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Dosimeters, calibration and uncertainty

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Presentation on theme: "Dosimeters, calibration and uncertainty"— Presentation transcript:

1 Dosimeters, calibration and uncertainty
Radcal Corporation Paul Sunde Monrovia, CA


3 Modern Dose Sensor Diversity
Solid State Multi-sensors (dose, kV, time, hvl, filt) Ion Chambers (nGy – kGy 10keV – MeV) Solid State Dose only

4 Radcal’s Mfg. and Calibration Laboratory
Certified ISO 9001:2008 (TUV since 1995) IEC ISO/IEC 17025:2005 ANSI/NCSL Z All products CE marked Direct traceability to NIST (USA) – Air Kerma PTB (Germany) – Air Kerma Swedish National Laboratory – High Voltage Proficiency Tested – NIST & Secondary Labs

5 Dose Calibration Chain
National Laboratory Radcal Primary Ion Chamber Standards Working/Secondary Standards Std1 Std2 Std3 Std4 Std5 Std6

6 Proficiency Testing National Laboratory Transfer Standard
Secondary Lab - Lab calibrates transfer std. - Ship to Radcal for calibration - Radcal calibrates & issues report - Return to lab for re-calibration - Lab Issues proficiency report Radcal Cal Lab

7 Customer Traceability Chain
National Laboratory Radcal Certificate of Conformance Certified Calibration Customer

8 Radcal Certificate of Conformance
Motivation Verify equipment meets published specifications Preserve component interchangeability Process Test electronics (control unit) – verify performance and accuracy over full dynamic range Independently test sensors with reference standard electronics for accuracy and energy dependence Verify sensor & electronics combination, but do not adjust.

9 Air Kerma Uncertainty Budget
NIST & PTB National Standard ~1% Radcal (1s) Transfer measurements 0.50% Long-term stability 1.00% Short-term reproducibility Temperature: 0.25 ºC 0.08% Pressure: 1mB 0.10% Geometry & positioning 0.20% Charge: 0.5% Beam quality* 0.25% Quadrature Sum 1.5% Total Uncertainty Radcal @ 95% Confidence (2s) 2.9% (1)NIST and PTB uncertainties vary depending on the type of measurement. The value listed is the worst case with a 95% a confidence level (2s). *Radcal ion chambers have documented energy response functions of <5% over their specified range of use. For x-ray beams, hvl can be determined to < 5% uncertainty. The resulting error for ion chamber calibration is < 0.25%. For non-Radcal chambers, which may have up to 20% energy response, the associated uncertainty is <1%. Note: this influence quantity only applies to x-ray measurements and is not present in 60Co or 137Cs measurements.

10 TUR (test uncertainty ratio)
Note: desirable TUR is >4 Implies RMS <1.25%

11 Beam standardization Geometry – IEC “Radiation conditions for use in the determination of characteristics”. Collimator 1 Trans Chamber 1 Added Filt Collimator 2 Laser Beam Splitter Prisim HVL Filters. Collimator 3 Trans Chamber 2

12 Radcal Air Kerma Standards
3-terminal guarding < 5e-15 A leakage with 300 VDC bias Negligible stem effects < ±5% energy dependence over rated energy range 2nGy to >20 Gy Rates from nGy/s to >mGy/s Energies from 10 keV to 2 MV

13 Recent Standard Chamber Set

14 Diagnostic Energy Response (RQR & RQA)

15 Ion Chamber Response for low energy x-rays
American College of Radiology: ±1% mm Al hvl The 6M ion chamber was designed specifically for the mammographic energy range . It’s response characteristics have been demonstrated to be completely independent of spectra from all anode filter combinations.

16 Mammographic Energy Response Radcal 10X5-6M (DeWerd et al.)

17 Diode Energy Response Let’s take a look at how a bare diode responds to conventional Mo-Mo 30u beams. Compensation can dramatically improve this response.

18 Diode Energy Response Expanding the y-axis of the previous graph shows the detail of the compensated response.

19 Diode Energy Response Adding other Anode filter combinations shows the effect of these different spectra on the Mo-Mo compensation.

20 Diode Energy Response If we apply different correction factors for each anode-filter condition, we have a much better result.

21 Diode Energy Response – Auto Corrected via kV & hvl
Finally if we use additional sensors to determine kVp and filtration, we can have a very good result for dose diode energy response.

22 Equipment Tolerances IEC – “Dosimeters with ionization chambers and/or semiconductor detectors as used in x-ray diagnostic imaging" Performance Characteristics Limit of Variation Air Kerma Reproducibility 1% Resolution < Stabilization 2% Leakage currents (<1%/min) --- Stability Accumulated dose stability RMS 3%

23 Equipment Tolerances IEC – “Dosimeters with ionization chambers and/or semiconductor detectors as used in x-ray diagnostic imaging" Influence Quantities Limit of Variation Radiation Quality 5% Air Kerma Rate 2% Incidence angle (± 5°) 3% Operating Voltage Air Pressure Temperature and Humidity EMC Field size RMS 9%

24 Equipment Tolerances IEC – “Dosimeters with ionization chambers and/or semiconductor detectors as used in x-ray diagnostic imaging"

25 Summary Select dose sensors to match your requirements
Acceptable uncertainty - QA checks vs. acceptance testing Energy range – Mammography vs. conventional, Geometric response - fluoro backscatter, CT & tomo Energy response – acceptable uncertainty Dynamic Range - scatter & leakage vs. pulsed fluoro

26 Summary Understand the sensor & associated electronics
Trigger thresholds – minimum dose rate Maximum dose rate – pulsed fluoro Opaque – interfere with AEC Linearity over the range of application Use of automatic vs. manual temp & pressure corrections Time to come to thermal equilibrium

27 Thank You! Paul Sunde Radcal Corporation Monrovia, California USA
ISO/IEC 17025:2005 ANSI/NCSL Z Accredited ISO 9001:2008 Certified Paul Sunde Radcal Corporation Monrovia, California USA

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