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Highly sensitive coulometer for the measurement of ionization chamber currents

18.07.2014

Currents from ionization chambers lie in the range from less than a femtoampere (10-15 A = 1 fA) up to a few nanoamperes (10-9 A) and the most reliable way of determining them is to measure the charge. Such an instrument capable of measuring the charge with several measuring ranges and an input bias current of less than 0.3 fA is not commercially available and has therefore now been developed. For quality assurance reasons, the charge-measuring instrument was compared with PTB's current standard. This comparison has shown that a current of 40 fA can be determined with a standard uncertainty of 0.1 fA.

One of the most important tasks in the field of dosimetry of ionizing radiation is the measurement of currents from ionization chambers. These currents lie in the range from less than a femtoampere (10-15 A) up to a few nanoamperes (10-9 A = 1 nA). Experience has shown that the most reliable way of determining currents in this range is to measure the charge by means of an operational amplifier used as an integrator. Since such a measuring instrument for charge with several measuring ranges is not commercially available, it has now been developed [1]. An important uncertainty component of such a measurement is the input bias current of the amplifier used. Since this current strongly depends on the temperature of the amplifier and decreases along with a decreasing temperature, the amplifier of the charge measuring instrument described here is cooled down to -10 °C. The remaining input bias current is smaller than 0.3 fA. Since it is not possible to correct for the input bias current during the measurement, this bias current has to be determined before and after the measurement to be able to determine the actual measured current. For quality assurance reasons, the charge-measuring instrument was compared with PTB's current standard.

Figure: Ratio of the measured current I to the standard current IN

 

The measurements were carried out with currents of 40 fA, 200 fA and 2000 fA. The figure shows the quotient from the mean measured current I, corrected for the input bias current, and the current from the standard IN. The error bars represent the relative standard error of the measured current. In the case of the 40 fA standard current, this standard error corresponds to approx. 600 electrons per second. All in all, the comparison shows that the charge-measuring instrument described here allows currents in the range of a few femtoamperes to be measured with an absolute accuracy of a few parts per thousand.

Literature:

  1. G. Buchholz:
    Hochempfindlicher Ladungsmesser
    PTB Bericht EW-15, ISBN 978-3-95606-048-9, 2013