Hunting for Neutrinos with Greek astrophysicists in the Mediterranean
We went on board the vessel that retrieved a neutrino telescope offshore Peloponnese .
A few weeks ago, I found myself in a peculiar situation. I went to Pylos, famous for its luxury hotels, to observe as a team of astrophysicists, with the help of oceanographers, retrieved a neutrino detector from the bottom of the sea. The experiment was comprised of two detectors placed at three kilometers underwater on the 28th of October 2015. It was a collaborative experiment between the National Center for Scientific Research, ‘Demokritos,’ the University of Athens and the Hellenic Institute for Marine Research.
Neutrinos are notoriously elusive and annoyingly puzzling. These subatomic particles are born in some of the universe’s most inaccessible locations. True to their name, they are neutral, infuriatingly so. With almost no mass and no electric charge, they can emerge, for example, in a Gamma-Ray Burst at a galaxy far far away and pass right through your fingertips without interacting with anything in between.
The hunt for neutrinos is thus very difficult, but crucial to the understanding of the dark corners of the universe. If one of the 65 billion neutrinos that pass through every square centimeter of the Earth every second reveals itself through interaction with its surroundings and you are there to observe it, it will be the same particle that left the Gamma-Ray Burst far far away.
To this end, scientists have constructed massive experiments all over the world. They have drilled into the ancient glaciers of Antarctica. They have created 50,000-ton ultrapure water tanks in abandoned subterranean mines. Now, they are building a cubic kilometer of detectors in the deep Mediterranean sea, KM3NeT.
The geography is important. The search for neutrinos is essentially a search for the light they emit, known as Cherenkov radiation. This faint, and to our eyes, eerie blue light is detected by photomultipliers. But there are many other sources of this type of Cherenkov radiation on the surface of the Earth, and that which comes from neutrinos is dim. To minimise noise from irrelevant Cherenkov emissions and maximise clarity, scientists head far from the surface of the Earth and into clear waters.
“The idea was to look for extremely high-energy neutrinos. Specifically those that come from Gamma-Ray Bursts. These are the most intense explosions that happen in our universe. Thankfully, they don’t happen in our galaxy.” Dr. Christos Markou, lead scientist on the GRBNeT experiment, explains.
When two neutron stars, the smallest and densest stars known, collide, they do so very violently. “They start approaching, and as they get closer their speed increases exponentially. They explode instantly when they come in direct contact. In a matter of 2–30 seconds, they release energy far exceeding the total of a galaxy.”
“The neutrinos that come from these explosions are consequently very high-energy themselves. The higher their energy, the larger the telescope has to be. The current plans for KM3NeT describe a telescope of one cubic square kilometer in volume, comprised of many detectors, or, more accurately, photomultipliers. Even this is not enough to trace neutrinos that come from Gamma-Ray Bursts. What is more, we use the Earth as a giant target. Instead of looking upwards into the stars, we look downwards. This filters out many of the neutrinos we are not interested in.”
The KM3NeT telescope will be connected to the shore with a cable, transmitting data and transporting energy. Therein lies the problem. “These can cost several million euros. What we wanted to test with GRBNeT is if we can design and build autonomous detectors. Rid of the need for cables, they will operate with batteries, lasting several months, or years. This way we can increase the volume of the telescope with little cost,” Christos Bagatelas described the goals. He was in charge of the mechanics of the detectors, “down to the last screw,” as he put it. I saw firsthand; they worked perfectly.
He constructed two crosses. Four photoelectric multipliers, the devices that detect the light, were attached in a cross formation, made of titanium rods. At the heart of the cross were the electronics, protected from the extreme pressure by glass domes. Underneath the electronics were the batteries.
A long, thick rope kept the two crosses together. At the bottom of the rope was an anchor that prevented the deep underwater currents from sweeping them away. Additional plastic domes filled with air are attached to the titanium rods to aid buoyancy.
From idea to realisation, the whole project took three years to come to life. “For long periods of time, we were just waiting. After designing the electronics, we had to order them and wait for the company to make them. But most of it we made in our own labs,” said Bagatelas.
The retrieval process starts with getting in the vicinity of the exact coordinates. Then, a sonar signal is sent into the water that triggers the release of the anchor. The crosses start emerging, and, in about 20 minutes, they are on the surface of the sea.
After about an hour on the boat, we reached the exact point where GRBNeT was. Bagatelas sent the sonar signal, and the wait began. At this point, they were all huddled in the “control room.” This small space in the vessel was filled with all sorts of equipment that looked alien to me.
The vessel belonged to the Hellenic Center for Marine Research. The scientists who work on it are out at sea for weeks at a time, monitoring waves and currents. They lend their services to other disciplines when needed. They are equal parts seamen and scientists.
Everyone was impatient. They kept checking where the telescope was. Until, finally, it announced that it was on the surface. But it was nowhere to be seen, we had to find it in the open sea. The engine started again, they knew it was somewhere close.
Bagatelas was the one to spot it on the horizon. The boat moved in its direction. It had been found. A smaller boat fell onto the sea, carrying a few men that would attach the construction on the crane on board. Within less than an hour, we were heading back to shore.
Dr. Markou was satisfied with the results. “I am extremely happy with the outcome of the endeavour. We proved that we can make this work. There was nothing simple about the mechanics of it. A small malfunction could have the whole thing heading towards Cairo. We had to build complicated and precise electronics that would function with little energy for a long time.”
As soon as the crosses were on safe ground, they had to be dismantled. After three years worth of planning, and one further underwater, GRBNeT had served its purpose. Now, all that mattered was the memory cards.
The data has to be extracted and analysed, and this is another time-consuming process. Intricate simulations are coded, describing what they expect to find and then compared with the actual data. This can take as much time and manpower as the development of the telescope itself.
They compare the “events,” with satellite data. “Astronomers are observing the stars 24/7. They know exactly when, and where a Gamma-Ray Burst has occurred. This way we can filter out noise in the data that comes from cosmic rays, or maybe shrimps.” Dr. Markou explained.
The adventure is over; hard work lies ahead. To me, it was hard to grasp the concepts these scientists see as trivial. What was even more uncanny was to be with them on this occasion, when years of effort were culminating in a single sea journey. It was something straight out of a children’s book. The adventurous scientist ventures out into the world, keen to solve the universe’s big mysteries as if they are fun puzzles. Yet simultaneously, it is obvious to me now, that it is far, far more complicated than that.