National Physical Laboratory

Photon beam calibration service for radiotherapy (air kerma and absorbed dose)

Traceable calibration of a therapy-level dosemeter in terms of absorbed dose to water or air kerma. The service is available for thimble chambers or thin-window chambers, and offered in the Spring or Autumn (subject to demand) of each year.


Thimble Chambers

Thimble Chamber
Typical air kerma and absorbed dose
thimble chambers sent in for calibration

Air kerma (low + medium energy X rays + 60Co) and absorbed dose

Service delivery

  • For instruments intended to be used as secondary standards or others required for measurements of the greatest accuracy
  • Calibration of an ionisation chamber with or without an electrometer
  • Ionisation chambers of type 2561, 2611, 2571, and other Farmer type chambers with a graphite wall and aluminum central electrode will be accepted after confirmation with NPL
  • Batch calibrations are undertaken between February and April each year

Absorbed dose to water

  • Calibration against NPL reference standards in terms of absorbed dose to water
  • Primary standard is the NPL graphite microcalorimeter
  • For 60Co γ-rays and X-rays generated between 4 MV and 25 MV, quality index (TPR) between 0.568 and 0.800
  • Beam qualities similar to those used for high energy photon radiotherapy.
  • Calibration conforms to IPSM Code of Practice (1990)

Air kerma

  • Calibration directly against the NPL air kerma primary standards
  • Calibration for X-rays generated between 50 kV and 280 kV and for 60Co γ-rays
  • Beam qualities similar to those used for radiotherapy treatments
  • Calibration with beam qualities selected from Table 1
  • Calibration conforms to IPEMB Code of Practice (1996)
Low & Medium X-Rays for Air Kerma
Table 1: Low + medium energy X-ray qualities used for therapy level air kerma calibrations


Thin-Window Chambers

Thin-Window Chamber
Typical low energy X-ray thin window
chamber sent in for calibration

Air kerma - very low energy X-rays

Service delivery

  • For instruments intended to be used as secondary standards or others required for measurements of the greatest accuracy.
  • PTW 23342, 0.02 cc, PTW 23344, 0.2 cc and other similar chambers are accepted after confirmation from NPL.
  • Calibration of an ionisation chamber with or without an electrometer.
  • Batch calibrations are undertaken between February and April each year.

Air kerma

  • Calibration directly against the NPL 50 kV air kerma primary standar.
  • Calibration for X-rays generated between 8.5 kV and 50 kV
  • Beam qualities similar to those used for radiotherapy treatments (see Table 2)
  • Calibration with beam qualities selected from Table 2
  • Calibration conforms to IPEMB Code of Practice (1996)
Very Low X-Rays for Air Kerma
Table 2: Very low-energy X-ray qualities used for therapy level air kerma calibrations

Contact

Customer Services tel: +44 20 8943 8695
E-mail: dosimetry_enquiries@npl.co.uk

Last Updated: 23 May 2012
Created: 17 Apr 2007