Traceability and Equivalence

Simon Jerome and Mike Woods

Centre for Ionising Radiation Metrology,
National Physical Laboratory

In most areas of measurement science, the ability to relate one’s measurement to a suitable measurement standard is a key activity in order to instil confidence of the end user in such data. This is especially so in the measurement of radioactivity. Thus, this traceability of measurement can be defined as:

‘The property of a result of a measurement whereby it can be related to appropriate standards, generally international standards, through an unbroken chain of comparisons.’

The harmonization of different National Measurement Systems under the recently agreed mutual recognition agreements depends on the ability to demonstrate Equivalence between the systems. Equivalence can be defined as:

‘The condition of being equivalent, that is equal for practical purposes in significance or worth. (Equivalence does not imply identity.)’

This can be put in more practical terms as follows;

‘The degree to which the value of a measurement standard is consistent with the key comparison reference value.’

The establishment of the procedures to achieve equivalence lies with the International Committee for Weights and Measures (CIPM) and it’s associated consultative committees. These have been developed by the International Bureau for Weights and Measures (BIPM). Central to this theme is the concept of key comparisons, which involve a series of intercomparisons both within and between regional metrology organisations (such as EUROMET and NORAMET). This, however, is impossible in the case of radionuclide metrology and has been achieved by alternative means which utilise currently available effort at National Metrology institutes and the large volume of data from previous intercomparison work in this area at an international level.

This paper discusses how this has been achieved in practice and the implications for traceability of measurements made by users and how this relates to currently available standards of radioactivity.