A new preprint on arXiv (2604.15406, submitted April 2026, not yet peer-reviewed) details the nEXO collaboration's exhaustive radioassay program, compiling some of the world's tightest constraints on natural radioactivity in materials critical for rare-event searches. Using a combination of high-purity germanium gamma spectroscopy, inductively coupled plasma mass spectrometry, neutron activation analysis, and alpha counting, the team—led by researchers including R. MacLellan and A. Piepke—tested dozens of components from metals and plastics to cables and photomultipliers. While exact sample sizes vary by material (often 10-100 samples per type), the study acknowledges limitations such as batch-to-batch variability and detection thresholds that still leave some ultra-trace contaminants hard to quantify at the required parts-per-trillion levels.

This is far more than a technical appendix. Neutrinoless double beta decay (0νββ) in xenon-136 would prove neutrinos are Majorana particles—meaning they are their own antiparticles—violating lepton number conservation. Such a discovery would bolster leptogenesis models, where CP-violating decays of heavy right-handed neutrinos in the early universe could generate the matter-antimatter asymmetry we observe today, explaining why matter survived annihilation after the Big Bang. nEXO aims for a half-life sensitivity beyond 10^28 years using five tonnes of enriched liquid xenon, building on its predecessor EXO-200 which set strong limits but fell short of discovery.

The preprint goes beyond typical tabulations by offering the most restrictive U/Th/K contamination bounds yet for several materials of broad interest to the low-background community. However, what both this paper and much prior coverage miss is the cross-pollination effect: these datasets are already being adopted by dark matter experiments like XENONnT and future DARWIN successors, which share xenon TPC technology and face identical radiopurity demands. Synthesizing this with the 2019 EXO-200 final results (Phys. Rev. C 2020, arXiv:1906.02723) showing background rates near 10^-3 counts/keV/kg/yr and a 2022 review by Dolinski, Poon, and Rodejohann in Annual Review of Nuclear and Particle Science on 0νββ prospects, a clear pattern emerges—the field has shifted from isotope competition (Xe-136 vs Ge-76 in LEGEND vs Te-130 in CUORE) toward shared infrastructure and screening libraries.

Critically supporting nEXO means recognizing both its elegance and its fragility. Barium tagging for near-zero background is innovative, yet the radioassay underscores a sobering reality: even parts-per-quadrillion thorium can swamp a potential signal. What others got wrong was treating material screening as mundane bookkeeping; it is, in fact, the experiment's foundation. Without this program, nEXO's projected sensitivity to the inverted neutrino mass hierarchy evaporates. Challenges remain—cosmogenic activation during surface handling and scaling purification techniques—but this compilation accelerates the entire rare-event field. If successful, nEXO won't just answer whether neutrinos are Majorana; it could illuminate why anything exists at all to ask the question.