CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS
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Transcript of CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS
CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS
V. Spasić Jokić, I. Župunski, B. Vujičić, Z. Mitrović, V. Vujičić, Lj.Župunski
Faculty of Technical Sciences, University of Novi Sad
SPECIFIC AIMS Purpose : trace the harmonization of
uncertainty evaluation within accreditation framework
Uncertainty estimation in accordance with the GUM but it is necessary to establish the method more suitable for the measurements in calibration laboratories
Good metrology practice : evaluation of Type B uncertainty is particularly important and requires proper use of the available information is based on experience and skill.
DOSIEMETERS BASED ON IONIZATION CHAMBERS
Reading device
• The typical order of magnitude of ion currents: (10-6 to 10-14 ) A
READOUT CONSIDERATION Voltmeter function: The input resistance of an
integrator is greater than 100 TΩ , the input offset current is less than 3fA.
Ammeter function: can detect currents as low as 1fA
Coulombmeter Function: Current integration and measurement as low as 10fC, has low voltage burden, (less than100μV).Currents as
low as 1fA may be detected using this function
Low current source
+
Ramp-+
C6C5
INPUT C1
C2
Relay
RLY1
C3 C4
R1
R2 R3
R4
U1
HVIC IC self capacitance = 100 pF
CURRENT INTEGRATOR
For charge measurement
For current measurement
• Capacitor in the feedback: (10-5 - 10-11) F (calibrated within 0.1%) •Conventional carbon resistors are available in values up to 108
CALIBRATION: WHICH SOLUTION IS THE ‘BEST’ • Ionization chambers are used together with current integrators and they should be calibrated together•Chamber is standard instrument•Integrator is standard instrument•Calibrated together as the same rank instrumentsGood reason for separate calibrations is that, one integrator is used with a number of chambers, so it would be inconvenient to calibrate it with every chamber.
• Assumes user has a calibration factor for exposure ND for the ion chamber/ integrator combination in use
But allows
IAEA 398 SOLUTION
Dw,Q= MQ ND,w,Qo kQ,Qo
beamqualityfactor
calibrationcoefficientat Qo
corrected instrument reading at Q
ion TP elec pol raw (C or rdg)M P P P P M• Pelec a factor allowing for separate calibration of the
integrtor - here 1
Calibration method for a current-measuring feedback-controlled
integrator
The output impedance of the current source must be large compared to R.
Verification of dosimeters used in health care and radiation protection is a legal requirement in Serbia
Verification is a subject of accreditation according to SRPS/ISO 17025
ISO 17025: GENERAL REQUIREMENTS FOR THE COMPETENCE OF TESTING AND CALIBRATION LABORATORIES
EUROMET Project n. 830, “Comparison of small current sources”
Laboratory for metrology at the Faculty of Technical Sciences, University of Novi Sad is accredited in terms of SRPS/ISO 17025 for verification of current integrators
CALIBRATION OF CURRENT INTEGRATORSSuitable direct current source that
simulate the output from ionizing radiation detectors.
Range: 100 fA - 100 mA (uncertainty better than 0.05 %), depending of chamber type
IEC 60731Calibration: using method of direct
measurement
SIMPLIFIED CALIBRATION SETUP
DMM
DC reference voltage source
Reference capacitor
Relay switching
unit
Device under test
V+V-
Ground
Guard
GPIB
- standard high impedance DC source Keithley 6220, - various standard resistors and capacitors and - digital multi-meter HP 3450 B
ACCREDITED METROLOGICAL LABORATORY FTN UNS
CONCEPT OF UNCERTAINTY ESTIMATION
Model function for uncertainty estimation in the calibration procedure for current integrator can be expressed as
Ix - current read by integrator under the test; δIx – error of reading obtained by integrator under
the test due to final resolution; Ie – preset current (on current source) derived from
the declaration of the manufacturer or calibration certificate
exxx IIIE )(
CONCEPT OF UNCERTAINTY ESTIMATION Sensitivity coefficient is derived from
expressions
1/ xxI IEcx
1/ exIe IEc
Calibration uncertainty for current integrator can be expressed as
2 2
x Ix x Ie eu E c u I c u I
RESULTS The main part of each calibration procedure
is uncertainty estimation and design of uncertainty budget
Uncertainty budget obtined during calibration procedure of current integrator type NP 2000 manufactured in OMH, Hungary
Preset value: 2 nA Rectangular probability distribution was assumed
THE UNCERTAINTY OF THE CURRENT SOURCE ITSELF Comes from several contributions: Capacitance calibration (5 ppm) Temperature coefficient (4 ppm/K) ac-dc difference Voltage reading (35 ppm) Triggering timing (1 ppm) Leak current compensation (2.10-5 I + 10 aA)
Preliminary uncertainty assessment for the current generated by the source
Only type B evaluation has been considered
I UB (I)( k=2)100 fA 13 aA1 pA 48 aA10 pA 420 aA100 pA 4.2 fA
ESTIMATION OF TYPE B UNCERTAINTY
ASSUMPTIONS I = 2 nA Lower and Upper limit values: (I- =I – Δ, I+ =I+Δ) Rectangular distribution: there is 100 % probability
that the true value is found in the interval
2 nAI- I+
Step 1. Probability density p(x) for the distribution of
current values as p(x)=C for I- Δ x I+Δ p(x)= 0 in all other cases
ESTIMATION OF TYPE B UNCERTAINTY
ESTIMATION OF TYPE B UNCERTAINTY Step 2: Calculation of the best
estimated value and variance
UNCERTAINTY BUDGET
Quantity ValueUncertainty
(Type) ci
Integrator under the test 2,004 nA
0,00802 nA (A)
Uncertainty due to final resolution of reading
100 fA28,9 fA
(B) 1
Preset current on DC source 2 nA 0,001 nA
(B)-1
Uncertainty of integrator 0,0081 nA
xE
UNCERTAINTY BUDGET OF THE CURRENT TO VOLTAGE CALIBRATION FOR THE 100 PA, 10 PA AND 1 PA
Uncertainty component 1 pA [ppm]
10 pA [ppm]
100 pA [ppm]
Voltage measurement 50 10 10
Resistor value 1350 190 70
1/f noise 1300 130 13
Current source 10 10 10
Combined standard uncertainty
1900 230 75
Expanded uncertainty (k=2) 3800 460 150
MEASUREMENT CAPABILITIES WITH UNCERTAINTY BUDGET
I dV/dt Reference capacitor
u95 source u95 integrator
calibration
100 fA 10 mV/s 1 pF 400 μA/A 2 %
100 pA 100 mV/s 1 pF 100 μA/A 1 mA/A
1 pA 100 mV/s 10 pF 20 μA/A 500 μA/A
10 pA 100 mV/s 100 pF 18 μA/A 90 μA/A
100 pA 100 mV/s 1 nF 10 μA/A 70 μA/A
The expanded uncertainty U with the coverage factor k = 2, correspondingto the 95% confidence level, is often used to represent theoverall uncertainty, which relates to the accuracy of the measurement ofthe quantity Q.
CONCLUSION The current uncertainty permits the
calibration of even the most accurate commercial meters present on the market.
The source is simple, portable and based on low-cost electronics and equipment typically present in most electrical metrology calibration laboratory, where it could be efficiently employed.