The IAEA/WHO Network, through Secondary Standards Dosimetry Laboratories (SSDLs) designated by the Member States, provides a direct linkage of national dosimetry standards to the international measurement system (SI, for Système International). Through the proper calibration of field instruments by the SSDLs, these measurements are traceable to the Primary Standards Dosimetry Laboratories (PSDLs) and the Bureau International des Poids et Mesures (BIPM). The Network has proven to be of value in improving national capabilities for instrument calibration and the level of awareness of the need for better accuracy and traceability . To ensure that the services provided by SSDL members to end users follow internationally accepted standards, the IAEA has set up a comparison programme using ionization chambers. In this way, the laboratories verify the integrity of their national standards and the procedures used for the transfer of the standards to the end users . During the implementation of the comparison programme, the IAEA identified important discrepancies in the way SSDLs report their calibration uncertainty. Although the general Guidance on the expression of Uncertainty in Measurement (GUM) was published by ISO in 1995 , no specific publication deals with the implementation of the ISO guidance in the calibration of dosimeters. The need to prepare a guidance document for SSDL members on this topic was recognized by the IAEA and supported by the IAEA Standing Advisory Group, the Scientific Committee of the IAEA/WHO SSDL Network organized in 2004 .
A guidance document on the assessment of measurement uncertainty will also benefit SSDLs that signed the Mutual Recognition Arrangement (MRA) of National Measurement Standards and of the Calibration and Measurement certificates Issued by the National Metrology Institutes. The IAEA signed the MRA in October 1999 and has, since then, contributed to strengthening the SSDL capabilities in this field. The signing by the IAEA of the MRA imposes stricter demands on its dosimetry comparisons and may require a modification of the criteria of acceptability of the level of performance achieved by SSDLs in these comparisons.
A common procedure for uncertainty assessment among all SSDL Network members would greatly facilitate the review of comparison results by the IAEA and other regional or international institutions.
The purpose of this publication is to provide guidance to SSDLs on how to assess and report measurement uncertainty related to their calibration services in a way that is consistent with the GUM. It includes a review of the calibration procedures and model equations used by SSDLs in Section 2, a tutorial part on how to assess the various uncertainty components in Section 3, and two practical examples of typical calibrations in 60Co gamma ray beams. The practical examples, linked to the concepts described in Sections 2 and 3, are given to help the user in preparing the uncertainty budget. Although the scope of this publication is limited to a model equation used in 60Co gamma beam for radiotherapy dosimetry, the concept can be extended to cover X ray dosimetry for radiotherapy and radiation protection dosimetry. For the calibration of instruments used in diagnostic radiology, the guidance given in IAEA technical Reports Series No. 457  should be followed.