Design And Construction of a Parallel Plate Ionization Chamber for Dosimetry in Conventional Radiography

Abstract

The main objective of every radiodiagnostic procudure is to produce an informative image with minimum radiation exposure to patients. To be able to minimize dose to patients, and ensure image quality, there must be a regular quality control on the entire X-ray systems, which involves routine measurement of exposure and exposure rate. The most employed dosimeters for the adjustment and control measurements is the parallel plate ionization chamber. It is less intrinsic to energy dependence, hence mostly recommended for low dose rate measurement. It is in view of this, that a portable, and less expensive detector (parallel plate ionization chamber) has been designed and constructed for dosimetry in diagnostic radiography.The chamber comprises of a body made of Perspex (1.7 mg/cm²), a bias electrode made of copper plate, a measuring electrode made of an aluminium plate, guard rings made of an aluminium plate an entrance window made of a paper coated with graphite (shading the paper with HB pencil until the paper became electrically conductive) with the uncoated side pasted to a piece of unexposed developed radiographic film. The chamber has a sensitive volume of 2.8 cc which was vented to the environment. The operational bias voltage of the constructed ionization chamber was found to range from 200 V – 400 V. Two different conceptual designs were developed and evaluated. The concept with the highest overall utility value was selected and developed. The completed chamber was subjected to several performance characteristic and quality control tests: energy dependence, response reproducibility and constancy, angular dependence, response linearity and leakage characteristics. The chamber was cross calibrated against diagnostic multimeter (Piranha) with traceability to a secondary standard dosimetry laboratory (Swedac. Ackredictering, Sweden) and found to have a calibration coefficient (NK) of 1.7 x109 mGy/A. Beam quality correction factor for chamber could be expressed with a fourth degree polynomial equation in terms of HVL (mmAl) using 100 kVp and 20 mAs (200 mA) as the reference exposure parameters. Response reproducibility and constancy, angular dependence, response linearity were all within the International Electrotechnical Commission (IEC) 61674 stipulated limit. A maximum deviation of 8.6% was observed at 90O clockwise of the gantry angle. This was as a result of cable leakage. A parallel plate ionization chamber has successfully been designed and developed and is applicable in a range of 50 -130 kVp.