[en] National regulatory authorities usually demand that monitoring instruments are regularly calibrated against reliable reference sources. There are many national and international standards describing the properties and quality of reference sources. National accreditation services (e. g. NAMAS in the UK and DKD in Germany) are authorized to approve laboratories for the supply of traceable reference sources, and sources covered by this arrangement are fully traceable to national standards of radioactivity. Under mutual recognition agreements, the traceability of sources from approved laboratories in one country can also be recognized in other participating countries