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Diamond Detectors for Medical Dosimetry

Diamond Detectors for Medical Dosimetry. CARAT Workshop, 13 – 15 December 2010. Jan U. Würfel , PTW-Freiburg, jan.wuerfel@ptw.de. Outline. Outline. Quick introduction to PTW What a Diamond detector is used for Some PTW test criteria for diamond dosemeters. Quick introduction to PTW.

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Diamond Detectors for Medical Dosimetry

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  1. Diamond Detectors for Medical Dosimetry CARAT Workshop, 13–15 December 2010 Jan U. Würfel, PTW-Freiburg, jan.wuerfel@ptw.de

  2. Outline Outline • Quick introduction to PTW • What a Diamond detector is used for • Some PTW test criteria for diamond dosemeters

  3. Quick introduction to PTW Company Profile 4 PTW PTW-Freiburg GmbH (1922) PTW-New York Corporation (1995) PTW-France SARL (2001) PTW-Asia Pacific Limited (2004) PTW-Latin America LTDA (2005) PTW-Beijing Limited (2007) PTW-UK Limited (2008) K&S Associates, Inc. (2009) Sales, consolidated: 39.6 million € Employees: approx. 260 4 More than 60 distributors worldwide 4 ISO 9001 and ISO 13485 certified

  4. Quick introduction to PTW Employees 4Freiburg4Non-permanent 4Subsidiaries 4Total 205947261

  5. Quick introduction to PTW Calibration Laboratories 4PTW-FreiburgSecondary Standard Dosimetry Laboratory (SSDL) Accredited by PTB since 1979 Member of DKD and IAEA network 12,000 Chamber calibrations per year 4K&S AssociatesAccredited Dosimetry Calibration Laboratory (ADCL) Accredited by AAPM since 1982 1,500 Chamber calibrations per year

  6. Quick introduction to PTW Consolidated Turnover

  7. Quick introduction to PTW Product Overview Electrometers Ionization Chambers Detector Arrays Water Phantoms X-Ray Test Tools OEM Components Calibration Benches Application Software

  8. What a Diamond detector is used for Outline • Quick introduction to PTW • What a Diamond detector is used for • Some PTW test criteria for diamond dosemeters

  9. What a Diamond detector is used for Measurements in Water • Propagation and interaction of photons in water is very similar to human tissue  Measurements in radiation therapy are performed in real or artificial (plastic) water • The quantity we are looking for is: How much energy per mass is deposited in water by the radiation? Name: Absorbed Dose to water, Dw Unit: 1 J/kg = 1 Gy

  10. What a Diamond detector is used for What Are We Actually Measuring? • Only very few photon interactions • The interacting photon transfers energy to an electron • The dose is deposited by these (“secondary”) electrons Processes for this photon energy transfer in radiation therapy beams: Photoelectric effect Compton effect Pair production low energy  500 keV high energy 25 MeV

  11. What a Diamond detector is used for Unfortunately... • Our detector is not made out of water • Photoelectric effect: mass attenuation coefficient ~ Z³ •  A detector with Z > Zwater will over-respond to scattered • (low-energy) radiation. • This is called (bad) energy response • In large treatment fields (> 10x10 cm²) silicon detectors (Z = 14) over-respond by several %.

  12. What a Diamond detector is used for What Happens at High Energies? • At high energies (> 1 MeV), the electron stopping power ratio (detector / water) is the important quantity

  13. What a Diamond detector is used for What’s so Special about a Diamond Detector? The perfect detector is infinitesimally small and made out of water

  14. What a Diamond detector is used for Only One Type of Diamond on the Market • The PTW diamond detector is the only usable diamond detector on the market • It’s a natural diamond • Limited resources (smaller than the market need) • Quite expensive

  15. What a Diamond detector is used for This is what a diamond detector is used for Linac g Profile PDD

  16. What a Diamond detector is used for Diamond “Quality” Depends on its intended Use • A diamond detector may react very fast to a single particle event but very slow to high-flux radiation therapy photons (mean flux density roughly 1010 1/(cm²s)) • Reaction to 30 keV X-rays can be completely different from reaction to 1 MeV gamma radiation. This includes response, priming (= pumping), and speed of response

  17. Some PTW test criteria for diamond dosemeters Outline • Quick introduction to PTW • What a Diamond detector is used for • Some PTW test criteria for diamond dosemeters

  18. Some PTW test criteria for diamond dosemeters Priming (Pumping) • Natural diamond, Co-60, 6.77 mGy/s • Dose for priming: when final signal ±0.5 % is reached • Here: 5 Gy

  19. Some PTW test criteria for diamond dosemeters Speed of Response • Reaction to beam-on and -off: • Signal rise: 99 % reached within  2 s • Signal drop: 1 % reached within  2 s

  20. Some PTW test criteria for diamond dosemeters Signal to Dark Current Ratio (SDR) • NOT the same as Signal to noise ratio (SNR) SDR_2s SDR_1min • SDR_2s  100 • SDR_1min  1000

  21. Some PTW test criteria for diamond dosemeters Why is the Speed of Response so Important? A slow speed can lead to tilted shoulders in profile measurements scan direction

  22. Some PTW test criteria for diamond dosemeters Radiation Hardness? • Diamond is less sensitive to radiation damage than silicon • But is it fully radiation resistant to 25 MV Linac radiation?

  23. Some PTW test criteria for diamond dosemeters Re-Priming Effect • For artificial diamond dosemeters it can happen that after a break of a few minutes, re-priming is necessary • This can lead to tilted shoulders • Re-priming dose after 3 min pause should be below  100 mGy

  24. The End Thank You for Your Attention

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