Preparation and Calibration of Carrier-Free Po Solution Standards.

Carrier-free Po solution standards have been prepared and calibrated. The standards, which will be disseminated by the National Institute of Standards and Technology as Standard Reference Material SRM 4326, consist of (5.1597 ±0.0024) g of a solution of polonium in nominal 2 mol · L hydrochloric acid (having a solution density of (1.031±0.004) g · mL at 22 °C) that is contained in 5 mL flame-sealed borosilicate glass ampoules, and are certified to contain a Po alpha-particle emission rate concentration of (85.42±0.29) s · g (corresponding to a Po activity concentration of (85.83 ±0.30) Bq · g) as of the reference time of 1200 EST 15 March 1994. The calibration was based on 4πα liquid scintillation (LS) measurements with two different LS counting systems and under wide variations in measurement and sample conditions. Confirmatory measurements by 2πα gas-flow proportional counting were also performed. The only known radionuclidic impurity, based on α- and photon-emission spectrometry, is a trace quantity of Po. The Po to Po impurity ratio as of the reference time was 0.00124 ±0.00020. All of the above cited uncertainty intervals correspond to a combined standard uncertainty multiplied by a coverage factor of = 2. Although Po is nearly a pure α emitter with only a weak electron capture branch to Bi, LS measurements of the Po a decay are confounded by an a transition to a 2.3 keV ( = 1/2) level in Pb which was previously unknown to be a delayed isomeric state.