[SLAC] [SLAC Pubs and Reports]
SLAC-PUB-8099
Air Kerma Calibration Factors and kch Values for PTW Soft X-ray, NACP and Roos Ionization Chambers at Very Low X-ray Energies (0.035 mm - 1.0 mm Al HVL)
Abstract
Several national and international protocols have been established for
the dosimetry of x-ray beams used in radiotherapy. For the very low
energy x-rays (0.035mm- 1.0 mm Al HVL) only two codes are available:
the UK IPEMB Code of Practice and the German standard, DIN 6809 Part
4. The measurement of very low energy x-ray beams is normally
performed with parallel plate ionization chambers calibrated at a
standards laboratory and characterized by an air kerma calibration
factor Nk. According to the IPEMB Code of Practice the
absorbed dose in the user's beam should be determined by taking
measurements with the parallel plate chamber positioned such that its
entrance window is at the surface of a full-scatter water equivalent
phantom. The absorbed dose to water can then be determined using an
equation which includes a factor, kch, which accounts for
the change in response of the ionization chamber between the
calibration in air and measurement at the surface of the
phantom. Nk and kch values for the PTW soft
X-ray, NACP and Roos ionization chambers are reported. It was found
that kch values varied from about 1.01 to 1.08 depending on the
chamber, beam quality and phantom material. It is recommended that the
IPEMB Code of Practice should be revised to incorporate these values.
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