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EVALUATION OF DMOS TRANSISTORS AS ELECTRON BEAMS DOSIMETER

M.A. Carvajal , F. Simancas, D. Guirado, J. Banqueri, S. Martínez-García, A.M. Lallena and A.J. Palma. EVALUATION OF DMOS TRANSISTORS AS ELECTRON BEAMS DOSIMETER. Summary. Introduction DMOS study Methods and materials Results and discussion Conclusion DMOS study CD 4007 characterization

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EVALUATION OF DMOS TRANSISTORS AS ELECTRON BEAMS DOSIMETER

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  1. M.A. Carvajal, F. Simancas, D. Guirado, J. Banqueri, S. Martínez-García, A.M. Lallena and A.J. Palma EVALUATION OF DMOS TRANSISTORS AS ELECTRON BEAMS DOSIMETER

  2. Summary • Introduction • DMOS study • Methods and materials • Results and discussion • Conclusion DMOS study • CD 4007 characterization • Thermal characterization • Characterization as dosimeter • Results and discussion • Conclusions • Acknowledgements

  3. Introduction • Intra Operative Radiotherapy (IORT): • Complementary technique applied just after the cancer extraction. • Electron beams provided by a LINAC. • Aims to destroy the remaining tumoral cells on the edge of the cancer. • Only one session.

  4. Introduction • IORT:

  5. Introduction • Treatment planning in IORT: • Software using MonteCarlo code • Dosimetry control very suitable • MOSFETs dosimeters provides immediate readout

  6. Materials and Methods • Dosimetric system: • Commercial pMOS transistors (non RADFETs) • Reader unit and dosimeters based on commercial pMOSFET developed by our research group • Thermal characterization need for thermal compensation • Readout techniques for linearity and resolution improvements

  7. Materials and Methods • Measurement method: • Two currents: Linear range improvement • Three currents: Thermal compensation (IZTC) Non thermal compensated 7

  8. Method And Materials: Block diagram of dosimetric system 8

  9. Method And Materials • Dosimetric system:

  10. Method And Materials: Experimental setup • Model DMOS tested (Unbiased mode): • BS250F, ZVP3306 and ZVP4525 • Four transistors per model • Irradiated by a Siemens Mevatron KDS: • 6 MV electrons • Field 25x25 cm2 • At the iso-center, 100 cm • Normal incidence

  11. Method And Materials: Thermal characterization • I-V characteristics at different temperatures • Extracted by a semiconductor analyzer (B1500, Agilent Technologies) • Temperature variations produced by a climate chamber (VCL4006 Vötosch Industryetedhnik, Germany)

  12. Method And Materials: Thermal characterization • The IZTC was not found for the studied DMOS transistor:

  13. Method And Materials: Thermal characterization • No IZTC then 2 currents algorithm to determine the DVT. • Numerical compensation: Determination the thermal coefficient of VT (aVT)

  14. Method And Materials: Thermal characterization • Thermal compensation • Thermal coefficients:

  15. Results and discussion Accumulate VT shift as dose function

  16. Results and discussion Accumulate VT shift as dose function

  17. Results and discussion • Average sensitivities:

  18. Results and discussion • ZVP3306: • Thermal coefficient: (-2.48 ± 0.16) mV/ºC • Average sensitivity: (3.7 ± 0.3) mV/Gy • Then, thermal drift: (0.60 ± 0.07) Gy/ºC

  19. Conclusions of DMOS study • ZVP3306 presented the highest sensitivity and lowest dispersion • However, the thermal dependence is too high to be used as dosimetry control in IORT. • A thermal compensation algorithm is needed or the sensitivity must be increased (for example using an external bias voltage) • To look for new dosimeter candidates: We have tested the CD4007 integrated circuit.

  20. New pMOS as dosimeter: Characterization of CD 4007 • Texas Instruments (USA) • 0.3 € and 100 nm of SiO2

  21. CHARACTERIZATION CD 4007: Thermal response, IZTC • I-V of five in saturation region, VGD =0 f from 15 to 45ºC

  22. CHARACTERIZATION CD4007: Thermal response, IZTC • VT thermal coefficient: (-2.0 ± 0.3) mV/ºC • Thermal compensation is needed. • IZTC found for CD4007: (137 ± 19) mA • Three current algorithm can be applied Five transistor studied:

  23. CHARACTERIZATION CD4007: Irradiation conditions Buildup (1.5 cm, and only for photon beams) CD4007 dosimeter modules (At the isocentre) Ionization chamber PTW23332

  24. CHARACTERIZATION CD 4007: Linearity, Photon beams 6 MV

  25. CHARACTERIZATION CD 4007: Sensitivity, Photon beams 6 MV

  26. CHARACTERIZATION CD 4007: Sensitivity, Photon beams 6 MV

  27. CHARACTERIZATION CD 4007: Linearity, Electron beams 6 MV

  28. CHARACTERIZATION CD 4007: Sensitivity, Electron beam 6 MV

  29. CHARACTERIZATION CD 4007: Sensitivity, Electron beam 6 MV

  30. CHARACTERIZATION CD 4007: Thermal considerations

  31. Conclusions of CD4007 • CD4007 presents a sensitivity of radiation of (4.62 ± 0.13) mV/Gy for photon beams of 6 MV • Thermal dose coefficient of 5 cGy/ºC • A good candidate to be use as IORT dosimeter • Need to study in depth thermal dependence, linearity and calibrations • Possible sensitivity increasing: biasing, stacking pMOS transistors.

  32. Acknowledgements • University Hospital San Cecilio (Granada, Spain). • For funding this work: Ministerio de Ciencia e Innovacion and the Junta de Andalucía. • And partially supported by European Regional Development Funds (ERDF)

  33. M.A. Carvajal, F. Simancas, D. Guirado, J. Banqueri, S. Martínez-García, A.M. Lallena and A.J. Palma Thank you very munch for your attention carvajal@ugr.es

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