Absolute Air-kerma Measurement in a Synchrotron Radiation Beam by Ionization Free-air Chamber

A dosimetric procedure to assure the traceability to the air-kerma standard of the measurements performed in synchrotron radiation beams, was implemented at the “Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti” of ENEA (INMRI-ENEA). To this purpose, absolute air-kerma measurements by the ENEA standard free-air chamber for low energy x-rays have been performed at the SYRMEP (SYnchrotron Radiation for Medical Physics) beamline of the ELETTRA light source in Trieste (Italy) where a program of clinical mammography based on phase contrast imaging has been developed in the framework of the SYRMA project (SYnchrotron Radiation for MAmmography). The synchrotron radiation beams to be characterized were monochromatic x-rays with energy in the range from 8 keV to 35 keV and the reference point for measurements corresponded to the centre of the breast when the patient lies on her support. The absolute air-kerma measurements were also used to calibrate two monitor chambers and an ionization free-air chamber specially designed and realized by ENEA to act as transfer standard in the synchrotron radiation beams. The irradiation conditions for measurements – both beam quality and geometry – were very different from the reference conditions at the INMRI-ENEA. According to the actual conditions, the diaphragm of the ENEA standard chamber was modified and some correction factors were re-determined by Monte Carlo calculations based on the PENELOPE code or by experimental measurements. In particular, the mass air attenuation coefficients of these monochromatic x-rays were experimentally determined to obtain the correction for photon attenuation in air.