= M. P. Unterweger
{c}
M. P. Unterweger coauthored a 2000 study of <measurement calibration> for <water> standards containing <tritium>. The paper combined measurements using a <proportional counter> with <liquid scintillation counting> comparisons. It also followed <massic activity> data spanning 38 years to estimate a <tritium> <half-life> of $4504\pm9$ days, with the uncertainty specified as one standard deviation.<Calibration of the National Institute of Standards and Technology tritiated-water standards by M. P. Unterweger and others>[Abstract, Applied Radiation and Isotopes 52 (2000), pp. 527–531; DOI 10.1016/S0969-8043(99)00205-5.]{ref}
The long time record makes a useful distinction visible. A reference sample's <radioactive activity> changes through <radioactive decay> even when its chemical identity is unchanged. In an ideal closed sample with constant <half-life> $T_{1/2}$,
$$
A(t)=A(0)\,2^{-t/T_{1/2}}.
$$
Measurements made at different times must therefore be related to a common time before differences can be interpreted as inconsistent standards. A <half-life> determination and a detector comparison address related but distinct parts of that problem.
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