CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS

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CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS

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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

- 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.

Reading device

- The typical order of magnitude of ion currents: (10-6 to 10-14 ) A

- 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

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

- 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.

- 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

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

- 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

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

- 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

- Sensitivity coefficient is derived from expressions

Calibration uncertainty for current integrator can be expressed as

- 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

- 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+

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

- 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

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

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.

- 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.