Hard at work building China’s Jiangmen Underground Neutrino Observatory (JUNO). JUNO hopes to be detecting neutrinos by the end of 2024. 正在努力建设中国江门地下中微子观测站（JUNO）。
China’s giant underground neutrino lab prepares to probe cosmic mysteries
中国巨大的地下中微子实验室准备探索宇宙奥秘
Due to come online this year, the JUNO facility will help to determine which type of neutrino has the highest mass — one of the biggest mysteries in physics.
由于今年上线，JUNO设施将有助于确定哪种类型的中微子具有最高的质量-物理学中最大的谜团之一。
Kaiping, China 中国，开平
Seven hundred metres below the rolling green landscape of Kaiping, southeast China, construction workers are furiously finishing a 35-metre-diameter orb-shaped detector that aims to observe ghostly subatomic particles known as neutrinos in exquisite detail. If all goes to plan, the US$376 million Jiangmen Underground Neutrino Observatory (JUNO) will be ready to start detecting by the end of this year, says JUNO’s on-site manager Yuekun Heng, a physicist at the Chinese Academy of Science’s Institute of High Energy Physics in Beijing.
在中国东南部的开平，在绵延的绿色景观之下700米处，建筑工人们正在紧张地完成一个直径35米的球形探测器，该探测器旨在观察被称为中微子的幽灵般的亚原子粒子。如果一切按计划进行，耗资3.76亿美元的江门地下中微子天文台（JUNO）将准备在今年年底前开始探测，JUNO的现场经理，中国科学院高能物理研究所的物理学家Yuekun Heng说。
That will make it the first of several ambitious new neutrino detectors currently being built around the world to go online. Two others — in Japan and the United States — are due to start collecting data in 2027 and 2031.
这将使它成为目前世界各地正在建造的几个雄心勃勃的新中微子探测器中的第一个上线。另外两个在日本和美国，将于2027年和2031年开始收集数据。
JUNO’s main goal will be to help researchers determine which type of neutrino has the highest mass and which has the least, one of the biggest mysteries in physics. Solving this problem could help physicists to understand what neutrinos are and why their mass is so small. Researchers at JUNO aim to do this by measuring neutrinos pouring in from two nuclear power stations located more than 50 kilometres away from the observatory. Another goal is to study neutrinos streaming in from other sources, including the Sun, atmosphere, exploding stars and natural radioactive decay processes within Earth.
JUNO的主要目标是帮助研究人员确定哪种类型的中微子质量最大，哪种质量最小，这是物理学中最大的谜团之一。解决这个问题可以帮助物理学家了解中微子是什么以及为什么它们的质量如此之小。JUNO的研究人员旨在通过测量从距离天文台50多公里的两个核电站涌入的中微子来实现这一目标。另一个目标是研究从其他来源流入的中微子，包括太阳，大气，爆炸的恒星和地球内的自然放射性衰变过程。
On 7 March, researchers at the observatory started to fill a miniature version of the detector with liquid scintillator — a cocktail of solvent and organic chemicals that emits light when neutrinos zip through it. This model will test whether the scintillator is pure enough to help researchers to crack the mass-order problem.
3月7日，天文台的研究人员开始在一个微型探测器中填充液体闪烁体–一种溶剂和有机化学物质的混合物，当中微子穿过它时会发光。这个模型将测试闪烁体是否足够纯净，以帮助研究人员破解质量序问题。
JUNO’s approach sets it apart from the other detectors being built. Japan’s planned Hyper-Kamiokande detector will use purified water as its neutrino-detecting medium, whereas the Deep Underground Neutrino Experiment in the United States will rely on liquid argon to measure the elusive particles, says Mary Bishai, a physicist at the Brookhaven National Laboratory in New York and co-spokesperson for the US observatory. Both of these future detectors will measure neutrinos beaming in from nearby particle accelerators rather than nuclear reactors.
