The Measure of All Things: From quetta to quecto

by Paddy Regan (University of Surrey & National Physical Laboratory, Teddington, UK)

IFIN-HH/ ELI-NP Training and Conference Center

IFIN-HH/ ELI-NP Training and Conference Center


An overview of the International Measurement System of Units (SI) will be presented, including the recent revisions of definitions for some of the base units linking them to fundamental constants of nature [1,2,3].
The importance of consistent and traceable measurement of physical quantities will be explained, including concepts of traceability, primary standards and the realisation of SI units.
The specific case of traceable measurements for radioactive decays to the becquerel (Bq) will be presented in terms of determining absolute values for isotope-specific radioactive sources using precision measurements of
radioactive decay half-lives [4,5,6] – all of which are required for safe and appropriate applications of radioactive materials in areas as diverse as nuclear medicine, energy production, forensics and national security.
The presentation will end with a mention of the current largest and smallest prefixes in the SI units which now covers 60 orders of magnitude with example of masses from quetta (Qg) to quecto (qg) grams [7].


[1] Stock, M., Davis, R., de Mirandés, E. and Milton, M.J., 2019.
The revision of the SI—the result of three decades of progress in metrology. _Metrologia_, _56_(2), p.022001 (2019)


[2] Brown,
RJC  A metrological approach to quantities that are counted
and the unit one. _Metrologia_, _58_(3), p.035014.(2021)


[3] Güttler, B., Bettin, H., Brown, R.J., Davis, R.S., Mester, Z., Milton, M.J., Pramann, A., Rienitz, O., Vocke, R.D. and Wielgosz, R.I.,
Amount of substance and the mole in the SI. _Metrologia_, _56_(4), p.044002.(2019)


[4] Regan, P.H., Judge, S.M., Keightley, J.D. and Pearce, A.K,
Radionuclide metrology and standards in nuclear physics. _Nuclear Physics News_, _28_(3), pp.25-29 (2018)


[5] Collins, S.M., Shearman, R., Keightley, J.D. and Regan, P.H., 2018.
Investigation of γ-γ coincidence counting using the National Nuclear Array (NANA) as a primary standard. Applied radiation and isotopes, 134, pp.290-296 (2018)


[6] Collins, S.M., Gilligan, C., Pierson, B., Ramirez, N., Goodwin, M.,
Pearce, A.K., Archambault, B.C., Haney, M.M. and Regan, P.H.,
Determination of the 161Tb half-life. Applied radiation and isotopes, 182, p.110140 (2022)


[7]  Brown, R.J., 2022. A further short history of the SI prefixes. _Metrologia_, _60_(1), p.013001 (2022)


[1] This work is supported by the UK Science and Technologies Facilities
Council (STFC) and UK Department for Business, Energy and Industrial
Strategy (BEIS) through the National Measurement System (NMS) programme