Japan’s ‘Super-Kamiokande’ is the world’s largest neutrino detector using water. It detects neutrinos using a tank containing 50,000 liters of water. Courtesy of Super-Kamiokande
Neutrinos are fundamental particles of the universe, also known as ‘ghost particles’, and have very little interaction with other matter. They are particles that are constantly passing through our bodies and the Earth at speeds close to light. They are mainly produced during nuclear fusion in the sun, supernova explosions, and the decay of unstable atomic nuclei.
Neutrinos are a key research subject in astrophysics because their minimal interaction with matter allows them to retain information regarding the events that created them. This makes them valuable tools for understanding the distant universe. Scientists are using Japan’s giant neutrino detector to search for neutrinos generated by supernova explosions in the distant universe.
According to a report published in the international academic journal Nature on the 10th (local time), a team led by Professor Masayuki Harada of the Department of Physics at the University of Tokyo announced their search for hints of supernova neutrinos using the Super-Kamiokande detector at the ‘Neutrino 2024’ conference held in Italy on the 20th of last month.
When a star collapses in a supernova, it releases an enormous number of neutrinos, estimated to be around 10^58. Neutrinos from a supernova have been observed only once so far. In 1987, the Kamiokande-2 detector, a predecessor to Super-K, successfully detected neutrinos from a supernova explosion in the nearby Large Magellanic Cloud.
It is estimated that neutrinos created by the collapse of countless stars spread throughout space, forming a ‘diffuse supernova neutrino background (DSNB).’ However, DSNB neutrinos, which are spread throughout space, have yet to be confirmed.
If we can identify and collect data on DSNB neutrinos, we can gain a deeper understanding of supernovae. We can also prove that neutrinos are stable particles that do not decay into other particles over billions of years. Measuring the spectrum of supernova neutrino energies can also provide insights into the number of supernovae that have exploded throughout the history of the universe.
The ‘Super-K’, a device that detects neutrinos 1km underground in the Kamioka mine in Japan, is a massive tank containing 50,000 tons of purified water. When an antineutrino, the antiparticle of a neutrino, collides with a proton in the water, the proton transforms into a pair of particles called a neutron and an antiparticle. An antiparticle has the same mass as its corresponding particle but with opposite spin, etc. This antiparticle moves quickly through the water and emits a flash of light. This flash of light can be captured by a sensor on the wall of the tank, confirming the existence of a neutrino.
In 2020, scientists “upgraded” the Super-K tank to enhance its sensitivity for detecting neutrinos. They added the rare earth metal gadolinium (Gd). When an antineutrino collides with water, a neutron is captured by a gadolinium nucleus, releasing energy instantaneously. Recording the two flashes produced by the antielectron and neutron separately improves detection sensitivity compared to previous methods. The higher the detection sensitivity, the easier it is to pinpoint the source of the neutrino.
The research team analyzed data collected from Super-K for 956 days, but they did not find a clear signal of DSNB neutrinos. “The neutrino signals may be mixed in from other sources,” the research team explained, adding, “We need to collect more data to detect a clear supernova signal.” The research team aims to find DSNB neutrinos within the next 10 years.
The Hyper-Kamiokande detector, the successor to Super-K, is scheduled to be completed in 2027. It is expected to be able to conduct experiments on a larger scale, which will improve experimental results. Hyper-K will also be filled with pure water initially, but like Super-K, gadolinium may be added later.
– doi.org/10.1038/d41586-024-02221-y
Japan’s Super-Kamiokande: In Search of Elusive Supernova Neutrinos
Japan’s ‘Super-Kamiokande’ is the world’s largest neutrino detector using water. It detects neutrinos using a tank containing 50,000 liters of water. Courtesy of Super-Kamiokande
The Ghostly Particles of the Universe
Neutrinos, often dubbed "ghost particles," are fundamental particles of the universe that interact minimally with other matter. These elusive particles constantly zip through our bodies and the Earth at speeds approaching the speed of light. Their primary origin lies in nuclear fusion within the sun, supernova explosions, and the decay of unstable atomic nuclei.
