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Improved current measurement on ionization chambers – a milestone for activity and half-life determinations

23.12.2020

The use of ionization chambers as secondary standards in radionuclide metrology is essential for the realization and dissemination of the becquerel unit. Ionization chambers are distinguished by excellent long‑term stability and – if carefully characterized – are able to cover an activity range spanning several orders of magnitude, in particular for gamma‑emitting radionuclides. Moreover, given their intrinsically good linearity, ionization chambers have long been used for half‑life measurements. In activity determinations, one factor limiting the uncertainty levels that can be attained is often found in the measurement of the ionization current. With commercial systems it is not uncommon to see nonlinearities in the percent range.

At Department 6.1 "Radioactivity," individual ionization chambers have been in nearly continuous operation for some 50 years now. They are primarily employed as secondary standards for relative activity determinations. The advantage of doing measurements with secondary standards is that the primary standard measurements which are required for initial calibration and involve a great deal of time and technical resources are only rarely necessary. For the initial calibration of an ionization chamber, the results of absolute measurements are retained in the form of calibration factors, which can then be used for carrying out relative measurements for many years or even decades afterwards, provided the long‑term stability of the ionization chamber systems is sufficient. This stability can be validated experimentally, for instance by using reference sources of long‑lived radionuclides.

In the past, two different current measurement systems have been deployed, each of which exhibit specific disadvantageous properties. The first system is based on measuring the current by charging a capacitor and then determining the ionization current from the voltage‑time relation. In this system, annual fluctuations of up to 0.4 % induced by environmental influences have been observed. This system was later replaced by a commercial ammeter, which was more stable with respect to environmental parameters but exhibited pronounced jumps when measurement range changes became necessary, so that no reduction of the measurement uncertainties could be achieved. In many cases, the act of measuring the current represented the dominant component of uncertainty.

The new ULCA (ultrastable low-noise current amplifier) system was developed at PTB by Departments 2.6 "Electrical Quantum Metrology" and 7.6 "Cyrosensors" for the purpose of measuring very small currents [1]. As part of a collaboration project, the system is currently being evaluated at Department 6.1. Results to date have shown that by combining the chambers with the ULCA device it is possible to reduce the current measurement uncertainties to a level where they no longer dominate the overall uncertainty. Depending on the radionuclide and the ionization chamber involved, the new system can improve uncertainty by approximately one order of magnitude. For this reason, PTB will soon be deploying the new ULCA system to measure currents on more of its secondary standard measuring facilities with ionization chambers. This will then allow calibration factors gained from new absolute measurements to be used for larger measurement ranges and longer periods of time, and it will significantly improve the quality of activity determination using secondary methods. The excellent linearity of the ULCA system can also serve to reduce uncertainties for half-life measurements and in this way make an important contribution to more accurately determining basic radionuclide data.

Fig. 1: Normalized and decay-corrected ionization current of a radium standard as measured by different electrometers. Until 1999: capacitor method, 1999-2019: commercial electrometer, since 2020: ULCA.

Literature

[1] Drung, D., Krause, C., Becker, U., Scherer, H., Ahlers, F. J.: Ultrastable low-noise current amplifier: A novel device for measuring small electric currents with high accuracy, Review of Scientific Instruments 86, 024703 (2015); https://doi.org/10.1063/1.4907358

Contact:

Opens local program for sending emailO. Nähle, Department 6.1, Working Group 6.11