Kamioka ObservatoryICRR, The University of Tokyo

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 Emeritus of the University of Tokyo, was awarded a Nobel Prize in Physics for this achievement.

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 neutrino observation. By the end of 1986, the detector modification was completed and the observation began.

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.)

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.

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.

SK-Gd aims to detect supernova neutrinos

In the summer of 2020, Super-Kamiokande has introduced the rare earth element gadolinium to start a new observation of supernova neutrinos from the beginning of the universe.The addition of gadolinium will also benefit the observation of supernova explosion in our galaxy, allowing us to determine the direction of these stars with the greater precision.

When a supernova explosion occurs in our galaxy, we can expect to observe about 10,000 neutrinos. However, only about 5% of them are electron-neutrino events that tell us the direction of the supernova explosion. By eliminating a large fraction of anti-electron neutrino interactions, SK-Gd can dramatically improve the accuracy of determining the direction of supernova explosions.