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On February 23, 1987, just before 30 years from today, the neutrinos emitted from the supernova explosion SN1987A in the Large Magellanic Cloud, approximately 160,000 light-years away, reached the earth. Kamiokande, the predecessor detector of Super-Kamiokande, detected the 11 emitted neutrinos. Worldwide, it was the first instance of the detection of the emitted neutrinos from the supernova burst, and it served a big step toward resolving the supernova explosion system. In 2002, Dr. Masatoshi Koshiba, a Special University Professor Emeriuts of the University of Tokyo, was awarded a Nobel Prize in Physics for this achievement.

Before the explosion of supernova SN1987A (right) and after the explosion (left) Anglo-Australian Observatory/David Malin

Kamiokande, the pioneer of neutrino research

Kamiokande detector was a cylindrical water tank (16 m in diameter and height) with 1000 of the world’s largest photomultiplier tubes inside it, and it was laid 1000 m underground in Kamioka-town, Yoshiki-gun, (currently Hida-city) Gifu Prefecture, Japan. (Currently the site of Kamiokande is used for KamLAND experiment.) Kamiokande was devised by Prof. Koshiba who started the observation in 1983. Originally, it was constructed for detecting the proton decay phenomenon, but it was modified for the solar neutirno observation. By the end of 1986, the detector modification was completed and the observation began.

Inside of Kamiokande detector

Overview of Kamiokande detector

Prof. Koshiba working in the tank

Prof. Kajita and Prof. Nakahata (then PhD students) tuning up the data aquision system in the mine

The day of detection of the supernova neutrinos

On February 25, 1987, two days after the observation of supernova SN1987A through naked eyes, a fax was sent from Pennsylvania University to the University of Tokyo to inform them about the supernova explosion. Soon after receiving the fax, Prof. Yoji Totsuka asked the researcher in Kamioka to send the magnetic tapes that recorded the Kamiokande data. (At that time, the information network was not developed, so the data was delivered physically).

The fax sent from Pennsylvania University to inform about the supernova explosion.

On February 27, when the magnetic tapes arrived at the laboratory in Tokyo, Prof. Masayuki Nakahata (currently the spokesperson of Super-Kamiokande experiment), who was then a PhD student immediately started the analysis. On the morning of February 28, while Prof. Nakahata printed out the analysis plot between the detection time and number of photo-sensors that detect the light, Ms. Keiko Hirata, a Master's student found a peak, obviously different from the noise in the distribution. It was the exact trace to detect the neutrinos from SN1987A. (A two minutes blank period due to a regular system maintenance is recorded in the plot, at a few minutes before the explosion. If the explosion occurred during this period, Kamiokande could not have detected the SN1987A neutrinos.) After a detailed analysis, it was clear that Kamiokande detected 11 neutrinos for 13 seconds after 16:35:35 on February 23, 1987.

THe magnetic tape recorded SN1987A data

The printout of Kamiokande data and the envelope which stores the printout in. "Keep carefully Y.T." written by Prof. Youji Totsuka.

The printout of the data. Horizontal axis shows time (from right to left and one line as 10 seconds) and the vertical axis shows the number of hit photo-sensors of each event (approximately proportional to the energy of the event). The obvious peak is the signal of neutrinos from SN1987A. The blank period due to the detector maintainance was recorded a few minutes before the signal.

When Prof. Nakahata finished the analysis and reported to Prof. Koshiba on the morning of March 2, Prof. Koshiba instructed him to investigate the entire data for the presence of similar signals. Under a gag rule, researchers analyzed the 43 days data of Kamiokande on March 2 to March 6, and obtained conclusive evidence that the occurrence of the peak was only from the signal of the supernova SN1987A; further, they published these findings as an article. Here are the the signatures of researchers who wrote the article.

The subsequent development of neutrino research

The Kamiokande’s detection of the supernova neutrinos became a trigger to recognize the importance of neutrino research, and the construction of Super-Kamiokande, whose volume is about 20 times larger than that of Kamiokande, was approved. Super-Kamiokande started observation from 1996 and discovered the neutrino oscillation in 1998. In 2015, Prof. Takaaki Kajita was awarded the Nobel Prize in Physics for this achievement. SN1987A made a worldwide breakthrough in neutrino research, including the K2K experiment, T2K experiment and KamLAND experiment.

