Detecting Neutrinos, The Slippery Ghost Particles That Don’t Want To Interact

Neutrinos are some of the most elusive particles that are well-known to science. These tiny subatomic particles have no electric charge and an extremely small mass, making them incredibly difficult to detect. They are produced in abundance by the sun, as well as by nuclear reactions on Earth and in supernovae. Despite their elusive nature, scientists are keen to detect neutrinos as they can provide valuable information about the processes that produce them.

Neutrinos interact with matter so rarely that it takes a very special kind of detector to catch them in the act. These detectors come in a few different flavors, each employing its unique method to spot these elusive particles. In this article, we’ll take a closer look at how these detectors work and some of the most notable examples of neutrino detectors in the world today.

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Detecting Anti-Neutrinos From Distant Fission Reactors Using Pure Water At SNO+

Although neutrinos are exceedingly common, their near-massless configuration means that their presence is rather ephemeral. Despite billions of them radiating every second towards Earth from sources like our Sun, most of them zip through our bodies and this very planet without ever interacting with either. This property is also what makes studying these particles that are so fundamental to our understanding so complicated. Fortunately recently published results by researchers behind the SNO+ neutrino detector project shows that we may see a significant bump in our neutrino detection sensitivity.

The Sudbury Neutrino Detector (Courtesy of SNO)
The Sudbury Neutrino Detector (Courtesy of SNO)

In their paper (preprint) in APS Physical Review Letters, the researchers describe how during the initial run of the new SNO+ neutrino detector they were able to detect anti-neutrinos originating from nuclear fission reactors over 240 kilometers away, including Canadian CANDU and US LWR types. This demonstrated the low detection threshold of the  SNO+ detector even in its still incomplete state between 2017 and 2019. Filled with just heavy water and during the second run with the addition of nitrogen to keep out radioactive radon gas from the surrounding rock of the deep mine shaft, SNO+ as a Cherenkov detector accomplished a threshold of 1.4 MeV at its core, more than sufficient to detect the 2.2 MeV gamma radiation from the inverse beta decays (IBD) that the detector is set up for.

The SNO+ detector is the evolution of the original Sudbury Neutrino Observatory (SNO), located 2.1 km below the surface in the Creighton Mine. SNO ran from 1999 to 2006, and was part of the effort to solve the solar neutrino problem, which ultimately revealed the shifting nature of neutrinos via neutrino oscillation. Once fully filled with 780 tons of linear alkylbenzene as a scintillator, SNO+ will investigate a number of topics, including neutrinoless double beta decay (Majorana fermion), specifically the confounding question regarding whether neutrinos are its own antiparticle or not

The focus of SNO+ on nearby nuclear fission reactors is due to the constant beta decay that occurs in their nuclear fuel, which not only produces a lot of electron anti-neutrinos. This production happens in a very predictable manner due to the careful composition of nuclear fuel. As the researchers noted in their paper, SNO+ is accurate enough to detect when a specific reactor is due for refueling, on account of its change in anti-neutrino emissions. This is a property that does not however affect Canadian CANDU PHWRs, as these are constantly refueled, making their neutrino production highly constant.

Each experiment by SNO+ produces immense amounts of data (hundreds of terabytes per year) that takes a while to process, but if these early results are anything to judge by, then SNO+ may progress neutrino research as much as SNO and kin have previously.

The WIMP Is Dead, Long Live The Solar Axion!

For decades scientists have been building detectors deep underground to search for dark matter. Now one of these experiments, the XENON1T detector, has found an unexpected signal in their data. Although the signal does not stem from dark matter it may still revolutionize physics.

Since the 1980s the majority of scientists believe that the most likely explanation for the missing mass problem is some yet undiscovered Weakly Interacting Massive Particle (WIMP). They also figured that if you build a large and sensitive enough detector we should be able to catch these particles which are constantly streaming through Earth. So since the early 1990s, we have been putting detectors made from ultrapure materials in tunnels and mines where they are shielded from cosmic radiation and natural radioactivity.

Over the decades these detectors have increased their sensitivity by a factor of about 10 million due to ever more sophisticated techniques of shielding and discriminating against before mentioned backgrounds. So far they haven’t found dark matter, but that doesn’t mean the high-end sensing installations will go unused.

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Towards A 3D-Printed Neutrino Detector

Additive manufacturing techniques like fused deposition modeling, aka 3D printing, are often used for rapid prototyping. Another advantage is that it can create shapes that are too complex to be made with traditional manufacturing like CNC milling. Now, 3D printing has even found its way into particle physics as an international collaboration led by a group from CERN is developing a new plastic scintillator production technique that involves additive manufacturing.

A scintillator is a fluorescent material that can be used for particle detection through the flashes of light created by ionizing radiation. Plastic scintillators can be made by adding luminophores to a transparent polymer such as polystyrene and are usually produced by conventional techniques like injection molding.

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Hunting Neutrinos In The Antarctic

Neutrinos are some of the strangest particles we have encountered so far. About 100 billion of them are going through every square centimeter on Earth per second but their interaction rate is so low that they can easily zip through the entire planet. This is how they earned the popular name ‘ghost particle’. Neutrinos are part of many unsolved questions in physics. We still do not know their mass and they might even be there own anti-particles while their siblings could make up the dark matter in our Universe. In addition, they are valuable messengers from the most extreme astrophysical phenomena like supernovae, and supermassive black holes.

The neutrinos on earth have different origins: there are solar neutrinos produced in the fusion processes of our sun, atmospheric neutrinos produced by cosmic rays hitting our atmosphere, manmade reactor neutrinos created in the radioactive decays of nuclear reactors, geoneutrinos which stem from similar processes naturally occurring inside the earth, and astrophysical neutrinos produced outside of our solar system during supernovae and other extreme processes most of which are still unknown. Continue reading “Hunting Neutrinos In The Antarctic”