Performance evaluation of a homemade extrapolation chamber in low energy radiation field.

Extrapolation chambers are widely used to detect the weakly-penetrating types of radiation beams. Normally, the sensitive volume of them is limited, only a few cubic centimeters. Their collecting electrodes use sheet structure, which lead to more backscattering photons. In this work, a large volume extrapolation chamber with (235–1413) cm3 was designed for the absolute measurement of air kerma strength of 125I brachytherapy seeds. The developed chamber has the following innovation: large sensitive volume and the Aluminized Polyethylene terephthalate (PET) collecting electrode foil. In order to give a more comprehensive performance evaluation for the designed chamber, a series of experiments were conducted. Evaluation parameters include leakage current, saturation curve, polarity effect, ion collecting efficiency, linearity of response, collecting area and zero point. Those tests were measured in the standard X-ray radiation field. A comparative experiment with the primary standard free air chamber (FAC) was performed. Moreover, extrapolation curve was measured in 125I radiation field. Evaluation results show a good behavior of the extrapolation chamber. The Al-coated PET collecting electrode influence the response of the chamber to 0.07% with the simulation of MCNP5 Monte Carlo. All the results indicate the usefulness of the extrapolation chamber for the absolute measurement for 125I seeds. • A large volume extrapolation chamber with (235–1413) cm3 was designed and fabricated. • A new design method of the collecting electrode is adopted. It is etched as the collecting electrode and the potential guard. • The Al-coated PET foil can be employed for fabricating the collecting electrode to replace the PMMA sheet structure. [ABSTRACT FROM AUTHOR]