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Technical Note on Unexpected Bias in NIST 4π Ionization Chamber Measurements



Michael P. Unterweger, Ryan P. Fitzgerald


The NIST 4π pressurized ionization chamber “A” (PIC “A”) has been the mainstay for secondary calibrations of liquid and gaseous gamma-ray emitting sources for the last 40-45 years. It has also been the main instrument used to measure the half-lives of these and similar radionuclide sources. The ionization chamber is the prototype for the TPA Mk.II ionization chamber manufactured by 20th¬ Century Electronics, Ltd. (Centronic), Croydon, England. The chamber and measurement method (Fig. 1) was described in Unterweger et al. (1992) and Calhoun (1987). The basic analysis of the decay data and the chi squared fitting technique are described in Hoppes and Schima (1982). In January of 2010, it was discovered that the position of the source holder (Fig. 2) used for calibrations in the NIST 4π ionization chamber (PIC “A”) has not been constant. The positioning ring that determines the height of the sample in the reentrant tube of the PIC “A” has slowly shifted during 35 years of use. Since the chamber response is height dependent, this
Applied Radiation and Isotopes


halflife, ionization chamber calibration


Unterweger, M. and Fitzgerald, R. (2012), Technical Note on Unexpected Bias in NIST 4π Ionization Chamber Measurements, Applied Radiation and Isotopes (Accessed July 24, 2024)


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Created April 4, 2012, Updated February 20, 2018