Speaker
Description
Rare event search experiments such as AMoRE require precise radioassay of materials to constrain experiment backgrounds. ICP-MS is sensitive only to long-lived nuclides, while HPGe gamma-ray counting is generally limited by low detection efficiency that scales poorly with sample size. To bypass the limitations of these long-established techniques, we are developing a decay energy spectroscopy technique that uses magnetic microcalorimeters for direct, high-efficiency assay of Ra-226 and Th-228 at the µBq/kg level.
In this method, radium or thorium is chemically extracted from bulk samples into a solution. The solution is deposited and dried on a 1.5 cm × 1.5 cm × 25 µm gold foil. The foil is subsequently folded to enclose the sample, forming a 4π calorimetric absorber. Radioactive decays occurring inside the gold absorber deposit their full decay energy, including that of the alpha particle and recoil nucleus, and are detected as temperature pulses by the magnetic microcalorimeter. Nuclide identification is performed by selecting alpha-decay energies and time-coincidences in accordance with decays in the Ra-226 and Th-228 chains. Energy calibration is established with a Gd-148 alpha source.
We will present the development and validation of this technique with an emphasis on Th-228 assay in NOSV copper samples. Thorium is extracted using UTEVA resin and measured with the MMC-based decay energy spectrometer, with potential to scale to the sub-μBq/kg level. In addition, the sample-deposition efficiency is evaluated using Th-229. These results demonstrate that chemical extraction combined with MMC-based decay energy spectroscopy is a promising technique for improving sensitivities in ultra-low radioactivity assays for rare-event search experiments.