Absolute activity measurement of 85 Kr with proportional counters

Transcription

1 Absolute activity measurement of 85 Kr with proportional counters J.Plagnard, M.Rosenzweig Laboratoire National Henri Becquerel - France -

2 System used for the gas measurement at LNHB Proportional counters HV & Signal counters Circulator P, T sensors Pump Amplifiers & Signal analysers ADC 2

3 Synoptic of the system 2364 cm cm cm 3 Sampling system 1080 cm 3 Measurement system with 3 proportional counters 4701 cm 3 3

4 Geometry of the differential proportional counters Cylinder diameter : 30 mm lenghts : short : 240 mm medium : 315 mm long : 390 mm Wire material : tungsten with a thin layer of gold Wire diameter : 16 µm Gas used : pure propane Pressure : 30 kpa High voltage : volts 4

5 Synoptic of the electronic system High voltage reference PA Amplifier Signal analyser Amplifier Signal analyser Amplifier Signal analyser PA corrected corrected corrected PA counting Gate ADC counting Gate ADC counting Gate ADC gate gate gate N N medium NF long N short l NF m NF s NF COMPUTER Extending dead time circuit correction = NF NFl 5

6 Method for activity measurement (example for 85 Kr) Subtraction of the counting results 2 by 2 : N long N short N long N medium N medium -N short N l-s N l-m N m-s Only 2 counters are necessary to make a measurement, the third counter is used to detect any discrepancy Short Compensation for non-uniform detection of ionising events which take place near to the ends of the counters Diff Functioning defaults of counters Long - short Long Counts Flat spectrum like theoretical response Channel 6

7 85 Kr activity measurement Activity determination & correction factors due to volumes used during the gas preparation procedure l-s = cm 3 ±0.21 cm 3 A tot = N l-s (or N l-m or N m-s ) V l-s (or V l-m or V m-s ) Determination of the volumic activity inside the system l-m = cm 3 ±0.23 cm 3 m-s = cm 3 ±0.19 cm 3 X Total volume 4701 cm 3 ±6 cm 3 X ( ) n Dilution volume Reference volume Number of dilution Correction due to the dilution operations 2364 cm 3 ±5 cm cm 3 ± 0.2 cm 3 X Package volume Reference volume Correction due to the packaging of the reference volume cm 3 ± 3.1 cm 3 7

8 Main corrections Counts missing due to the low level thresold Method : - Energy calibration of the detection threshold with an 37 Ar source, using the 218 ev L X- ray Auger. Counts determination of the count losses due to the low level thresold determination of the average level Spectrum long-short - Extrapolation to 0 energy Correction : thresold level Channel Counts missing due to the non ionising β particles near the cathode Correction :

9 Result of the 85 Kr activity measurement Budget of uncertainties : Statistic (L-S) : 0.3 % Correction due to the extrapolation to 0 : 0.2 % Correction due to non ionising β particles : 0.1 % Dilution volume : 0.3 % Reference volume : 0.2 % Preparation volume : 0.3 % Total volume : 0.1 % Quadratic sum : 0.6 % (k=1) Container volume : 0.2 % Remark : The result was obtained from the difference between the long and short counters 9

10 Correction of the non ionising beta particle (explanation ) It can be demonstrated : Energy calibration with a 37 Ar source For 37 Ar : Maximum amplitude correspond to the creation of n = 7.01 n electrons Counts maximum 37 Ar 218 ev L X-ray Auger For 85 Kr : Flat spectrum means that the number of electrons created is constant N1 N2 85 Kr flat spectrum P(n) = constant = N(0) n n 3n 5n 7n 9n Channel Average number of electron created by a β particule with N(0) = fraction of non ionising β particle and n = average number of electrons created by a β particle It also can be show : N1 + N2 N T N(0) = = 7.01 x n x P(n) N1 + N2 N T x

11 Correction of the non ionising beta particle (explanation) N(0) = N1 + N2 N T x The back scattering of the β particle on the cathode are not taken into account The Rubidium ions are not taken into account 85 Kr Rb + β + ν Estimation of the back scattering : 40 % (this proportion is ionising on the counters) 79.2 % of the Rb atom has a positive charge (not detected) and 20.8 % of the Rb atom lose one or several electrons. They will be detected on the counters Consequently, the fraction of non ionising β particle is : N(0) = N1 + N2 N T x x 0.6 x