Characterization of the Angular Response of an “Isotropic” Mosfet Dosimeter

Abstract

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.

In Vivo Dosimetry Using a Linear Mosfet-Array Dosimeter to Determine the Urethra Dose In 125I Permanent Prostate Implants

“…The deviations at other angles were within the observed deviation at 0 . These results are comparable to those of other studies (5,8,9).…”

“…Studies performed with single dosimeters (5,8) and with the linear array at 60 Co energies (9) demonstrated a small angular dependence. To test the angular response of the linear MOSFET array for 125 I sources, the detector was placed at the center of a polymethylmethacrylate (PMMA) phantom made in-house (Fig.…”

Lack of backscatter factor measurements in HDR applications with MOSkins

“…The maximum radial angular response for the MOSFET detectors used in this study was 12.9% which is consistent with the reported range of 2 to 15% for commercially available detectors [3,4,10,15,16]. New 'dual MOSkins', that take an average of the measurements from two MOSkins attached face to face, have reported angular variations between ±2.5% [5] and will be the subject of future investigations.…”

Is it time to retire the CTDI for CT quality assurance and dose optimization?

“…TLDs are rather laborious for multiple routine measurements, 13 and the recent advent of metal-oxidesemiconductor field-effect transistor ͑MOSFET͒ solid-state dosimeters [18][19][20] suggests another practical alternative. MOS-FET dosimeters are small ͑active volume 0.2ϫ 0.2 mm͒, sensitive at doses of a few milliGrays, 20 have a linear response at these doses, 18,20 have immediate readout, and are convenient for simultaneous multidetector use.…”