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 IC self capacitance = 100 pF IC low as 1fA may be detected HV using this function C2 R2 R3 R1 Relay + C1 INPUT C6 C5 Low current source R4 RLY1 C3 C4 + - U1 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= MQ ND,w,Qo kQ,Qo corrected instrument reading at Q M Pion PT P Pelec Ppol M raw • calibration coefficient at Qo (C or rdg) Pelec a factor allowing for separate calibration of the integrtor here 1 beam quality factor 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 DC reference voltage source V+ V- Relay switching unit Device under test Reference capacitor DMM 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 E x ( I x I x ) I e 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 c I x E x / I x 1 c Ie E x / I e 1 Calibration uncertainty for current integrator can be expressed as u Ex c u I x c Ie u I e 2 Ix 2 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 I 100 fA 1 pA 10 pA 100 pA UB (I)( k=2) 13 aA 48 aA 420 aA 4.2 fA Only type B evaluation has been considered 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 I- 2 nA I+ 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 Ex UNCERTAINTY BUDGET Quantity Integrator the test Value under 2,004 nA Uncertainty (Type) 0,00802 nA (A) ci Uncertainty due to final resolution of reading 100 fA 28,9 fA (B) 1 Preset current on DC source 2 nA 0,001 nA (B) -1 Uncertainty integrator of 0,0081 nA UNCERTAINTY BUDGET OF THE CURRENT TO VOLTAGE CALIBRATION FOR THE 100 PA, 10 PA AND 1 PA Uncertainty component Voltage measurement 1 pA [ppm] 50 10 pA [ppm] 10 100 pA [ppm] 10 Resistor value 1350 190 70 1/f noise 1300 130 13 10 10 10 1900 230 75 3800 460 150 Current source Combined standard uncertainty Expanded uncertainty (k=2) 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, 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 lowcost electronics and equipment typically present in most electrical metrology calibration laboratory, where it could be efficiently employed.