Microliter source preparation for absolute assays of massic activity by decay energy spectrometry

If a radioactive source can be embedded within an absorber and coupled to a low-temperature detector, a type of single decay event microcalorimetry can be achieved. In such a configuration, it is possible to measure all of the energy from the decay event, rather than just the energy of one or another decay products (alpha particles, electrons, x-rays, or gamma-rays) and so this method has been referred to as “Q Spectrometry” or “Decay Energy Spectrometry” (DES). In cases where the radioactive source is entirely contained within the volume of the absorber, very high efficiencies can be achieved, with only neutrinos and high-energy gamma rays escaping, and even then inducing clear and interpretable events that can be counted. In contrast to other spectroscopic approaches to quantifying radioactivity, DES provides simple energy spectra (one peak per radionuclide for ideal alpha decay) and truly remarkable resolution (e.g., < 1 keV FWHM on a 5 MeV alpha peak). The impact of this powerful technique will be magnified if the radioactive sources embedded in DES detectors can be gravimetrically linked to bulk samples such that the measurand becomes massic activity. We describe advances with a microliter gravimetric dispensing technique to prepare deposits of aqueous radioactive solutions on gold foils for DES measurements. We start with a well-characterized Am-241 standard reference material and perform liquid scintillation-based primary activity assays in parallel as a radiometric check of the gravimetric dispensing and as a rigorous check of the massic activity of the bulk solution.