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

Carrier-free Po-209 solution standards have been prepared and calibrat ed. The standards, which will be disseminated by the National Institut e 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 (L S) measurements with two different LS counting systems and under wide variations in measurement and sample conditions. Confirmatory measurem ents by 2?ra gas-flow proportional counting were also performed. The o nly known radionuclidic impurity, based on alpha- and photon-emission spectrometry, is a trace quantity of Po-208. The Po-208 to Po-209 impu rity ratio as of the reference time was 0.00124 +/- 0.00020. All of th e 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 a n 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.