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Diamond Detectors for Medical Dosimetry Jan U. Würfel, PTW-Freiburg, CARAT Workshop, 13–15 December 2010.

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Presentation on theme: "Diamond Detectors for Medical Dosimetry Jan U. Würfel, PTW-Freiburg, CARAT Workshop, 13–15 December 2010."— Presentation transcript:

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

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

3 Company Profile Quick introduction to PTW  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  More than 60 distributors worldwide  ISO 9001 and ISO certified

4 Employees Quick introduction to PTW  Freiburg  Non-permanent  Subsidiaries  Total

5 Calibration Laboratories Quick introduction to PTW  PTW-Freiburg Secondary Standard Dosimetry Laboratory (SSDL) Accredited by PTB since 1979 Member of DKD and IAEA network 12,000 Chamber calibrations per year  K&S Associates Accredited Dosimetry Calibration Laboratory (ADCL) Accredited by AAPM since ,500 Chamber calibrations per year

6 Consolidated Turnover Quick introduction to PTW

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

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

9 Measurements in Water What a Diamond detector is used for  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, D w Unit: 1 J/kg = 1 Gy

10 What Are We Actually Measuring? What a Diamond detector is used for  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 Unfortunately... What a Diamond detector is used for  Our detector is not made out of water  Photoelectric effect: mass attenuation coefficient ~ Z³  A detector with Z > Z water 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 Happens at High Energies? What a Diamond detector is used for  At high energies (> 1 MeV), the electron stopping power ratio (detector / water) is the important quantity

13 What’s so Special about a Diamond Detector? What a Diamond detector is used for Type of detector Size for same Low High energy Air filled ionisation chamber 1excellent not perfect but corrections available Diode  1000 x smaller badgood Diamond  1000 x smaller good The perfect detector is infinitesimally small and made out of water

14 Only One Type of Diamond on the Market What a Diamond detector is used for  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 This is what a diamond detector is used for What a Diamond detector is used for  PDD Profile Linac

16 Diamond “Quality” Depends on its intended Use What a Diamond detector is used for  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 /(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 Outline Some PTW test criteria for diamond dosemeters  Quick introduction to PTW  What a Diamond detector is used for  Some PTW test criteria for diamond dosemeters

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

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

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

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

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

23 Re-Priming Effect Some PTW test criteria for diamond dosemeters  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 Thank You for Your Attention The End


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