JUNO的方法使其与其他正在建造的探测器区别开来。纽约布鲁克海文国家实验室的物理学家、美国天文台的联合发言人玛丽·比沙伊说，日本计划中的超级神冈探测器将使用纯净水作为中微子探测介质，而美国的深层地下中微子实验将依靠液态氩来测量难以捉摸的粒子。这两个未来的探测器都将测量从附近的粒子加速器而不是核反应堆射来的中微子。
Like telescopes that view the cosmos at different wavelengths, having several neutrino detectors that use distinct techniques to observe neutrinos from various sources, such as the Sun and nuclear power stations, will allow researchers to develop a better understanding of neutrino characteristics and the role of these particles in the Universe, says Bishai. “It gives us a unique way of checking that our picture is consistent,” she says.
Bishai说，就像在不同波长下观察宇宙的望远镜一样，拥有几个中微子探测器，使用不同的技术来观察来自不同来源的中微子，如太阳和核电站，将使研究人员能够更好地了解中微子的特性和这些粒子在宇宙中的作用。她说：“这给了我们一种独特的方法来检查我们的图片是否一致。”
The liquid scintillator must contain only minuscule traces of uranium and thorium, radioactive elements that can mimic neutrino events when their decay accidentally coincides with other signals and can destroy experiment results. If levels of these elements are too high, it will be almost impossible to measure neutrinos with the sensitivity needed to solve the mass-ordering problem, says JUNO team member Alberto Garfagnini, a physicist at the University of Padua, Italy. The team is therefore filling the miniature version of JUNO — called OSIRIS — to test the fluid’s radiopurity before it is pumped straight into the main detector next door. It’s important to get this step right, because there’s no going back once JUNO is filled with 20,000 tonnes of the liquid. “It has to be pure from the beginning,” says Garfagnini.
液体闪烁体必须只含有微量的铀和钍，这些放射性元素可以模拟中微子事件，当它们的衰变意外地与其他信号重合时，可能会破坏实验结果。如果这些元素的水平太高，几乎不可能以解决质量排序问题所需的灵敏度测量中微子，JUNO团队成员Alberto Garfagnini说，他是意大利帕多瓦大学的物理学家。因此，该团队正在填充JUNO的微型版本-称为OSIRIS -以测试流体的放射性纯度，然后将其直接泵入隔壁的主探测器。重要的是要正确地完成这一步，因为一旦JUNO装满了20，000吨液体，就没有回头路了。“它必须从一开始就是纯净的，”Garfagnini说。
Photomultiplier tubes will detect flashes of energy produced when neutrinos interact with matter.Credit: Institute of High Energy Physics, Chinese Academy of Sciences
光电倍增管将探测中微子与物质相互作用时产生的能量闪光。图片来源：中国科学院高能物理研究所
Ghostly particles 幽灵粒子
Observing a neutrino sounds like it should be easy, given that they are the most abundant particles that have mass in the Universe, with billions of them passing through every cubic centimetre of Earth each second. But their properties remain mostly a mystery, because most of them barely interact with matter while they glide through the cosmos, making it difficult to detect them directly. However, neutrinos might hold clues about how the Universe evolved, says Garfagnini. “They are an important ingredient in cosmology,” he says.
观察中微子听起来应该很容易，因为它们是宇宙中最丰富的粒子，每秒有数十亿个中微子穿过地球的每立方厘米。但它们的性质仍然是一个谜，因为它们中的大多数在宇宙中滑行时几乎不与物质相互作用，因此很难直接探测到它们。然而，Garfagnini说，中微子可能包含宇宙如何演化的线索。“它们是宇宙学的重要组成部分，”他说。
A giant orb 一个巨大的球体
JUNO is located beneath a granite hill, which will act as a shield against cosmic rays — supercharged particles from space that can drown out faint neutrino signals. Every day, fluorescent-vested researchers and construction workers take a 15-minute cable-car ride down a steep 1.3-kilometre tunnel to continue building the detector inside a pristine, temperature-controlled hall. The acrylic sphere, which is roughly two-thirds complete, will soon be submerged in 35,000 tonnes of high-purity water, which will further shield the detector from background radiation. Once the liquid scintillator has passed its radiopurity test, it will be funnelled into the main detector. The entire process will take six months, says Heng.
