氙-124创造了半衰期新记录

諸平
据美國化學會主辦的《化學工程與新聞》（C&EN）網站2019年5月4日報道，科學家在暗物质实验研究過程中，意外發現了氙-124（¹²⁴Xe）罕见的放射性衰变半衰期。

Credit: XENON Collaboration

与¹²⁴Xe的半衰期相比，整个宇宙的历史不过是一个转瞬即逝的瞬间。令人震惊的是，¹²⁴Xe的半衰期达到了1.8×10²²年，是迄今为止直接测量到的最长的半衰期——大约是宇宙年龄的1万亿倍。相關研究結果于2019年4月24日已經在《自然》（Nature）杂志網站發表——XENON Collaboration. Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature, 2019,  volume 568, pages 532–535 (Published: 24 April 2019). DOI: 10.1038/s41586-019-1124-4。

（XENON1T detector at Italy’s Gran Sasso National Laboratory，如图所示)。在今年早些时候退役之前，该探测器包含3.2吨液态氙，其目的是在寻找暗物质粒子的迹象。暗物质粒子被认为是宇宙中大量不可见物质的来源。每1000个氙原子中只有一个是¹²⁴Xe同位素，它被预测会通过一种罕见的途径衰变为碲-124（¹²⁴Te），这种途径被称为双中微子双电子捕获(two-neutrino double electron capture)。当原子核中的2个质子同时捕获原子自身的2个电子时，它们就转变成中子并释放出2个中微子。这个过程也会产生x射线和电子，这些电子可以被XENON1T接收。该装置在一年内探測到126个此類事件，使研究人员能够计算此同位素（¹²⁴Xe）的半衰期。苏黎世大学（University of Zurich）的Laura Baudis是XENON1T的负责人之一，她说:“这显示了探测器前所未有的灵敏度。”虽然实验尚未发现任何暗物质，但一个更敏感的后续物质8.4吨氙正在形成，它可能会发现一个更罕见的衰变途径，预计为¹³⁶Xe。更多信息請注意瀏覽原文或者相關報道。

Abstract

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude¹. Until now, indications of 2νECEC decays have only been seen for two isotopes^2,3,4,5, ⁷⁸Kr and ¹³⁰Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance^6,7. The 2νECEC half-life is an important observable for nuclear structure models^{8,9,10,11,12,13,14} and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass^15,16,17. Here we report the direct observation of 2νECEC in ¹²⁴Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 10²² years (statistical uncertainty, 0.5 × 10²² years; systematic uncertainty, 0.1 × 10²² years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments^18,19,20.