diamond detectors for medical dosimetry jan u. würfel, ptw-freiburg,...
TRANSCRIPT
Diamond Detectors for Medical Dosimetry
Jan U. Würfel, PTW-Freiburg, [email protected]
CARAT Workshop, 13–15 December 2010
Outline
Outline
Quick introduction to PTWQuick introduction to PTW
What a Diamond detector is used for
Some PTW test criteria for diamond dosemeters
Company Profile
Quick introduction to PTW
PTWPTW-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 13485 certified
Employees
Quick introduction to PTW
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reib
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1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
FreiburgNon-permanent Subsidiaries Total
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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 1982 1,500 Chamber calibrations per year
Consolidated Turnover
Quick introduction to PTW
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1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
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 forWhat 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, Dw
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 > 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 %.
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
response
@ Low
energy
@ High
energy
Air filled ionisation chamber
1 excellent
not perfect but
corrections available
Diode1000 x
smallerbad good
Diamond1000 x
smallergood 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
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-100 -50 0 50 100Pos [mm]
Sig
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%]
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0 50 100 150 200 250 300 350Depth [mm]
Sig
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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 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
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 dosemetersSome PTW test criteria for diamond dosemeters
Priming (Pumping)
Some PTW test criteria for diamond dosemeters
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I [nA
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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
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1485 1495 1505 1515 1525 1535 1545 1555 1565 1575 1585t [s]
I [nA
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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
0.000
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3030 3040 3050 3060 3070t [s]
I [nA
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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
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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