Abstract
We experimentally evaluated the proton beam dose reproducibility, sensitivity, angular dependence and depth‐dose relationships for a new Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector. The detector was fabricated with a thinner oxide layer and was operated at high‐bias voltages. In order to accurately measure dose distributions, we developed a practical method for correcting the MOSFET response to proton beams. The detector was tested by examining lateral dose profiles formed by protons passing through an L‐shaped bolus. The dose reproducibility, angular dependence and depth‐dose response were evaluated using a 190 MeV proton beam. Depth‐output curves produced using the MOSFET detectors were compared with results obtained using an ionization chamber (IC). Since accurate measurements of proton dose distribution require correction for LET effects, we developed a simple dose‐weighted correction method. The correction factors were determined as a function of proton penetration depth, or residual range. The residual proton range at each measurement point was calculated using the pencil beam algorithm. Lateral measurements in a phantom were obtained for pristine and SOBP beams. The reproducibility of the MOSFET detector was within 2%, and the angular dependence was less than 9%. The detector exhibited a good response at the Bragg peak (0.74 relative to the IC detector). For dose distributions resulting from protons passing through an L‐shaped bolus, the corrected MOSFET dose agreed well with the IC results. Absolute proton dosimetry can be performed using MOSFET detectors to a precision of about 3% (1 sigma). A thinner oxide layer thickness improved the LET in proton dosimetry. By employing correction methods for LET dependence, it is possible to measure absolute proton dose using MOSFET detectors.PACS number: 87.56.‐v
Highlights
327 Kohno et al.: Proton dose distribution measurements using Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector a p-channel enhanced MOSFET constructed on a negatively doped (n-type) silicon substrate
Dose sensitivity The sensitivity of the TN-252RD MOSFET detector was 0.72 ± 0.01 and the corresponding reproducibility was ± 1.4%. The sensitivity of this detector was lower than the TN-502RD MOSFET with a thicker oxide layer, its reproducibility was within 2%
We experimentally evaluated the proton beam dose reproducibility, angular dependence and depth-dose relationships for a new TN-252RD MOSFET detector at high-bias voltages
Summary
327 Kohno et al.: Proton dose distribution measurements using MOSFET detector a p-channel enhanced MOSFET constructed on a negatively doped (n-type) silicon substrate. The holes drift toward the substrate under an appropriate bias voltage and are semipermanently trapped at the interface, resulting in a shift in the gate voltage required for source-drain conductivity that is proportional to the radiation dose. The MOSFET response is strongly dependent on the degree of linear energy transfer (LET) occurring through columnar recombination. This is due to the significant reduction in charge recombination when the electric field applied to the MOSFET is perpendicular to the plasma track, leading to faster drift of electron-hole pairs. As a result of the LET dependence and the columnar recombination effect, quantitative proton dose measurements are difficult to accurately perform using MOSFET detectors. A lower angular dependence would be desirable when using MOSFET detectors for in vivo proton dosimetry
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