Md. Shakilur Rahman

TLD Postal Dose Quality Audit Intercomparison for Megavolts Photon Beam of Radiotherapy Centers in Bangladesh

A postal dose intercomparison program based on the thermo-luminescence dosimeter (TLD) was carried in order to investigate and reduce uncertainties involved in the measurement of absorbed dose, and to improve the accuracy of dose measurement at different radiotherapy centers in Bangladesh. The intercomparison program was organized by the Secondary Standard Dosimetry Laboratory (SSDL) with the assistance of the IAEA/WHO postal dose intercomparison network. A set of three TLDs (encapsulated LiF powder) including a control was sent to the participating radiotherapy centers for irradiation with 2 Gy of absorbed dose to water. The measurements were made for 60Co and megavolts X-ray beam. Thirteen beams in total were checked. Deviation of ±5% between stated and evaluated dose was considered to be acceptable for 60Co and 6MV X-ray beam. The result of 60Co beam of two centers exceeds the acceptance limit. The maximum deviation was -8.6% with an uncertainty of 1.8% i.e., the ratio of intercomparison dose to locally measured dose was 1.09. We discussed the immediate and corrective action to minimize the discrepancies. For ten 60Co beam measurements, a mean valued of the ratio of intercomparison dose to locally stated dose was 0.988 with a standard deviation of 0.031. The importance of a proper ongoing quality assurance program is essential in maintaining the consistency and the uniformity of absorbed dose measurement for the precision dose delivery to the patient.

Measurement of the Neutron Total Cross Sections of Natural Palladium by Using Pulsed Neutrons at Pohang Neutron Facility

The neutron total cross-sections of palladiums are measured in the neutron energy region from 0.1 eV to 500 eV by using time of flight method at the Pohang Neutron Facility. The Pohang Neutron Facility consists of an electron linac, a water-cooled tantalum target with a water moderator, and a 12 m long time-of-flight path, and is producing pulsed neutron beam from 65 MeV electron as 10~15 Hz repetition rate. The neutron detector is using a 6Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm. Natural palladium metal plates with 5 × 5 cm2 in area and 0.5 mm, 1 mm in thickness, are used for the neutron transmission measurement. The background level is determined by using a notch-filter of Co, In, and Cd sheets. The measurements are compared with other measurements and the evaluated data in ENDF/B-VII.0.

Influence of the PMMA Slab and ISO Water Phantom in Calibrating Personal Dosimeter in the Energy Range of 36-662 keV

Personnel dosimeter calibration for the determination of operational quantities used in individual monitoring requires placement on a phantom that provides a reasonable approximation to the backscatter properties of the part of the body on which it is worn. The personal dose equivalent Hp(d) is defined in the human body which is not a measurable quantity. The reference International Commission on Radiation Units and Measurements (ICRU) tissue is not readily available; hence a phantom of alternative must be used for calibration. The well recognized polymethyl-methacrylate (PMMA) slab phantom of size 30 × 30 × 15 cm3 are still being used in calibrating personal dosimeters. The International Organization for Standardization (ISO)-4037(3) has proposed another phantom of the same size as ICRU, which is named as ISO water phantom. In the present study, calibrated X-ray fields are characterized for National Institute of Advanced Industrial Science and Technology (AIST) middle beam spectrum series by which the influence of these proposed phantoms has been studied by the thermoluminescence dosimetry (TLD) technique. Discrepancies of responses within 8% have been observed at lower energies up to 42 keV. There is a very good agreement in responses is observed for ISO water slab and PMMA slab phantom for photon energies above 42keV. Experimentally obtained correction factors for AIST radiation qualities of M50 and M60 for PMMA slab to ISO water phantom is proposed. Measured backscatter factor results from PMMA to ISO water-filled phantoms over the photon energy range 36 to 662 keV are reported. This shows that the new ISO water phantom is a better substitute of ICRU tissue phantom than the PMMA slab.