Diamond Detectors for Medical Dosimetry CARAT Workshop 13

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

Outline 4 Quick introduction to PTW 4 What a Diamond detector is used for 4 Some PTW test criteria for diamond dosemeters

Quick introduction to PTW Company Profile 4 PTW-Freiburg Gmb. H (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

Quick introduction to PTW Employees 4 Freiburg 4 Non-permanent 4 Subsidiaries 205 9 47 4 Total 261

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

Quick introduction to PTW Consolidated Turnover

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

What a Diamond detector is used for Outline 4 Quick introduction to PTW 4 What a Diamond detector is used for 4 Some PTW test criteria for diamond dosemeters

What a Diamond detector is used for Measurements in Water 4 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 4 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

What a Diamond detector is used for What Are We Actually Measuring? 4 Only very few photon interactions 4 The interacting photon transfers energy to an electron 4 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 high energy 500 ke. V 25 Me. V

What a Diamond detector is used for Unfortunately. . . 4 Our detector is not made out of water 4 Photoelectric effect: mass attenuation coefficient ~ Z³ A detector with Z > Zwater will over-respond to scattered (low-energy) radiation. 4 This is called (bad) energy response 4 In large treatment fields (> 10 x 10 cm²) silicon detectors (Z = 14) over-respond by several %.

What a Diamond detector is used for What Happens at High Energies? 4 At high energies (> 1 Me. V), the electron stopping power ratio (detector / water) is the important quantity

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 Type of detector Size for same response @ Low energy @ High energy Air filled ionisation chamber 1 excellent not perfect but corrections available Diode 1000 x smaller bad good Diamond 1000 x smaller good

What a Diamond detector is used for Only One Type of Diamond on the Market 4 The PTW diamond detector is the only usable diamond detector on the market 4 It’s a natural diamond 4 Limited resources (smaller than the market need) 4 Quite expensive

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

What a Diamond detector is used for Diamond “Quality” Depends on its intended Use 4 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)) 4 Reaction to 30 ke. V X-rays can be completely different from reaction to 1 Me. V gamma radiation. This includes response, priming (= pumping), and speed of response

Some PTW test criteria for diamond dosemeters Outline 4 Quick introduction to PTW 4 What a Diamond detector is used for 4 Some PTW test criteria for diamond dosemeters

Some PTW test criteria for diamond dosemeters Priming (Pumping) 4 Natural diamond, Co-60, 6. 77 m. Gy/s 4 Dose for priming: when final signal ± 0. 5 % is reached 4 Here: 5 Gy

Some PTW test criteria for diamond dosemeters Speed of Response 4 Reaction to beam-on and -off: 4 Signal rise: 99 % reached within 2 s 4 Signal drop: 1 % reached within 2 s

Some PTW test criteria for diamond dosemeters Signal to Dark Current Ratio (SDR) 4 NOT the same as Signal to noise ratio (SNR) SDR_2 s SDR_1 min 4 SDR_2 s 100 4 SDR_1 min 1000

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

Some PTW test criteria for diamond dosemeters Radiation Hardness? 4 Diamond is less sensitive to radiation damage than silicon 4 But is it fully radiation resistant to 25 MV Linac radiation?

Some PTW test criteria for diamond dosemeters Re-Priming Effect 4 For artificial diamond dosemeters it can happen that after a break of a few minutes, re-priming is necessary 4 This can lead to tilted shoulders 4 Re-priming dose after 3 min pause should be below 100 m. Gy

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