Neutrinos: A Window into the Universe
Neutrinos hold a prominent position in astrophysics research due to their unique characteristic of limited interaction with matter. This quality enables them to retain information regarding the events that generated them, offering a valuable glimpse into the mysteries of the distant universe.
Super-Kamiokande: Hunting for Supernova Neutrinos
Scientists, driven by the potential of neutrinos to unlock cosmic secrets, are utilizing Japan’s massive neutrino detector, Super-Kamiokande, to search for neutrinos produced by supernova explosions in the distant universe. A recent report in the journal Nature highlighted the efforts of a team led by Professor Masayuki Harada from the University of Tokyo, who presented their findings at the Neutrino 2024 conference held in Italy.
A Glimpse into the Past: Supernova Neutrinos and the Diffuse Supernova Neutrino Background (DSNB)
When a star collapses in a supernova, it releases a staggering number of neutrinos, estimated to be around 10^58. A single observation of supernova neutrinos was recorded in 1987 by the Kamiokande-2 detector – a precursor to Super-K – during a supernova explosion in the nearby Large Magellanic Cloud.
Scientists believe that neutrinos generated by countless stellar collapses have dispersed throughout space, forming a "diffuse supernova neutrino background (DSNB)." However, the identification and analysis of these dispersed DSNB neutrinos remain elusive.
The Significance of DSNB Neutrinos
The discovery and characterization of DSNB neutrinos offer the potential to unravel several cosmic mysteries:
- Understanding Supernovae: DSNB neutrinos provide insights into the properties and characteristics of supernovae.
- Proving Neutrino Stability: Observing DSNB neutrinos would confirm that neutrinos are stable particles, meaning they don’t decay into other particles over billions of years.
- Unveiling the History of the Universe: Measuring the energy spectrum of supernova neutrinos might reveal the number of supernovae that have exploded throughout the history of the universe.
Super-Kamiokande: Detecting Neutrinos in a Giant Water Tank
Located 1 kilometer underground in Japan’s Kamioka mine, Super-K is an impressive detector that consists of a massive tank filled with 50,000 tons of purified water. When an antineutrino – the antiparticle of a neutrino – collides with a proton in the water, the proton transforms into a neutron and an antiparticle. This antiparticle races through the water, generating a faint flash of light that is captured by sensors lining the tank walls, indicating the presence of a neutrino.
Gadolinium Enhancement: Boosting Detection Sensitivity
In 2020, scientists significantly enhanced the sensitivity of Super-K by introducing gadolinium (Gd) to the tank. When an antineutrino collides with water, the neutron produced is captured by a gadolinium nucleus, causing an instantaneous release of energy. The separate detection of these two flashes – one from the antielectron and the other from the neutron – improves detection accuracy compared to previous techniques.
The Ongoing Quest for DSNB Neutrinos
Despite the advancements in Super-K’s capabilities, the research team analyzed data gathered over 956 days without finding a clear signal of DSNB neutrinos. The team attributes this to the possibility of neutrino signals being mixed with signals from other sources. They plan to continue collecting data in hopes of eventually detecting a clear supernova signal. The research team aims to achieve this goal within the next decade.
Hyper-Kamiokande: The Future of Neutrino Detection
Super-K’s successor, the Hyper-Kamiokande detector, is scheduled for completion in 2027. This larger-scale detector is expected to improve experimental results significantly. While Hyper-K will initially be filled with pure water, plans are underway to add gadolinium in the future, mirroring the enhancements made to Super-K.
The Significance of Supernova Neutrino Research
The pursuit of DSNB neutrinos represents a monumental endeavor in the realm of neutrino and astrophysical research. It holds the key to unlocking the secrets of supernovae, understanding the stability of neutrinos, and illuminating the vast history of the universe. The relentless pursuit of these elusive particles promises to revolutionize our understanding of the cosmos.
