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Improved current measurements for ionization chambers using the ULCA: A benchmark experiment to determine the F-18 half-life
Ole Nähle1), Dietmar Drung 1), Karsten Kossert 1), Hansjörg Scherer1), Marcell Péter Takács 1)
1) Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany
Corresponding author’s e-mail: ole.j.naehle@ptb.de
Ionization chambers play an important role in radionuclide metrology and are used for secondary activity standardization as well as for half-life determinations of a number of radionuclides. In both cases, the actual measurand is the ionization current which is often determined by means of electrometers with insufficient accuracy. In particular when considering a wide current range over several orders of magnitude (typically about 5 nA down to a few 10 fA) most electrometers suffer from range switches and/or significant non-linearities that may even be influenced by the preceding measurements. Thus, when using such devices, corrections must be applied, and additional uncertainty components must be taken into account when determining activities or half-lives. These effects can only be studied by measuring short half-lives and will harm the determination of long half-lives in an unpredictable way.
In this paper we present results of a benchmark experiment using a modified experimental setup that makes use of the ultrastable low-noise current amplifier (ULCA) [1, 2]. The voltage readout of this device was measured by means of a Keysight 3548A voltmeter. During the installation, great emphasis was placed on excellent shielding of the cables and connectors and a dedicated data acquisition software was developed. All test measurements so far underline a significant improvement in linearity of the instrument readings. Moreover, the readings do not show any disturbing unsteadiness that was previously found as a consequence of range switches.
As an important benchmark experiment, we present here the determination of the half-life of 18F. To this end, a 18F sample was measured for 25 half-lives with unchanged sample position until background current was reached. The analysis yields a half-life (preliminary result: 109.711 min) that is shorter than current values from nuclear data evaluations. This somewhat shorter half-life is confirmed by independent measurements using liquid scintillation counting systems to apply the triple-to-double coincidence ratio (TDCR) method. In this case, valuable information on the sample/counter stability can be derived from triple and double coincidence counting rates.
Details on the analyses and uncertainty assessments will be presented.
References
[1] Drung, D., Götz, M., Pesel, E., Scherer, H., 2015a. Improving the Traceable Measurement and Generation of Small Direct Currents. IEEE Trans. Instrum. Meas. 64, 3021–3030.
[2] Drung, D., Krause, C., Becker, U., Scherer, H., Ahlers, F.J., 2015b. Ultrastable low-noise current amplifier: a novel device for measuring small electric currents with high accuracy. Rev. Sci. Instrum. 86, 024703.