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First application of a high resolution silicon detector for proton beam Bragg peak detection in a 0.95 T magnetic field.

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PURPOSE To report on experimental results of a high spatial resolution silicon based detector exposed to therapeutic quality proton beams in a 0.95 T transverse magnetic field. These experimental results… Click to show full abstract

PURPOSE To report on experimental results of a high spatial resolution silicon based detector exposed to therapeutic quality proton beams in a 0.95 T transverse magnetic field. These experimental results are important for the development of accurate and novel dosimetry methods in future potential real-time MRI-guided proton therapy systems. METHODS A permanent magnet device was utilised to generate a 0.95 T magnetic field over a 4 × 20 × 15 cm3 volume. Within this volume, a high-resolution silicon diode array detector was positioned inside a PMMA phantom of 4 × 15 × 12 cm3 . This detector contains two orthogonal strips containing 505 sensitive volumes spaced at 0.2 mm apart. Proton beams collimated to a circle of 10 mm diameter with nominal energies of 90 MeV, 110 MeV and 125 MeV were incident on the detector from an edge-on orientation. This allows for a measurement of the Bragg peak at 0.2 mm spatial resolution in both the depth and lateral pro le directions. The impact of the magnetic field on the proton beams, i.e. a small deflection was also investigated. A Geant4 Monte Carlo simulation was performed of the experimental setup to aid in interpretation of the results. RESULTS The nominal Bragg peak for each proton energy was successfully observed with a 0.2 mm spatial resolution in the 0.95 T transverse magnetic field in both a depth and lateral profile. The proton beam deflection (at 0.95 T) was a consistent 2 ± 0.5 mm at the centre of the magnetic volume for each beam energy. However, a pristine Bragg peak was not observed for each energy. This was caused by the detector packaging having small air gaps between layers of the phantom material surrounding the diode array. These air gaps act to degrade the shape of the Bragg peak, and further to this the non-water equivalent silicon chip acts to separate the Bragg peak into multiple peaks depending on the proton path taken. Overall, a promising performance of the silicon detector array was observed, however with a qualitative assessment rather than a robust quantitative dosimetric evaluation at this stage of development. CONCLUSIONS For the first time, a high-resolution silicon-based radiation detector has been used to measure proton beam Bragg peak deflections in a phantom due to a strong magnetic field. Future efforts are required to optimise the detector packaging to strengthen the robustness of the dosimetric quantities obtained from the detector. Such high resolution silicon diode arrays may be useful in future efforts in MRI-guided proton therapy research.

Keywords: resolution; proton; detector; bragg peak; silicon; magnetic field

Journal Title: Medical physics
Year Published: 2019

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