks=1).
To verify the stability o the ionization chambers during the calibration procedure, the reference kerma in air is measured twice, once before and once after the measurement with the chamber to be calibrated.
The reference conditions used at LNK Gent for the calibration of ionization chambers in terms of air kerma in 60Co beam are in agreement with the current protocols used by the radiotherapy departments:
| Influence quantity | Recommended reference value or reference characteristic |
| Source-chamber axis distance (SCD) | 100 cm |
| Air temperature | 20°C |
| Air pressure | 101.325 kPa |
| Reference point of the ionization chamber | on the chamber axis at the centre of the cavity volume (for cylindrical chambers) |
| Filed size at the position of the reference point of the chamber | 10 cm x10 cm |
| Relative humidity | 50 % |
| Polarizing voltage and polarity | No reference values are recommended but the values used are stated in the calibration certificate |
| Air kerma rate | 29 Gy/h ( at 2011-01-01) |
Bibliography
[1]. IAEA TRS-398, "Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water", Technical Report Series 398 (Vienna IAEA) 2000.
[2]. IAEA TRS-277 "Absorbed dose determination in photon and electron beams - An international code of practice for dosimetry", Technical Report Series 277 (Vienna IAEA) 1997
[3]. NCS-18 -Netherlands Commission on Radiation Dosimetry – " Code of practice for absorbed dose determination in high energy photon and electron beams" – January 2008