ABSTRACT
Standards are needed for radioactive materials to permit their accurate measurement for purposes of health, worker protection, and public safety. The National Bureau of Standards developed a program for standards and calibrations of radium-226 in the early part of the century, but this program was limited mainly to naturally-occurring radionuclides in the uranium and thorium series until the late 1940s. Derived standards of radioactivity differ from the base quantities of time, length, and mass in two important respects. First, radioactivity is an ephemeral quantity. Second, there are a few thousand radionuclides, and they are found in many matrices (gas, liquid, solid, soil, air filters), so that choices must be made on the select few nuclides and matrices for "national standards." Radionuclides have been used in therapy for many years, but in the recent past improvements in sealed source preparation and radiopharmaceutical targeting strategies have led to much greater use and, accordingly, demands for standards.
