Radionuclide neutron sources are of fundamental importance to neutron metrology. They are the reference standards which allow quantities such as neutron fluence and neutron dose equivalent to be realized in laboratories around the world. In order to do this it is essential that the sources themselves are well characterized in terms of neutron emission rate, emission anisotropy and spectral fluence ΦE of emitted neutrons. A summary of the types of radionuclide neutron sources is given followed by detailed descriptions of the primary standardization techniques in use today, focusing on the manganese bath technique. Methods for determining the anisotropy of emission are discussed, followed by a brief description of the spectral fluence of the different source types. Finally, an overview of international comparison exercises in this area is included.

Section III (neutron measurements) of the Comité Consultatif des Rayonnements Ionisants, CCRI, conducted a key comparison of primary measurements of the neutron emission rate of an 241 Am-Be(α,n) radionuclide source. A single 241 Am-Be(α,n) source was circulated to all the participants between 1999 and 2005. Eight laboratories participated-the CIAE (China), CMI (Czech Republic), KRISS (Republic of Korea), LNMRI (Brazil), LNE-LNHB (France), NIST (USA), NPL (UK), and the VNIIM (Russian Federation)-with the NPL making their measurements at the start and repeating them near the end of the exercise to verify the stability of the source. Each laboratory reported the emission rate into 4π sr together with a detailed uncertainty budget. All participants used the manganese bath technique, with the VNIIM also making measurements using an associated particle technique. The CMI, KRISS, VNIIM, and later the NPL, also measured the anisotropy of the source although this was not a formal part of the comparison. The first draft report was released in May 2006 and having been discussed and modified by the participants and subsequently reviewed by the CCRI(III), the present paper is now the final report of the comparison.

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