Determination of backscatter factors and air kerma to dose equivalent conversion factors for diagnostic x-ray beams by experimental and Monte Carlo methods

Backscatter measurements are required for the determination of the Dose equivalent quantity H' (0.07) in ICRU sphere or H (0.07) in practical phantoms both for area monitoring and personal monitoring. These values were determined for a Siemens Polymat 501 diagnostic x-ray machine used in the Division for calibrations. The spectra of 40, 60 and 81 kV beams were earlier determined in terms of exposure units and photon fluence units by attenuation analysis method. A 3 cc spherical Exradin chamber and a 0.2 cc. Nuclear Enterprises type 2577 chamber, both calibrated by free air chamber, were used for the determination of backscatter factors of perspex 30 cm x 30 cm x 15 cm slab and reference 30 cm x 30 cm x 25.6 cm water phantoms respectively. The Monte Carlo code for Neutrons and Photons (MCNP 1983) was also used to find out the backscatter factors and backscatter spectra for the above two phantoms. Results showed that the ratio of measured to Monte Carlo determined backscatter values were within ±3% at the three beams used. (author)