Abstract
Ionization chambers are the most employed dosimeters for precise measurements, as those required in radiotherapy. In this work, a new graphite ionization chamber was developed and characterized in order to compose a primary standard system for the beam dosimetry of the 60 Co sources. This dosimeter is a cylindrical type ionization chamber, with walls and collecting electrode made of high-purity graphite, and the insulators and stem made of Teflon ® . The walls are 3.0 mm thick, and it has a sensitive volume of 1.40 cm 3 . The characterization was divided in two steps: experimental and Monte Carlo evaluations. This new dosimeter was evaluated in relation to its saturation curve, ion collection efficiency, polarity effect, short- and medium-term stabilities, leakage current, stabilization time, linearity of response and angular dependence. All results presented values within the established limits. The second part of the characterization process involved the determination of the correction factors, obtained by Monte Carlo simulations. Comparing these correction factors values with those from other primary standard laboratories, the highest differences were those for the wall and stem correction factors. The air kerma rate of the 60 Co source was determined with this new dosimeter and with the IPEN standard system, presenting a difference of 1.7%. These results indicate that this new dosimeter may be used as a primary standard system for 60 Co gamma beams.
Highlights
Primary air-kerma standards for 60Co beams are based on graphite-walled cavity ion chambers
Free-air ionization chambers are commonly utilized to measure the air kerma in x-ray beams produced with tube voltages of up to around 400 kV [1]
In order to measure the air kerma at higher photon energies some cavity ionization chambers were developed. These ionization chambers present some advantages in relation to free-air chambers such as: small size, easy to use and they could measure multidirectional irradiation fields
Summary
Primary air-kerma standards for 60Co beams are based on graphite-walled cavity ion chambers. These ionization chambers generally present cylindrical or plane-parallel design. Free-air ionization chambers are commonly utilized to measure the air kerma in x-ray beams produced with tube voltages of up to around 400 kV [1]. It is not possible to use free-air chambers, and cavity ionization chambers, or calorimeters, are employed. In order to measure the air kerma at higher photon energies some cavity ionization chambers were developed. These ionization chambers present some advantages in relation to free-air chambers such as: small size, easy to use and they could measure multidirectional irradiation fields
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