朱诺位于一座花岗岩山丘下，这座山丘将作为宇宙射线的屏障-来自太空的超荷粒子可以淹没微弱的中微子信号。每天，穿着荧光衣的研究人员和建筑工人乘坐15分钟的缆车沿着1.3公里长的陡峭隧道向下，继续在一个原始的温控大厅内建造探测器。大约完成了三分之二的丙烯酸球体将很快被淹没在35，000吨高纯度水中，这将进一步保护探测器免受背景辐射。一旦液体闪烁体通过了其放射性纯度测试，它将被注入主探测器。恒说，整个过程将需要六个月。
Safeguarding JUNO’s sensitivity has been no easy feat. When construction started in 2015, the team was hoping to finish the building work in three years. But removing the huge volumes of groundwater resulted in delays. “Water was a big problem,” says Heng. To address this, the team installed a system that pumps 500 cubic metres of water out of the snaking underground tunnels every hour. To control levels of radon — a radioactive gas produced naturally by granite and other rocks that doesn’t play well with sensitive neutrino experiments — the cavernous facility is dotted with whirring, cylinder-shaped fans.
保护JUNO的敏感性并非易事。当2015年开始施工时，该团队希望在三年内完成建筑工作。但是，大量地下水的抽取导致了延误。“水是一个大问题，”Heng说。为了解决这个问题，该团队安装了一个系统，每小时从蜿蜒的地下隧道中抽出500立方米的水。为了控制氡的水平–一种由花岗岩和其他岩石自然产生的放射性气体，在敏感的中微子实验中效果不佳–这个洞穴般的设施点缀着呼呼作响的圆柱形风扇。
The reason for its difficult location lies on the surface. JUNO sits between two nuclear power stations, each located 53 kilometres away, that will supply the detector with a steady stream of electron antineutrinos, which have the same mass as neutrinos. The sheer number of them churned out by these power plants will give researchers a chance of measuring them with the precision needed to determine their mass order, says Heng.
它的困难位置的原因在于表面。JUNO位于两座核电站之间，每座核电站相距53公里，将为探测器提供稳定的电子反中微子流，这些反中微子与中微子质量相同。Heng说，这些发电厂大量生产的核燃料将使研究人员有机会给予机会，以确定它们的质量顺序所需的精度测量它们。
Neutrinos cannot be detected directly, so to figure out their mass, physicists measure the energy of other particles produced on the rare occasion that a neutrino interacts with matter. In JUNO’s case, when an electron antineutrino bumps into a proton in the liquid scintillator, the interaction will produce a positron and a neutron, a process called inverse beta decay. The energy from the positron results in a flash of light, while the neutron produces another flash when it is captured by a proton. These telltale flashes — 200 microseconds apart — will be measured by more than 40,000 bubble-shaped photomultiplier tubes that will cover the sphere. The time difference between these flashes will help researchers to separate neutrinos from cluttering background signals, says Garfagnini. “It’s a clear signature,” he says. The researchers hope to detect 100,000 neutrinos over the next six years.
中微子不能直接被探测到，所以为了计算出它们的质量，物理学家测量了中微子与物质相互作用时产生的其他粒子的能量。在朱诺的例子中，当一个电子反中微子与液体闪烁体中的质子相撞时，相互作用将产生一个正电子和一个中子，这个过程称为逆β衰变。来自正电子的能量导致闪光，而中子被质子捕获时产生另一个闪光。这些信号性的闪光–间隔200微秒–将被覆盖在球体上的40,000多个气泡状光电倍增管测量。Garfagnini说，这些闪光之间的时间差将有助于研究人员将中微子从杂乱的背景信号中分离出来。“这是一个明确的签名，”他说。研究人员希望在未来六年内探测到10万个中微子。
Its size, shielded environment and proximity to nuclear power sources will make it one of the most sensitive neutrino detectors in the world, says Geoffrey Taylor, a physicist at the University of Melbourne in Australia. This gives it a good chance of solving the mass order of neutrinos before other experiments get off the ground, he adds. “It’s on track to be a winner.”
澳大利亚墨尔本大学的物理学家杰弗里·泰勒说，它的大小、屏蔽环境和靠近核动力源的位置将使它成为世界上最灵敏的中微子探测器之一。他补充说，这给了它一个很好的机会，在其他实验开始之前解决中微子的质量顺序。“这是一个赢家的轨道。”