A brand new view into the historical past of the universe

Two perspectives of the bulbs in Super-K

With an improve to the Tremendous-Kamiokande detector, neutrino physicists will achieve entry to the supernovae of the previous.

In 1987, the explosion of a big star created an excellent gentle present throughout the Massive Magellanic Cloud, a small, satellite tv for pc galaxy orbiting the Milky Method. The cataclysmic occasion, also called a supernova, was seen from telescopes on Earth. 

However earlier than the sunshine from the stellar blast reached our planet, three observatories, together with the Kamiokande neutrino observatory in Japan, picked up indicators from one other kind of particle produced within the blast: neutrinos. 

Neutrino particles, although elusive, carry away nearly the entire power launched by these exploding stars. By analyzing them, physicists can higher perceive the properties of neutrinos and probe the internal workings of supernovae. 

“There’s actually no technique to look inside the guts of a dying star besides through neutrinos,” says Mark Vagins, an experimental physicist at College of Tokyo’s Kavli Institute for the Physics and Arithmetic of the Universe.

For the final three a long time, physicists have patiently waited for the following close by supernova. Fortunately, ready is now not the one choice. 

The successor to Kamiokande, referred to as Tremendous-Kamiokande, is about to get an improve. Including the uncommon earth aspect gadolinium to Tremendous-Okay will enable scientists to seek for neutrinos not solely from future supernovae, but in addition from stellar explosions in our universe’s historical past. 

“Each few seconds, a supernova occurs someplace within the universe, and they’re all producing neutrinos,” says Masayuki Nakahata, the spokesperson for Tremendous-Okay. “Through the use of this new expertise, we will detect these neutrinos.” 

Separating sign from noise 

The Tremendous-Okay observatory lies underneath Mount Ikeno, in a mine 3300 toes under the bottom in central Japan. The detector is encased inside a cylindrical stainless-steel tank as tall because the Statue of Liberty. Its inside is full of 50,000 tons of ultra-pure water and lined with roughly 13,000 photosensors—golden bulbs that detect the flashes of sunshine produced as neutrinos move by.

Illustration of the Super-K detector located ~1000 meters under Mt. Ikeno and about the same height as the Statue of Liberty

Illustration by Sandbox Studio, Chicago with Steve Shanabruch

Within the early 2000s, the Tremendous-Okay collaboration tried to detect neutrinos from previous supernovae, that are collectively often known as the diffuse supernova neutrino background. In idea, Tremendous-Okay was massive sufficient to search out these particles. However the sign was being hid by “background noise” produced by different processes. 

Neutrinos are available three totally different “flavors”: electron neutrino, muon neutrino and tau neutrino. Supernovae launch each neutrinos and their antimatter counterparts, antineutrinos, in numerous flavors, however the ones that mostly work together inside detectors like Tremendous-Okay are electron antineutrinos. When one these particles is available in contact with hydrogen molecules within the Tremendous-Okay detector, it releases a positron, together with one other particle. This course of creates a flash of sunshine that Tremendous-Okay’s sensors can establish.

The issue is, a lot of different particles—together with the electron neutrinos that continuously stream from the solar and move by Tremendous-Okay way more usually than electron antineutrinos from supernovae—produce the identical sign.

Whereas at a neutrino convention in Munich in 2002, Vagins and his colleague John Beacom, a theoretical physicist now on the Ohio State College, got here up with an answer to this drawback. “John and I made a decision that there needed to be a technique to see these darn issues,” Vagins says. “We talked about many various approaches and fairly shortly realized that we had been going to have to make use of gadolinium.” 

The duo realized that gadolinium, a uncommon earth steel, can be a precious addition to Tremendous-Okay attributable to a sure property. Gadolinium is uniquely efficient at gobbling up the opposite type of particle an electron antineutrino produces when it hits the purified water in Tremendous-Okay detector: a neutron. 

If gadolinium had been added to the Tremendous-Okay detector, it could work together with a neutron launched by an electron antineutrino to generate a second pulse of sunshine. 

Twin gentle flashes, which Vagins and Beacom have dubbed “gadolinium heartbeats,” would come solely from electron antineutrinos—they’d not be produced by electron neutrinos from the solar or in interactions with different particles that till now have obscured the detection of supernova electron antineutrinos. 

“We anticipate the background to be lowered by an element 10,000,” Vagins says. “It’s an incredible achieve.” 

Convincing the group 

When Beacom and Vagins initially pitched the concept of including gadolinium to Tremendous-Okay to the collaboration, it was not greeted with the extent of enthusiasm they anticipated. Their colleagues had been impressed by the potential of this method, however they anxious that gadolinium may hurt the multi-million-dollar detector. 

Researchers anxious that gadolinium may corrode the metal, alter the transparency of the water, or introduce radioactivity into the detector. “There was an enormous listing of points, and we needed to undergo these separately and present that no, it was not an issue,” Vagins says. 

