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.