MEASUREMENT OF BACKSCATTERING FACTOR FOR X RAY CALIBRATION BEAM WITH VARIOUS PHANTOMS USING IONIZATION CHAMBER AND THERMOLUMINESCENT DOSIMETER

Abstract

Radiation hazard is one of the critical issues at different medical centers, industries and nuclear facilities around the world. To protect radiation workers, public and environment, a reliable dose measurement system is required that should be complied with international recommendations. X-rays and gamma rays are a highly penetrating radiation and are widely used as a calibration source at different of medical facilities, industries and nuclear facilities. X-ray beam characterization for the calibration of radiation measuring equipment is necessary to meet the recommendations by international organizations. In the present study, characterization of X-ray beam generated from X-ray irradiator, Model: X80-225KV, Hopewell Design, Inc of Secondary Standard Dosimetry Laboratory (SSDL) has been performed in accordance with the ISO 4037-1 narrow–spectrum series. The beam characterization was done by the determination of Half-Value Layer (HVL), the Effective Energy, Homogeneity Coefficient (HC), Beam Quality Index and output air Kerma values. Experimentally measured HVL is compared with the values recommended by ISO-4037 and very small deviation is found. Filter thickness error or positioning of ionization chamber may be reason for this. The homogeneity coefficient is between 0.46 to 0.50. The homogeneity coefficient should be 1 but due to beam hardening, it is not possible. The Effective Energy, Eeff for ISO narrow spectrum series were calculated by empirical relation derived from Hubble mass attenuation coefficients. Hence, a set of conversion coefficients has been established for the effective energies of photon beam from air Kerma to dose equivalent (Sv/Gy) i.e., for ambient dose equivalent, H*(10) and personal dose equivalent, HP(10) & HP (0.07) for ISO beam code N40, N60, N80, N100, N120, N150 and N200 by empirical mathematical relationship applicable for the photons with energies between 10 keV to 10 MeV. The measured dose H*(10), HP(10) & HP(0.07) could be used for implementation of the new ICRU operational unit in radiation monitoring around the radiation facilities to protect human and environment in Bangladesh. An evaluation of backscattering factors for ISO water phantom and ICRU slab phantom has been conducted by MCNPX Code (version 2.6.0). for tube iii potential N80, N100, N120 and N140. The calculated values were compared with the values of experimental values. It is found that in personal dose monitoring, the backscattering factor contributes a considerable amount to the total absorbed dose.