Comparison: Co-60 Absorbed Dose for High-Dose Dosimetry

Summary

NIST participated in an international comparison of standards for absorbed dose to water for ⁶⁰Co gamma radiation at radiation-processing dose levels organized by the Bureau International des Poids et Mesures (BIPM), CCRI(I)-S(2). At the request of NIST, the comparison was preceded by a NIST-led comparison to demonstrate NIST-recommended modifications to the BIPM protocol.

Description

The NIST comparison demonstrated the need to know the participant's calibration scheme and dose rates, as well as the inclusion of an additional dose level (1 kGy, in addition to 5 kGy, 15 kGy, and 30 kGy).  The rationale for modifying the protocol was based on the recent NIST characterization of a rather obscure and previously unknown dose rate effect in alanine.  The effect was such that its greatest impact was on National Measurement Institute (NMI) metrology.  The discovery of this effect and its contribution to international comparison data is a demonstration of the quality of NIST measurement capabilities and associated facilities, and is a direct result of the quality system being implemented at NIST in the last decade. 

Eight laboratories offering high-dose irradiation services took part in the comparison; the Czech Metrology Institute Inspectorate for Ionizing Radiation (CMI-IIR, Czech Republic), the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti (ENEA-INMRI, Italy), the Laboratoire National Henri Becquerel (LNE-LNHB, France), the National Institute of Metrology (NIM, China), the National Institute of Standards and Technology (NIST, USA), the National Physical Laboratory (NPL, UK), the High Dose Reference Laboratory of the Danish Technical University (Risø-HDRL, Denmark) and the Institute for Physical-Technical and Radiotechnical Measurements, Rostekhregulirovaniye of Russia (VNIIFTRI, Russian Federation). All laboratories hold primary standards with the exception of the CMI-IIR and the Risø-HDRL that hold secondary standards traceable to the International Atomic Energy Agency (IAEA) and the National Physical Laboratory (NPL), respectively. In addition, the BIPM, although it does not offer a high-dose service, took part at the lowest dose level (1 kGy) to provide a direct link to the international reference for absorbed dose to water in ⁶⁰Co. Two transfer dosimeters were used for the comparison; the alanine/ESR dosimetry system of the NIST and that of the NPL.

These eight national standards for absorbed dose to water in ⁶⁰Co gamma radiation at the dose levels used in radiation processing were compared over the range from 1 kGy to 30 kGy using the alanine dosimeters of the NIST and the NPL as the transfer dosimeters

It was found that the national standards are in general agreement within the standard uncertainties, which are in the range from 1 to 2 parts in 10². The dose-rate effect was observed at the highest doses for the laboratories that used low-rate gamma sources. A manuscript that details the comparison and its analysis has been published in Metrologia.