If a supernova explosion in our galaxy occurs now, Super-Kamiokande will detect approximately 8,000 neutrinos, almost 1000 times greater than those detected 30 years ago. Further, it is expected that the detailed mechanism of supernova explosion will be revealed and we will understand the stars or our universe in depth. In our galaxy, the supernova explosion is expected to occur once in every 30-50 years. It may occur at this very moment. The neutrinos from the supernova will be detected in mere 10 seconds. Super-Kamiokande continues the observation and will not miss any explosion moment.

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Nine facts about neutrinos

We don’t know much about neutrinos, but what we do know points to renegade particles that, despite their prevalence, are hard to pin down. Here are, in a nutshell, nine neutrino nuggets that scientists have figured out so far.

Neutrinos are super abundant. The shining sun sends 65 billion neutrinos per second per square centimeter to Earth. Neutrinos are the second most abundant particle in the universe. If we were to take a snapshot, we’d see that every cubic centimeter has approximately 1,000 photons and 300 neutrinos

Neutrinos are almost massless. No one yet knows the mass of neutrinos, but it is at least a million times less massive than the lightest particle we know, the electron. We do know that each is so lightweight and so abundant that the total mass of all neutrinos in the universe is estimated to be equal to the total mass of all of the visible stars. (To learn more about the neutrino masses click here)

Neutrinos are perfect probes for the weak force. All other fundamental particles interact through the strong, electromagnetic or weak force or through some combination of the three. Neutrinos are the only particles that interact solely though the weak force. This makes neutrinos important for nailing down the details of the weak force.

Neutrinos are really hard to detect. On average, only one neutrino from the sun will interact with a person’s body during his or her lifetime. Since neutrino interactions are so rare, neutrino detectors must be huge. Super Kamiokande in Japan is as tall as Wilson Hall and holds 50,000 tons of ultrapure water. IceCube is buried between 1.5 and 2.5 kilometers under pure and clear ice in Antarctica, instrumenting a full cubic kilometer of ice.

Neutrinos are like chameleons. There are three flavors of neutrinos: electron, muon and tau. As a neutrino travels along, it may switch back and forth between the flavors. These flavor “oscillations” confounded physicists for decades.

Neutrinos of electron flavor linger around electrons. When neutrinos travel through matter, they see dense clouds of electrons. Electron neutrinos will have trouble traversing these dense clouds, effectively slowing down while muon and tau flavors travel through unimpeded. The NOvA experiment is using this phenomenon to deduce more information about the neutrino masses.

Neutrinos let us see inside the sun. The light that reaches Earth takes 10,000 to 100,000 years to escape the thick plasma of the sun’s core. When light reaches the solar surface, it freely streams through open space to our planet in only 8 minutes. Neutrinos provide us a penetrating view into the core, where nuclear fusion powers the sun. They take only 3.2 seconds to escape to the solar surface and 8 minutes to reach Earth.

Neutrinos may have altered the course of the universe. Why is everything in the universe made predominantly of matter and not antimatter? Cosmologists think that at the start of the universe there were equal parts of matter and antimatter. Neutrino interactions may have tipped this delicate balance, enabling the formation of galaxies, stars and planets like our own Earth.

Neutrinos dissipate more than 99 percent of a supernova’s energy. Certain types of stellar explosions lose nearly all of their energy through neutrinos. These “core collapse” supernovae end as either a black hole or a neutron star. Neutrinos are used to understand how supernovae explode and tell us more about other astronomical objects like active galactic nuclei.

Helium nuclei were created in the Big Bang and contain two protons and two neutrons each. Helium is the second most abundant element, comprising roughly one quarter of the mass of the Universe. This animation zooms into a standard helium atom, showing its protons (green), neutrons (white), and electrons (blue). 

Excitation of atom by photon

Electron excitation is the transfer of a bound electron to a more energetic, but still bound state. This can be done by photoexcitation (PE), where the electron absorbs a photon and gains all its energy or by electrical excitation (EE), where the electron receives energy from another, energetic electron.

When an excited electron falls back to a state of lower energy, it undergoes electron relaxation. This is accompanied by the emission of a photon (radiative relaxation) or by a transfer of energy to another particle. The energy released is equal to the difference in energy levels between the electron energy states.