Preparation and Calibration of Carrier-Free 209Po Solution Standards

Carrier-free {sup 209} 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 {center_dot} L{sup {minus}1} hydrochloric acid (having a solution density of (1.031 {+-} 0.004) g {center_dot} mL{sup {minus}1} at 22 C) that is contained in 5 mL flame-sealed borosilicate glass ampoules, and are certified to contain a {sup 209}Po alpha-particle emission rate concentration of (85.42 {+-} 0.29) s{sup {minus}1} {center_dot} g{sup {minus}1} (corresponding to a {sup 209}Po activity concentration of (85.83 {+-} 0.30) Bq {center_dot} g{sup {minus}1}) as of 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{pi}{alpha} gas-flow proportional counting were also performed. The only known radionuclide impurity, based on {alpha}- and photon-emission spectrometry, is a trace quantity of {sup 208}Po. The {sup 208}Po to {sup 209}Po impurity ratio as of the reference time was 0.00124 {+-} 0.00020. All of the above cited uncertainty intervalsmore » correspond to a combined standard uncertainty multiplied by a coverage factor of k = 2. Although {sup 209}Po is nearly a pure {alpha} emitter with only a weak electron capture branch to {sup 209}Bi, LS measurements of the {sup 209}Po {alpha} decay are confounded by an {alpha} transition to a 2.3 keV (J{sup {pi}} = 1/2{sup {minus}}) level in {sup 205}Pb which was previously unknown to be a delayed isomeric state.« less