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New Standardization of Ra-228

Summary:

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.  

Description:

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.