There have been additionally different challenges to beat, reminiscent of determining easy methods to dissolve the gadolinium into the water. This doesn’t occur naturally. (The reply: Mix it with sulfate to make gadolinium sulfate, a salt.) 

To check the feasibly of their plan, Tremendous-Okay scientists constructed a miniature model of the detector referred to as Evaluating Gadolinium’s Motion on Detector Methods, or EGADS. This scaled-down model of the detector was lined with 240 photosensors and had room for 200 tons of water. The workforce stuffed the prototype tank with gadolinium-loaded ultrapure water, then left it closed for round two-and-a-half years whereas operating checks to evaluate the detector’s capabilities. 

In the meantime, Beacom and his workforce have been gearing up for the forthcoming improve by conducting theoretical assessments, reminiscent of analyzing the main points of the background indicators throughout the gadolinium-loaded detector and the indicators that may be seen inside it. 

The Tremendous-Okay collaboration authorised the gadolinium improve in June 2015. However the last take a look at got here in 2017, when a gaggle of scientists, together with Vagins, donned protecting bunny fits (as human pores and skin is extremely radioactive, not less than compared to ultrapure water) and opened EGADS as much as assess whether or not there have been any indicators of harm. 

“That was a reasonably nervous second,” Vagins remembers. “However once we opened up, all the pieces was nonetheless shiny and fairly. That was just about the ultimate promoting level for everyone.” 

Upgrading the detector 

The Tremendous-Okay collaboration will begin loading the detector with gadolinium subsequent spring. They plan to begin at 0.01% gadolinium and steadily add extra. At a focus of 0.01%, gadolinium will already have the ability to seize round half of the neutrons that seem within the detector.

Bulbs inside of Super-K laid out as a cropped version of the periodic table of elements

Illustration by Sandbox Studio, Chicago with Steve Shanabruch

To organize for the addition of gadolinium, final 12 months, scientists opened up Tremendous-Okay for the primary time in 12 years to do some repairs. This included changing damaged phototubes, including new piping, cleansing the inside, and sealing a leak. Because it first began operating, Tremendous-Okay has been shedding round 1 ton of water per day, Nakahata says. This was not an issue when the tank was full of water. Now that gadolinium is being added, nevertheless, they’ll want to verify the liquid doesn’t seep into the surroundings. 

Gadolinium poses about the identical well being danger as desk salt, Vagins says. “A typical particular person must straight devour ounces of gadolinium to have issues, and since at full loading the water in Tremendous-Okay will likely be simply 0.1% gadolinium, one may drink a gallon a day proper out of the tank with out hassle,” he says. “Despite the fact that gadolinium is comparatively innocent, we don’t wish to probably be leaking that into the mountain vary or the neighborhood.” 

Gadolinium-loaded Tremendous-Okay will seek for neutrinos from all previous supernovae within the universe directly. Every supernova makes an incredible variety of neutrinos, however the probabilities of detecting one from supernovae exterior the Milky Method are tiny, Beacom explains. However by wanting on the complete diffuse supernova neutrino background, it’ll be potential to identification round two to 6 neutrinos per 12 months. 

These neutrinos will enable physicists to deal with a number of the many unsolved mysteries about supernovae. For instance, by making it potential to look at neutrinos from supernovae all through our universe’s historical past, the upgraded detector will assist scientists higher establish the traits of a typical stellar explosion. 

Gadolinium may also make Tremendous-Okay far more delicate to proton decay—a phenomena which has but to be noticed—and higher in a position to separate neutrinos from antineutrinos.

“I believe the gadolinium loading of Tremendous-Okay is a really thrilling improvement,” says André de Gouvêa, a theoretical particle physicist at Northwestern College who just isn’t concerned with the improve. “I’m assured we’ll study one thing attention-grabbing in regards to the historical past of the universe, supernova explosions, and the properties of neutrinos.” 

Ultimately, Vagins hopes the Hyper-Kamiokande collaboration—which in 2018 was granted seed funding towards the development of a successor to Tremendous-Okay that can maintain a whopping 260,000 tons of water—may also add gadolinium to its detector. 

Within the meantime, there are already a lot of different detectors which are planning to make use of gadolinium in the same means. These embody the XENONnT experiment at Gran Sasso Nationwide Laboratory in Italy, which searches for darkish matter particles, and the Water Cherenkov Monitor of Antineutrinos (WATCHMEN), a US- and UK-funded experiment primarily based in UK’s Boulby mine that can take a look at the feasibility of figuring out nuclear reactors by monitoring the telltale antineutrinos they produce. 

Vagins expects to see much more enthusiasm for gadolinium as soon as Tremendous-Okay scientists show its price. “I believe as soon as we’re operating Tremendous-Okay with gadolinium, and other people get used to the physics benefits, it’ll be exhausting to cease future experiments from doing it,” he says.