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

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

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

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

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

Consolidated Turnover Quick introduction to PTW

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

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

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

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

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 %.

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

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

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

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

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

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

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

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

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

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

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?

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

Thank You for Your Attention The End