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CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS

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### CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS

V. SpasićJokić, 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

IC

HV

IC self capacitance = 100 pF

+

Low current source

Current integrator

For current measurement

For charge 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 instruments

Good 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.

IAEA 398 solution

- Assumes user has a calibration factor for exposure ND for the ion chamber/ integrator combination in use
- But allows

Dw,Q=MQND,w,QokQ,Qo

corrected instrument reading at Q

calibrationcoefficientat Qo

beamqualityfactor

- 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 integrators

- Suitable 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 60731
- Calibration: using method of direct measurement

Simplified calibration setup

- standard high impedance DC source Keithley 6220,
- various standard resistors and capacitors and
- digital multi-meter HP 3450 B

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

Concept of uncertainty estimation

- Sensitivity coefficient is derived from expressions

Calibration uncertainty for current integrator can be expressed as

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 (2.10-5I + 10 aA)

Preliminary uncertainty assessment for the current

generated by the source

Only type B evaluation has been considered

2 nA

I-

I+

Estimation of type B uncertaintyASSUMPTIONS

- 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

Estimation of type B uncertainty

- 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

- Step 2: Calculation of the best estimated value and variance

Uncertainty budget of the current to voltage calibration for the 100 pA, 10 pA and 1 pA

Measurement capabilities with uncertainty budget

The expanded uncertainty U with the coverage factor k = 2, corresponding

to the 95% confidence level, is often used to represent the

overall uncertainty, which relates to the accuracy of the measurement of

the 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.

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