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Denis E. Bergeron is a NIST research chemist in its Radioactivity Group whose work concerns physical radioactive activity measurements and standards. His official profile identifies his work in that group; an author account in the Journal of Research of NIST documents the comparison of primary activity standards over several decades.^[ref][ref]

In the 2014 coauthored review, Bergeron and colleagues examined ten fluorine-18 standardizations from 1982 to 2013. A new standard differed by about 4 percent from results obtained in1998–2008. The authors compared multiple primary measurement methods and possible sources of bias, while explicitly reporting that the older discrepancy was not fully explained.^[ref]

A simple ionization chamber example shows why a calibration difference matters. Suppose measured electric current is $I=10\mathrm{pA}$ and the response model is $I=kA$. A hypothetical factor $k=2pA/MBq$ gives activity $A=5\mathrm{MBq}$. Replacing it by $1.92pA/MBq$ gives approximately $5.2083\mathrm{MBq}$ for the same current, an increase of about $4.17\%$.

These numbers are invented, not the review's calibration table. They separate the detector observation from the inferred radioactive activity. Bergeron's standards work demonstrates why cross-method comparisons and an explicit measurement uncertainty budget matter even when repeated readings are consistent.

Liquid scintillation counting measures radiation through light produced in a liquid scintillator containing the radioactive sample. Energy deposited in the medium leads to optical emission, which photomultipliers detect as pulses. The apparatus is a scintillation counter, but the measured pulse rate still needs a model connecting it to radioactive activity.^[ref]

In an idealized one-count-per-decay model with background subtracted and other corrections negligible,where $A$ is radioactive activity and $\epsilon$ is detection efficiency. A hypothetical net rate of $100{\mathrm{s}}^{-1}$ with $\epsilon =0.40$ corresponds to $A=250\mathrm{Bq}$. At the same activity, an efficiency of $0.20$ would produce only $50{\mathrm{s}}^{-1}$. A smaller count rate need not mean that fewer decays occur.

A motivating measurement example is L. L. Lucas's comparison of tritium-containing water standards. Liquid scintillation counting supplied massic activity ratios that could be compared with independently recorded mass-based dilutions. Agreement within stated measurement uncertainties supports consistency of the standards; it does not turn the counting apparatus into an uncertainty-free activity meter.^[ref]