Characterization of the angular response of an "isotropic" MOSFET dosimeter.

MOSFET dosimeters are seeing increased use for various dosimetry applications. Previously available commercial MOSFET dosimeters demonstrate significant anisotropies in the response when irradiated from certain directions at diagnostic x-ray energies. The angular response of a more recently introduced MOSFET dosimeter that claims an isotropic response to incident radiation is characterized. Measurements of the Thomson-Nielsen model TN-502RDI dosimeter were made for rotations of 360 degrees in 15 degrees increments about both the axial and normal-to-axial axes. These measurements demonstrated that the model TN-502RDI dosimeter has a nearly isotropic response at diagnostic x-ray energies with variations of less than 3% deviation from the mean response for radiation incident from most directions. Only two specific orientations showed significant deviation from the overall isotropic response. These correspond to the exposure scenarios where radiation is incident directly along the axis where the wire leads attach to the MOSFET device and the distal tip, the 90 degrees and 270 degrees orientations for normal-to-axial rotations, respectively. The largest deviations from an isotropic response occur when the dosimeter is irradiated free-in-air. Irradiations performed at the center of a tissue equivalent cylinder resulted in smaller deviations in angular response. The improved angular response of the TN-502RDI allows greater freedom in placement and use of MOSFET dosimeters in diagnostic radiology applications.