PREPARATION AND CALIBRATION OF CARRIER-FREE PO-209 SOLUTION STANDARDS

Carrier-free Po-209 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(-1) hydrochloric acid (having a solution density of (1.031 +/- 0.004) g . mL(-1) at 22 degrees C) that is contained in 5 mL flame-sealed borosilicate glass ampoules, and are certifed to contain a Po-209 alpha-particle emission rate concentration of (85.42 +/- 0.29) s(-1) . g(-1) (corresponding to a the reference time of 1200 EST 15 March 1994. The calibration was based on 4 pi alpha liquid scintillation (LS) measurements with two different LS counting systems and under wide variations in measurement and sample conditions. Confirmatory measurements by 2?ra gas-flow proportional counting were also performed. The only known radionuclidic impurity, based on alpha- and photon-emission spectrometry, is a trace quantity of Po-208. The Po-208 to Po-209 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 k = 2. Although Po-209 is nearly a pure a emitter with only a weak electron capture branch to Bi-209, LS measurements of the Po-209 alpha decay are confounded by an alpha transition to a 2.3 keV (J(pi) = 1/2(-)) level in Pb-205 which was previously unknown to be a delayed isomeric state.