Dosimeters, calibration and uncertainty

Radcal CorporationPaul SundeMonrovia, CA

Modern Dose Sensor Diversity

Ion Chambers (nGy – kGy 10keV – MeV)

Solid State Multi-sensors (dose, kV, time, hvl, filt)

Solid State Dose only

Radcal’s Mfg. and Calibration Laboratory

Certified ISO 9001:2008 (TUV since 1995) IEC ISO/IEC 17025:2005 ANSI/NCSL Z540-1-1994 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

Dose Calibration Chain

National Laboratory

Radcal Primary Ion Chamber Standards

Working/Secondary Standards

Std1 Std2 Std3 Std4 Std5 Std6

Proficiency Testing

National LaboratoryTransfer Standard Secondary Lab

RadcalCal Lab

- Lab calibrates transfer std.

- Ship to Radcal for calibration

- Radcal calibrates & issues report

- Return to lab for re-calibration

- Lab Issues proficiency report

Customer Traceability Chain

National Laboratory

Radcal

Customer

Certificate of Conformance

Certified Calibration

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.

Air Kerma Uncertainty BudgetNIST & PTB

National Standard ~1%

Radcal (1s)

Transfer measurements 0.50%

Long-term stability 1.00%

Short-term reproducibility 0.50%

Temperature: 0.25 ºC 0.08%

Pressure: 1mB 0.10%

Geometry & positioning 0.20%

Charge: 0.5% 0.50%

Beam quality* 0.25%

Quadrature Sum 1.5%

Total Uncertainty Radcal

@ 95% Confidence (2s) 2.9%

TUR (test uncertainty ratio)

Note: desirable TUR is >4Implies RMS <1.25%

Typical Dose measurmentDisplay resolution 0.10%Calibration uncertainty of standard (1sigma) 1.50%Reproducibility of reference readings 0.10%Positioning uncertainty (80 cm ±0.1) 0.25%Temperature (±0.5 °C) 0.34%Pressure (±.2 kPa) 0.40%

RMS 1.61%

Published Uncertainty 5%TUR 3.1

Beam standardization

Geometry – IEC-61267 “Radiation conditions for use in the determination of characteristics”.

Collimator 1

Trans Chamber 1

Added FiltCollimator 2

HVL Filters.

Collimator 3

Trans Chamber 2

Laser

Beam Splitter

Prisim

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

Recent Standard Chamber Set

Diagnostic Energy Response (RQR & RQA)

0.90

0.95

1.00

1.05

1.10

0.01 0.10 1.00 10.00

Re

spo

nse

hvl (mm Al)

RC6M(PTB Calibration)

Orange: W-anode Blue: Mo-anode Pink Area = Radcal Spec, Light Orange=Typical Mammo Energies

10 40

7.5 keV

Ion Chamber Response for low energy x-rays

American College of Radiology: ±1% 0.2-0.6 mm Al hvl

Mammographic Energy ResponseRadcal 10X5-6M (DeWerd et al.)

±2%

Diode Energy Response

Diode Mo-Mo 30uEnergy Response

0.01.02.03.04.05.06.07.0

20 25 30 35 40 45 50

Tube Voltage (kV)

No

rma

lize

d R

es

po

ns

e

Uncompensated Compensated

Diode Mo-Mo 30uEnergy Response

0.80

0.90

1.00

1.10

1.20

20 25 30 35 40 45 50

Tube Voltage (kV)

No

rma

lize

d R

es

po

ns

e

Uncompensated Compensated

Diode Energy Response

Diode Energy Response

0.80

0.90

1.00

1.10

1.20

1.30

1.40

20 22 24 26 28 30 32 34 36 38 40

Re

spo

nse

Tube Voltage (kVp)

Compensated Diode ResponseUnknown Anode-Filter

Mo/Mo

Mo/Rh

Rh/Rh

W/Ag

W/Rh

Diode Energy Response

0.80

0.90

1.00

1.10

1.20

1.30

1.40

20 22 24 26 28 30 32 34 36 38 40

Re

spo

nse

Tube Voltage (kVp)

Compensated Diode ResponseKnown Anode-Filter

Mo/Mo

Mo/Rh

Rh/Rh

W/Ag

W/Rh

Diode Energy Response – Auto Corrected via kV & hvl

0.90

0.95

1.00

1.05

1.10

20 22 24 26 28 30 32 34 36 38 40

Re

sp

on

se

Tube Voltage (kVp)

Diode Energy ResponseKV & Filtration Auto Corrected

Mo/Mo

Mo/Rh

W/Ag

Equipment TolerancesIEC 61674 – “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 < 1%

Stabilization 2%

Leakage currents (<1%/min) ---

Stability 2%

Accumulated dose stability 1%

RMS 3%

Equipment TolerancesIEC 61674 – “Dosimeters with ionization chambers and/or semiconductor detectors as used in x-ray diagnostic imaging"

Influence Quantities Limit of VariationRadiation Quality 5%Air Kerma Rate 2%Incidence angle (± 5°) 3%Operating Voltage 2%Air Pressure 2%Temperature and Humidity 3%EMC 5%Field size 3%

RMS 9%

Equipment TolerancesIEC 61674 – “Dosimeters with ionization chambers and/or semiconductor detectors as used in x-ray diagnostic imaging"

Limit of VariationInfluence Quantities 9%

Performance Characteristics 3%

Total RMS 10%

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

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

Paul SundeRadcal CorporationMonrovia, California USAwww.radcal.com

Thank You!

ISO/IEC 17025:2005 ANSI/NCSL Z540-1-1994

Accredited

ISO 9001:2008 Certified