Unterweger M P, Fitzgerald R
National Institute of Standards and Technology, Radioactivity Group, C114 Radiation Physics Building, 100 Bureau Drive Stop 8462, Gaithersburg, MD 20899-8462, USA.
Appl Radiat Isot. 2012 Sep;70(9):1892-3. doi: 10.1016/j.apradiso.2012.02.023. Epub 2012 Apr 4.
In January of 2010, it was discovered that the source holder used for calibrations in the NIST 4πγ ionization chamber (IC) has not been stable. The positioning ring that determines the height of the sample in the reentrant tube of the IC has slowly shifted during 35 years of use. This has led to a slow change in the calibration factors for the various radionuclides measured by this instrument. The changes are dependent on γ-ray energy and the time the IC was calibrated for a given radionuclide. A review of the historic data with regard to when the calibrations were done has enabled us to approximate the magnitude of the changes with time. This requires a number of assumptions, and corresponding uncertainty components, including whether the changes in height were gradual or in steps as will be shown in drawings of sample holder. For calibrations the changes in calibration factors have been most significant for low energy gamma emitters such as (133)Xe, (241)Am, (125)I and (85)Kr. The corrections to previous calibrations can be approximated and the results corrected with an increase in the overall uncertainty. At present we are recalibrating the IC based on new primary measurements of the radionuclides measured on the IC. Likewise we have been calibrating a new automated ionization-chamber system. A bigger problem is the significant number of half-life results NIST has published over the last 35 years that are based on IC measurements. The effect on half-life is largest for long-lived radionuclei, especially low-energy γ-ray emitters. This presentation will review our results and recommend changes in values and/or uncertainties. Any recommendation for withdrawal of any results will also be undertaken.
2010年1月，人们发现美国国家标准与技术研究院（NIST）4πγ电离室（IC）校准用的源支架不稳定。在35年的使用过程中，决定电离室凹腔管内样品高度的定位环逐渐发生了偏移。这导致该仪器测量的各种放射性核素的校准因子缓慢变化。这些变化取决于γ射线能量以及电离室针对给定放射性核素进行校准的时间。通过回顾校准时间的历史数据，我们得以大致估算出随时间变化的幅度。这需要一些假设以及相应的不确定度分量，包括高度变化是渐进的还是阶梯式的，这将在样品支架的示意图中展示。在校准方面，对于低能γ射线发射体，如（133）Xe、（241）Am、（125）I和（85）Kr，校准因子的变化最为显著。可以对先前校准进行近似校正，并在总体不确定度增加的情况下校正结果。目前，我们正在根据对电离室测量的放射性核素进行的新的一级测量对电离室重新校准。同样，我们也一直在校准一套新的自动电离室系统。一个更大的问题是，过去35年里NIST发表的大量半衰期结果是基于电离室测量的。对于长寿命放射性核素，尤其是低能γ射线发射体，对半衰期的影响最大。本报告将回顾我们的结果，并建议对数值和/或不确定度进行更改。还将对撤回任何结果的建议进行探讨。
