Calibration of current integrators used with ionization chambers
Download
1 / 28

CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS - PowerPoint PPT Presentation


  • 113 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' CALIBRATION OF CURRENT INTEGRATORS USED WITH IONIZATION CHAMBERS' - orinda


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Calibration of current integrators used with ionization chambers

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
Specific aims CHAMBERS

  • 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
Dosiemeters CHAMBERS based on Ionization chambers

Reading device

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


Readout consideration
Readout consideration CHAMBERS

  • 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
Current integrator CHAMBERS

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
Calibration: which solution is the ‘best’ CHAMBERS

  • 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
IAEA 398 solution CHAMBERS

  • 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
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
Calibration of current integrators testing and calibration laboratories

  • 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
Simplified calibration setup testing and calibration laboratories

  • standard high impedance DC source Keithley 6220,

  • various standard resistors and capacitors and

  • digital multi-meter HP 3450 B


Accredited metrological laboratory ftn uns
Accredited metrological laboratory FTN UNS testing and calibration laboratories


Concept of uncertainty estimation
Concept of uncertainty estimation testing and calibration laboratories

  • 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 estimation1
Concept of uncertainty estimation testing and calibration laboratories

  • Sensitivity coefficient is derived from expressions

Calibration uncertainty for current integrator can be expressed as


Results
results testing and calibration laboratories

  • 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
The uncertainty of the current source itself testing and calibration laboratories

  • 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 testing and calibration laboratories

generated by the source

Only type B evaluation has been considered


Estimation of type b uncertainty

2 testing and calibration laboratoriesnA

I-

I+

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


Estimation of type b uncertainty1
Estimation of type B uncertainty testing and calibration laboratories

  • 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 uncertainty2
Estimation of type B uncertainty testing and calibration laboratories

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


Uncertainty budget
Uncertainty budget testing and calibration laboratories



Measurement capabilities with uncertainty budget
Measurement capabilities with uncertainty budget the 100

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
conclusion the 100

  • 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.


ad