Astronomers have long been fascinated by neutrinos, tiny massless particles that travel near the speed of light throughout the cosmos. First studied in the 1950s, these elusive particles have become the subject of numerous intriguing observatories, such as IceCube in Antarctica, which utilizes a cubic kilometer of ice at the South Pole as its detection medium. Additionally, KM3Net, a deep-sea neutrino detector, is under construction in the Mediterranean Sea. Existing detectors are found all around the globe.
Now, a group of Chinese scientists is developing an ambitious deep-sea neutrino observatory that will surpass any existing technology. Chen Mingjun, the lead researcher at the Chinese Academy of Sciences, has announced plans for the largest neutrino observatory ever built. “The detector will have a 30-cubic-kilometer volume, featuring over 55,000 optical modules suspended along 2,300 strings,” Chen explained.
The Importance of Neutrino Research
Neutrinos are produced by a variety of cosmic events, such as massive stellar explosions. The presence of neutrinos often signals that a supernova has occurred, as they arrive on Earth before the light from the explosion. Neutrinos, as well as cosmic rays, can also originate from the Sun, stellar explosions, blazars, and even the Big Bang itself. On Earth, neutrinos are emitted from decaying radioactive materials underground, nuclear reactors, and particle accelerators. Neutrino astronomy allows scientists to investigate the sources and underlying physics of these particles, offering a unique glimpse into processes that are otherwise hidden, such as the core of the Sun, galaxy centers, gamma-ray bursts, and starburst galaxy events.
Capturing and measuring these elusive, near massless particles is no simple feat. Neutrinos barely interact with normal matter, making them hard to track. They can traverse vast distances through space, rarely interacting with interstellar gas, dust, or celestial bodies. However, when they do interact, they produce other detectable reactions and particles. As a result, neutrino detectors must possess a large “collecting area” to gather enough data for study. Early neutrino observatories were built underground to minimize interference from local radiation sources. Detection necessitates highly sensitive equipment, and even the most advanced detectors on Earth only capture a relatively small number of neutrinos.
Some neutrino observatories use tetrachloroethylene, commonly known as dry cleaning fluid, to detect neutrinos. When a neutrino collides with a chlorine 37 atom, it converts it into an argon-37 atom, which instruments then detect. Another detection method involves Cherenkov detectors, named for the Cherenkov radiation emitted when charged particles, such as electrons or muons, travel through water, heavy water, or ice. Facilities like IceCube, KM4Net, and Lake Baikal use this approach. The Chinese underwater observatory aims to refine this technique and greatly expand the search for neutrinos.
Unraveling the Connection Between Neutrino and Cosmic Ray Sources
The primary goal of constructing such an immense telescope is to detect high-energy neutrinos. However, Chen suspects that there may be a connection to cosmic rays. He hopes that the neutrinos detected by the observatory will help solve the longstanding scientific mystery surrounding the origin of cosmic rays. In the early 1900s, scientists discovered that Earth is constantly bombarded by energetic particles. Since then, astronomers have monitored neutrinos and gamma rays from space. In 2021, China’s Large High-Altitude Air Shower Observatory (LHAASO) in Sichuan province identified 12 gamma-ray sources, which are likely linked to some cosmic ray origins. Chen mentioned a popular hypothesis suggesting that high-energy neutrinos and gamma rays may be produced simultaneously during the generation of high-energy cosmic rays. “If we can detect both particles together, we can pinpoint the origin of the cosmic rays,” said Chen. The researchers plan to observe whether neutrino collisions in their detector yield secondary particles that emit light signals detectable by their underwater instruments. Existing research hints at this possibility, and Chen believes neutrino detection could help trace the source of this enigmatic space radiation.
Many team members have dedicated years to studying cosmic rays, particularly through the LHAASO project. Now, they are preparing to investigate neutrinos with an entirely new facility. Building an underwater neutrino observatory presents unique challenges, including the high costs of underwater equipment and operations. Furthermore, the team must develop a completely waterproof detector. Nevertheless, progress is being made, with the team recently completing their first sea trial to test the detection system at a depth of 1,800 meters underwater.