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Updated: May 23, 2026
07:31
Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator
Published on: May 9, 2014
Unexpected bias in NIST 4πγ ionization chamber measurements
1National Institute of Standards and Technology, Radioactivity Group, C114 Radiation Physics Building, 100 Bureau Drive Stop 8462, Gaithersburg, MD 20899-8462, USA. unterweg@nist.gov
Summary
A NIST ionization chamber (IC) calibration issue was found due to a shifting sample holder, affecting radionuclide calibration factors. This impacts past half-life measurements, especially for low-energy gamma emitters, requiring corrections and uncertainty adjustments.
Area of Science:
- Metrology
- Nuclear Physics
- Radiochemistry
Background:
- A long-term instability in the NIST 4πγ ionization chamber (IC) source holder was identified.
- A positioning ring shift over 35 years has altered sample height, impacting calibration accuracy.
Purpose of the Study:
- To assess the impact of the source holder instability on past radionuclide calibrations.
- To correct historical calibration factors and published half-life data.
- To re-evaluate uncertainties associated with NIST measurements.
Main Methods:
- Review of historical calibration data and analysis of positional changes in the ionization chamber source holder.
- Approximation of calibration factor changes based on gamma-ray energy and calibration time.
- New primary measurements for recalibrating the ionization chamber and an automated system.
Main Results:
- Calibration factors for radionuclides, particularly low-energy gamma emitters (e.g., Xe-133, Am-241), have been significantly affected.
- Historical half-life results, especially for long-lived, low-energy gamma-emitting radionuclides, are impacted.
- Corrections to previous calibrations can be approximated, but with increased overall uncertainty.
Conclusions:
- The NIST ionization chamber source holder instability necessitates corrections to past calibration factors and published half-life data.
- Adjustments to values and/or uncertainties are recommended for affected measurements.
- Consideration will be given to withdrawing any NIST results found to be significantly compromised.
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