After a long period of development and testing, a group of researchers in Brazil is in the midst of handing over its neutrino detection system to a large international particle physics project led by Fermilab, the main American particle physics laboratory, in Batavia. , on the outskirts of Chicago.
It is, literally, a set of traps to capture signals from these elusive neutral particles, whose existence has been known for decades, but remains shrouded in mystery.
The idea for the photodetection system was born with the couple Ettore Segreto and Ana Amélia Bergamini Machado, he is Italian, she is Brazilian, both researchers at Unicamp (State University of Campinas).
The design they came up with basically acts like a light trap, allowing individual photons (the particles that light is made of) to enter, but not leave, until they are absorbed and recorded by a silicon detector inside. Hence the name Arapuca, in reference to the word that, in Tupi-Guarani, means “trap to catch birds”.
Initial estimates indicated that it could be 10 times more sensitive than other proposals for the photodetection system (detection by light), which excited the team responsible for Dune. The acronym for Deep Underground Neutrino Experiment, this large project, when it goes into operation, should be the most sensitive detector of these particles in the world.
Arapuca, which started with an idea by the couple, ended up becoming the centerpiece of the largest and most international of the consortia involved in the project. “In the photodetection group, there are more than 300 researchers, in 47 universities, in the USA, in Latin America, in Europe and now we have an adhesion from South Korea”, says Segreto. “Ten countries in all.”
In recognition of the importance of the contribution, the researcher entered the race to be one of the leaders of Dune in 2022.
In 2017, during a visit to Cern (the European center for particle physics), in Geneva, Switzerland, the model was perfected by the duo and renamed X Arapuca. And now, Segreto and Machado’s team is about to complete the delivery of 40 of these X Arapucas, two meters long by 12 cm wide and 2 cm thick.
“In April we finished delivering them. They were conceived, assembled and tested in Brazil. Most of the parts were manufactured in the state of São Paulo”, highlights Machado. These 40 boards will be used in the Proto-Dune, a prototype of the final equipment, in the process of being installed at Cern.
The work is funded by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) and it is estimated that US$ 3 million have been invested in the initiative on the national side.
The saga of the neutrinos
Although these elementary particles were first detected in 1956, there is still a lot we don’t know about them. That’s because they are extremely light and small. With neutral charge, they interact very little with conventional matter, which makes their detection a challenge. Despite this, they play a key role in the physics that make stars shine and even explode at the end of their lives.
We know that they exist in three different types, called electron-neutrino, muon-neutrino, and tau-neutrino, and that they can “oscillate”, that is, change between one type and another over time. Nobody knows for sure how or why. Dune aims to explore exactly these questions.
While the prototype is being installed at ground level, the final equipment (which will feature 1,500 X Arapucas, to be produced over the next two years) will be accommodated underground, 1.6 km from the surface, in Lead, North Dakota. South (USA).
The idea of ”burying” the experiment is to block as much as possible cosmic rays capable of activating the detectors. As neutrinos pass through matter more easily, the deeper you go, the more prevalent they become.
When completed, Dune will consist of four large underground pools each filled with 17,000 tonnes of liquid argon (which can only be kept in that state if kept at -187°C). The tanks will be surrounded by the detection systems.
And there will be three main objectives of the project: to seek, in the study of neutrino oscillations, evidence of something that physicists call a charge-parity violation (which in turn may clarify why everything we see in the Universe is made of matter, not antimatter), try to detect the decay of a proton (something we don’t know if it actually occurs) and capture neutrino emissions from supernovas (such stellar explosions are known to emit copious amounts of them and it may be possible to detect them even before let the light of the detonation reach us).
As you can’t rely on supernovas to deliver neutrinos on demand, Dune will rely on another source to fulfill its other goals: Fermilab, in Batavia. It will generate a beam that will cross 1,300 km underground until it reaches the experiment tanks at Lead.
With each fortuitous collision of a neutrino with an argon atom, two signals will appear: a faster one, called scintillation, which consists of the production of light resulting from the collision, and a slower one, ionization, which involves the perturbation of an electron. bound to argon and detected by an electrical grid.
Arapuca deals with the light signal, which indicates the exact moment of the collision. It is also his responsibility to detect neutrinos that may come from supernovae, as well as to indicate whether a proton has decayed. The ionization signal indicates the trajectory and type of particle that hit the detector.
Schedule
More than half of the parts for the Proto-Dune have already been to Europe, and installation is expected to be completed in August, so the tank can be filled with liquid argon, and the prototype experiment can be started in September. And it is already possible to get some science with this preliminary version of the system.
“He will do characterization of the argon response, calibrations, tests of the technology, of the detectors”, says Segreto. “Now, if there’s a supernova out there and it’s working, it might be able to detect it,” adds Machado.
Meanwhile, work continues in full swing for Dune. At the moment, the underground rooms that will receive the equipment are being built. In Brazil, the X Arapucas for him should be produced between 2023 and 2024. The equipment integration follows, which should take a few years. The experiment should be ready to start scientific data taking in 2029.