Arnquist I J, Avignone F T, Barabash A S, Barton C J, Bhimani K H, Blalock E, Bos B, Busch M, Buuck M, Caldwell T S, Christofferson C D, Chu P-H, Clark M L, Cuesta C, Detwiler J A, Efremenko Yu, Ejiri H, Elliott S R, Giovanetti G K, Goett J, Green M P, Gruszko J, Guinn I S, Guiseppe V E, Haufe C R, Henning R, Hervas Aguilar D, Hoppe E W, Hostiuc A, Kim I, Kouzes R T, Lannen V T E, Li A, López-Castaño J M, Massarczyk R, Meijer S J, Meijer W, Oli T K, Paudel L S, Pettus W, Poon A W P, Radford D C, Reine A L, Rielage K, Rouyer A, Ruof N W, Schaper D C, Schleich S J, Smith-Gandy T A, Tedeschi D, Thompson J D, Varner R L, Vasilyev S, Watkins S L, Wilkerson J F, Wiseman C, Xu W, Yu C-H, Alves D S M, Hebenstiel L, Ramani H
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
IU Center for Exploration of Energy and Matter, and Department of Physics, Indiana University, Bloomington, Indiana 47405, USA.
Phys Rev Lett. 2023 Oct 13;131(15):152501. doi: 10.1103/PhysRevLett.131.152501.
^{180m}Ta is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known β and electron capture decays due to the large K-spin differences and small energy differences between the isomeric and lower-energy states. Detecting its decay presents a significant experimental challenge but could shed light on neutrino-induced nucleosynthesis mechanisms, the nature of dark matter, and K-spin violation. For this study, we repurposed the Majorana Demonstrator, an experimental search for the neutrinoless double-beta decay of ^{76}Ge using an array of high-purity germanium detectors, to search for the decay of ^{180m}Ta. More than 17 kg, the largest amount of tantalum metal ever used for such a search, was installed within the ultralow-background detector array. In this Letter, we present results from the first year of Ta data taking and provide an updated limit for the ^{180m}Ta half-life on the different decay channels. With new limits up to 1.5×10^{19} yr, we improved existing limits by 1-2 orders of magnitude which are the most sensitive searches for a single β and electron capture decay ever achieved. Over all channels, the decay can be excluded for T_{1/2}<0.29×10^{18} yr.
^{180m}钽是一种罕见的核同质异能素，其衰变从未被观测到。由于同质异能态与低能态之间存在较大的K自旋差异和较小的能量差异，其显著长的寿命超过了所有其他已知β衰变和电子俘获衰变的半衰期。探测其衰变面临重大的实验挑战，但可能有助于揭示中微子诱发的核合成机制、暗物质的本质以及K自旋违反情况。在本研究中，我们将用于使用高纯锗探测器阵列对^{76}锗进行无中微子双β衰变实验搜索的Majorana Demonstrator探测器进行了重新利用，以搜索^{180m}钽的衰变。超过17千克（有史以来用于此类搜索的最大量钽金属）被安装在超低本底探测器阵列内。在本信函中，我们展示了第一年钽数据采集的结果，并给出了不同衰变通道上^{180m}钽半衰期的更新限制。新的限制高达1.5×10^{19}年，我们将现有限制提高了1 - 2个数量级，这是对单个β衰变和电子俘获衰变最灵敏的搜索。在所有通道上，对于半衰期T_{1/2}<0.29×10^{18}年的衰变可以排除。
