SU‐E‐T‐219: Ion Chamber Dosimetry Modification under Strong Magnetic Field Conditions; Air and Liquid Filled Chambers Study

Purpose: To examine the dose response of ion chambers in an external high magnetic field, conducting measurements and Monte Carlo simulations. To examine a liquid filled ion chamber (LION) as a low B field dependant on‐time dosimeter. Methods: This work investigates the response of a 0.6cm3 Farmer chamber to a 11.17 mCi 137Cs source under the influence of a 1.5 T magnetic field. A special setup allowed measurements for transverse and longitudinal field directions inside a 1.5T MRI bore. Monte Carlo simulations were conducted using FLUKA, a particle physics transport package. A PTW 0.002 cm3 MicroLion chamber is offered as a non‐correctional on‐line B field measurement alternative, since it introduces a small cavity and no air phantom voids. Results: Higher Farmer chamber currents were obtained when a transverse (to chamber central axis) magnetic field was applied, compared to zero B field conditions. Lower readings were obtained when a longitudinal field was present. Measurements have shown that the electric field between the electrodes has a very small impact on the ion collection, and can be neglected for simulation purposes. A microLion chamber was found not to have response impact due to the presence of a magnetic field. Conclusions: The dose response of a Farmer chamber in an external magnetic field varies with the average charged particle trajectory length. The average trajectory depends on the B field magnitude and direction, and on the energy spectrum of the primary photon beam. Thus, an alignment with the B field direction would be essential. A liquid‐filled chamber is offered as an alternative due to its homogeneous structure and small cavity that results in no B field correction needed.