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Novel Multilayer Detector Design Using Polycrystalline CdTe for Radiation Therapy Imaging Applications

D Shvydka

D Shvydka*, V Karpov, N Paudel, E Parsai, University of Toledo Medical Center, Toledo, OH

WE-G-500-8 Wednesday 4:30PM - 6:00PM Room: 500 Ballroom

Purpose: In radiation therapy treatments the standard approach of taking a planar portal or CIAO image offers setup verifications crucial to accurate radiation delivery. A substantial detector thickness necessary for MV photon absorption is a limitation due to large signal spreading; the requirements of large areas (up to 40x40 cm) and commercial viability dictate the use of non-crystalline semiconductors. We propose a detector based on thin-film CdTe technologies recently developed for photovoltaic applications. It utilizes a stack of CdTe photovoltaic layers, each having a built-in electric field. This 3D structure adds a depth signal reading to the traditional 2-D pixilation allowing for the photon energy and position resolution.

Methods: We modeled the energy deposition under mono-energetic photon sources of 0.5 to 18MV with depth by Monte Carlo simulation package MCNP5. The results yield the charge carrier generation profiles in structures having 3 to 30 layers, and CdTe thickness of 2-10 microns. The profiles were used as input for modeling of current-voltage (I-V) characteristics with device operation modeling software SCAPS-1D.

Results: Based on the modeled I-V we established variations of the output signal in each layer with depth (layer number), and developed an algorithm for extracting the position dependent source energy. Our design includes an original read-out approach where, under the global short circuit conditions, the voltages across individual layers are exponentially sensitive to their corresponding generation rates, a ~1% difference in deposition energies translates into ~100% difference in the corresponding voltage readings.

Conclusion: The micron-range thickness of layers overcomes known limiting factors of the low hole mobility and electron trapping typical of thick crystalline CdTe detectors. A new algorithm allows extracting information on radiation intensity and spectra from the data on the layer photo-voltages leading to enhancement in spatial resolution.

Acknowledgment: this research is supported through NRC grant No.: NRC-HQ-12-G-38-0042

Funding Support, Disclosures, and Conflict of Interest: This project is part of a faculty development award supported by the NRC grant No.: NRC-HQ-12-G-38-0042

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