The standardization of 99Tc by several primary methods was investigated. This was performed to support a new 99Tc transfer standard that has been developed and will be disseminated by the National Institute of Standards and Technology (NIST) as Standard Reference Material SRM 4288B. This 99Tc solution standard is contained in a 5 mL flame-sealed borosilicate glass ampoule; consists of (5.9945 ± 0.0003) g of a carrier-free nominal 0.001 molL-1 potassium hydroxide solution; has a density of (0.997 ± 0.002) gmL-1 at 22.5 C; and is certified to contain a massic activity (31.55 ± 0.21) kBqg-1 as of the reference time 1200 EST, 1 May 2008. All of the uncertainties cited above correspond to standard uncertainties multiplied by a coverage factor k = 2. The standardization for the 99Tc content of the solution was based on 4 liquid scintillation (LS) measurements with 3H-standard efficiency tracing. Confirmatory determinations were performed by 4(LS)-(NaI) live-timed anticoincidence (LTAC) counting and an LS-based 4 triple-to-double coincidence ratio (TDCR) method. The certified value was in agreement with the confirmatory determinations to within -0.22 % and +0.06 % for the LTAC and TDCR methods, respectively. The relative k = 2 expanded uncertainties on the two confirmatory determinations are approximately 0.7 % and 0.3 %, respectively. The preparation of LS counting sources parameters and 99Tc beta spectrum shape were studied. It was found that water is not an efficient quenching agent since large amounts of it are needed to obtain a wide range of efficiencies and that the addition of Aliquat® 336 (a surfactant and complexing agent for the pertechnetate ion) is not necessary. Analyses were performed assuming that the beta transitions were either allowed, or 2nd forbidden with or without an experimentally determined shape factor.
Citation: Applied Radiation and Isotopes
Pub Type: Journals
anticoincidence counting, beta counting, CIEMAT/NIST method, efficiency tracing, liquid scintillation (LC), radioactivity, SRM, standards, technetium-99