Rare-earth element upgrade to help observatory detect ancient neutrinos
The Super-Kamiokande neutrino observatory in Japan has received a relatively simple upgrade that should allow it to look further back in time. A rare-earth element called gadolinium has been added to the water in the huge underground facility, which will make it more sensitive to neutrinos from more distant and ancient supernovae.
Neutrinos are elementary particles that are extremely light and rarely interact with regular matter. As such, they usually fly through most things unimpeded – in fact, billions are streaming through your body every second. But occasionally they will strike electrons in atoms, and under the right circumstances these collisions can be observed and studied.
And that’s where observatories like Super-K come in. Neutrino detectors work best when buried deep under rock or ice, which works to filter out other radiation. Super-K is located 1 km (0.6 mi) below Mount Ikeno in Japan, where it’s been quietly waiting and detecting neutrinos since 1996.
The active instrument of the facility is a huge tank standing 40 m (130 ft) tall, filled with some 50 million L (13 million gal) of ultra-pure water and with walls lined with 13,000 photomultiplier tubes. When a neutrino enters the tank and strikes a water molecule, it creates a tiny flash of light, which the photomultipliers amplify to help optical sensors pick up.
These neutrinos have different “fingerprints” of flashes depending on their origin, which can include the Sun, supernova explosions, artificial experiments, nuclear reactors, or from the decay of protons.
Supernovae are particularly fascinating, and while Super-K can detect those occurring in our own galaxy, these events don’t happen very often. Expanding the search to other galaxies can increase the number of neutrinos picked up, but these more distant signals are much weaker and don’t stand out from the background noise very well.
So the new upgrade is designed to help amplify neutrino signals from those distant supernovae. In July around 13 tons of a gadolinium compound was added to the water in the detector, creating a gadolinium concentration of about 0.01 percent. Some neutrino interactions produce neutrons, and gadolinium interacts with these particles to produce a gamma ray flash, which the optical sensors can easily spot. Importantly, this doesn't negatively impact the detections of other neutrino events.
“With a gadolinium concentration of 0.01 percent Super-Kamiokande should detect neutrons from neutrino collisions with 50 percent efficiency,” says Masayuki Nakahata, project overseer. “We plan to increase the concentration in a few years to increase efficiency. I hope we can observe neutrinos from ancient supernovae within a few years.”
The team says that this upgrade could allow Super-K to detect neutrinos from supernovae that occurred as far back as 10 billion years. This could teach us more about particle physics and the distant history of the universe.
Source: University of Tokyo