Standardization of ²⁴³Am

Americium's longest-lived isotope, americium-243, has a half-life of 7367(23) years. It decays into neptunium-239 through alpha decay. Americium has a few practical uses: as a monitor for radioactive contamination and as a tracer for radiochemical processes. An ²⁴³Am solution was standardized by 4παβ liquid scintillation (LS) spectrometry with two commercial LS counters. The beta detection efficiencies for the ²⁴³Am daughter (²³⁹Np) were calculated for the CIEMAT/NIST efficiency tracing (CNET) and the triple-to-double coincidence ratio (TDCR) methods using a modified MICELLE2 code and accounting for contributions from the delayed (T_1/2 = 193 ns) γ-ray emission from an excited state of ²³⁹Pu. Five confirmatory measurements of the activity were performed by 1) LS counting comparative measurements against the previous version of this Standard Reference Material (SRM) 4332e, which was standardized in 2008 by CIEMAT/NIST efficiency tracing using ³H as the detection efficiency monitor; 2) live-timed anticoincidence (LTAC) measurements; 3) triple-to-double coincidence ratio counting; 4) gamma spectrometry with high resolution HPGe detectors; and 5) decay energy spectrometry (DES) using transition edge sensors (TES). Alpha spectrometry measurement results gave a combined activity of ^{239, 240}Pu of 0.087(2) Bq·g^-1. The activity ratio of ²⁴¹Am/²⁴³Am was obtained by alpha spectrometry, gamma-ray spectrometry, mass spectrometry, and DES, with an average value of 0.00168(3). Mass spectrometry was also used to obtain values for ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu impurities.