New Standardization of Ra-228

Measurements of radium isotopes in the naturally-occurring radioactive decay chains (i.e., ²³⁵U, ²³⁸U, and ²³²Th) are exceedingly important for various applications related to geochemical studies.  

Radium-228 has a half-life of (5.75 years) and decays by emitting a beta particle.  It is a radioactive decay product in the thorium-232 decay series, a very difficult and complex decay chain: ²³²Th→ ²²⁸Ra → ²²⁸Ac → ²²⁸Th → ²²⁴Ra → ²²⁰Rn → ²¹⁶Po → ²¹²Pb → ²¹²Bi → ²⁰⁸Tl → ²⁰⁸Pb(stable); with a sub-branch of ²¹²Bi → ²¹²Po → ²⁰⁸Pb(stable).  The chain is especially difficult due to the presence of the volatile ²²⁰Rn.  Radium-228 gives rise to many additional short-lived radionuclides, resulting in a wide spectrum of alpha, beta and gamma radiations.  It is mainly used as a monitoring tracer in the quality assessment of water supplies.  It has also been used to provide estimates of the age of formation of the radiobarite (a common constituent of scale and sludge deposits that form in oil-field production equipment) contaminant.  The availability of a ²²⁸Ra is of great interest and necessity to the international environmental-measurements communities.  During the last year a Th(NO₃)₂ solution which was last milked (²²⁸Ra separated) in 1982 was used as the mother solution for the current milking of the ingrowth of ²²⁸Ra from the ²³²Th parent.  This intricate separation involved several steps roughly summarized here: co-precipitation of ²²⁸Ra with Pb(NO₃)₂; precipitation of PbS with H₂S; filtration; and N₂ bubbling to expel excess H₂S.  Filtrate was evaporated and residue dissolved with HNO₃.  The resulting solution was then eluted through an anion exchange column with HNO₃ to remove any Th leftover.  The standardization of a new ²²⁸Ra Standard Reference Material (SRM 4339B), after its separation, is currently underway at NIST.  The certification of this standard will be based on HPGe g-ray spectrometry (γ-SPECT) with confirmatory measurements by 2pa spectrometry using Si surface barrier detector (α-SPECT).  Liquid scintillation measurements (4ηαβ LS spectrometry) will be performed in order to confirm the dilution factor.