1. RAD 2012 SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR PORTABLE INSTRUMENTATION Alberto J. Palma Professor of the Department of Electronics University of Granada (Spain)

2. AGENDA  Research group  Introduction  Low cost pMOS as dosemeter  Procedures of measurement Pulsed biasing (PB) Two currents method (2CM) Three currents method (3CM)  Portable dosimetry system  Conclusions  Acknowledgements RAD 2012, Niš (Serbia)

3. RESEARCH GROUP  Interdisciplinary Spanish group: Department of Electronics Department of Atomic, Nuclear and Molecular Physics Radiotherapy Units of several Hospitals in Spain (Granada and Málaga) RAD 2012, Niš (Serbia)

4. RESEARCH GROUP  Our interest in dosimetry: MOSFET: electrical and thermal characterization Monte Carlo simulation of radiation-matter interaction Measurement science Design and testing of electronic instrumentation RAD 2012, Niš (Serbia)

5. INTRODUCTION  Scope: Dose verification systems (DVS) based on MOSFETs mainly for medical use.  High reliable commercial available DVS: Best Medical Canada Systems (BM) One Dose by Sicel Tech.? (OD) REM Oxford Ltd. (REM)  Some small disadvantages Wired sensors for bias purposing (BM, REM) Short dose range (OD) Expensive specific RADFETs (BM, REM, OD) RAD 2012, Niš (Serbia)

6. INTRODUCTION  Our approach Use of low-cost general-purpose MOSFET as dosemeters Wireless and reusable sensors without bias during irradiation  Drawbacks of our approach: Low sensitivity to radiation. Low signal-to-noise ratio (SNR). Low linear range.  Objective: overcoming the above problems RAD 2012, Niš (Serbia)

7. INTRODUCTION Irradiation set-up  Irradiation conditions Theratron-780 with a 60 Co source and a field of 25 x 25 cm 2 MOSFETs, in electronic equilibrium condition, located at 80 cm of the isocentre RAD 2012, Niš (Serbia)

8. LOW COST pMOS  3N163 (Vishay-Siliconix, USA) Response to radiation from I D -V GS curves Sensitivity 30 mV/Gy RAD 2012, Niš (Serbia)

9. PROCEDURES OF MEASUREMENT State of the art  Ionizing radiation creates charge in the oxide: Threshold voltage, V t,is the dosimetric parameter  Measurement of V t at constant drain current (V GD = 0). (BM, REM, OD) RAD 2012, Niš (Serbia) S G B D

10. PROCEDURES OF MEASUREMENT State of the art  Thermal compensation techniques Biasing the pMOS at I ZTC Two identical pMOS with different (REM, OD)sensibilities (BM) RAD 2012, Niš (Serbia) During irradiation Read-out configuration

11. PROCEDURES OF MEASUREMENT Pulsed biasing (PB)  SNR improvement Read-time instabilities caused by low frequency noise (LFN) (due to near-interface and interface traps) How can LFN be reduced? Chopping the drain current as in other experimental techniques (i.e. spectroscopy) and averaging RAD 2012, Niš (Serbia)

12. PROCEDURES OF MEASUREMENT Two currents method  Linear range improvement Accounting for  effect in  V t pre- and post-irradiation parameters at constant current I: RAD 2012, Niš (Serbia)

13. PROCEDURES OF MEASURMENT Two currents method  Linear range improvement Using two drain currents during read-out phase: Threshold voltage shift without neglecting  RAD 2012, Niš (Serbia)

14. PROCEDURES OF MEASUREMENT Results (PB+2CM)  Linearity improvement Sensitivity decay coefficient, mean sensitivity and the linear limit (up to 5%). RAD 2012, Niš (Serbia) DC modesPB I ZTC 2CMI ZTC 2CM m (mV/Gy 2 ) -0.153-0.095-0.113-0.057 Mean Sensitivity (mV/Gy) 20.619.720.019.2 Linear Range (Gy)6.8 10.38.8 16.8 20 % reduction of SD

15. PROCEDURES OF MEASUREMENT Three currents method (3CM)  Thermal compensation Starting hypothesis: Experimentally verified RAD 2012, Niš (Serbia)

16. PROCEDURES OF MEASUREMENT Three currents method (3CM)  Thermal compensation  |V t | thermal compensated Additional current: I 3 Simplification I 1 = I ZTC RAD 2012, Niš (Serbia)

17. PROCEDURES OF MEASUREMENT Results (3CM)  Thermal compensation: Reduction in a factor of 50 in the thermal drift Compensated RAD 2012, Niš (Serbia)

18. PORTABLE DOSIMETRY SYSTEM  Sensor Module JFET as a switch (shorted during irradiation and storage and open during read-out) RAD 2012, Niš (Serbia)

19. PORTABLE DOSIMETRY SYSTEM  Reader Unit RAD 2012, Niš (Serbia)

20. PORTABLE DOSIMETRY SYSTEM  Dose measurement process Zeroing: Measurement and storage of pre-irradiation V S at the two (three) drain currents Irradiation Timeout for short-term fading (120 -180 s) Dose measurement Recovery of the pre-irradiation values from the memory. Measurement and storage of the post-irradiation V S at the two (three) currents Dose calculation (calibration is required) RAD 2012, Niš (Serbia)

21. PORTABLE DOSIMETRY SYSTEM Results  Sensitivity per session A pre-irradiation of 30 Gy is required for reproducibility of S RAD 2012, Niš (Serbia)

22. PORTABLE DOSIMETRY SYSTEM Calibration curves  Radiation response Dpre=30 Gy RAD 2012, Niš (Serbia)

23. PORTABLE DOSIMETRY SYSTEM Technical specifications RAD 2012, Niš (Serbia) * with recalibrations every 15 Gy

24. CONCLUSIONS  We have presented methods of dose measurement for linearity and SNR improvement and thermal drift reduction  Portable dosimetry system based on commercial and standard MOSFET sensor Wireless sensor Reusable sensor (up to 80 Gy with recalibration each 15 Gy) Thermal compensation without additional devices (3 mGy/ºC) RAD 2012, Niš (Serbia)

25. AKNOWLEDGEMENTS  University Hospitals “San Cecilio” and “Virgen de las Nieves” of Granada (Spain) for permitting us to use their installations  Spanish and Andalusian (regional) Governments for funding, partially supported by European Regional Development Funds RAD 2012, Niš (Serbia)

26. RAD 2012 SOME ADVANCES IN DOSE MEASUREMENT WITH MOSFET FOR PORTABLE INSTRUMENTATION Alberto J. Palma Professor of the Department of Electronics University of Granada (Spain)