In Ontario, Canada, a container of completely pure water lay buried beneath kilometers of rock. When an undetectable particle quickly collided with its molecules, the water tank flashed.
According to Science Alert, this marks the initial instance of water being employed to identify an antineutrino particle, which emerged from a nuclear reactor situated over 240 kilometers (150 miles) away. This momentous discovery presents the possibility of conducting affordable and safe neutrino experiments and monitoring using easily accessible materials.
What Are Antineutrinos?
Neutrinos are plentiful particles in the Universe that could provide valuable information about the cosmos, as per the US Department of Energy. However, they are barely visible because of their lack of mass, charge, and interaction with other particles. Often called ghost particles, the properties of neutrinos pose a challenge for researchers to differentiate between the particle and its antiparticle, antineutrino.
Meanwhile, Science Alert reports that antineutrinos act as the opposite of neutrinos, and scientists can only distinguish the two based on the presence of either an electron or positron accompanying each particle.
Neutrinos and antineutrinos are created during nuclear beta decay, whereby a neutron eliminates a proton to generate an electron, and either a neutrino or antineutrino. By reacting with a proton, electron antineutrinos produce a neutron and a positron in a mechanism referred to as inverse beta decay.
To identify that reaction, large liquid containers containing photomultiplier tubes are utilized to observe the glow of Cherenkov radiation created by charged particles traveling faster than light through the liquid. Thus, the photomultiplier tubes have high sensitivity to weak light.
Despite being abundant in the universe, the nature of neutrinos, as well as their mass and interaction properties, make them challenging to observe. Antineutrinos have opposite characteristics to neutrinos, and researchers solely differentiate between the two by the presence of an accompanying electron or positron.
Electron antineutrinos are released during nuclear beta decay and can stimulate inverse beta decay by interacting with a proton to produce a neutron and positron. The observation of this mechanism is performed in large liquid containers fitted with photomultiplier tubes, which are highly sensitive to weak light.
Pure Water Used To Detect Antineutrinos
The SNO+ experiment as discussed in the study, titled "Evidence of Antineutrinos from Distant Reactors Using Pure Water at SNO+" published in the Physical Review Letters, is the first observation of reactor antineutrinos with a water Cherenkov detector.
The experiment, which is conducting a high-sensitivity search for neutrinoless double beta decay using a tellurium-loaded liquid scintillator, also measured neutrino mass splitting using reactor antineutrinos.
The SNO+ detector has the lowest reported rates of background-inducing muons from the atmosphere among water Cherenkov detectors, which significantly improved the team's chances of detecting antineutrinos, Phys.org reported. The detector is large and made of acrylic, with ultra-pure water providing a large volume for particles coming in and producing light.
Physicists are searching for a coincidence signal, which indicates the presence of antineutrinos when trying to detect them from reactors. The SNO+ collaboration searched for this signal in data collected during 190 days, using two independent analyses.
They achieved this by having high neutron detection efficiency and therefore detecting the 2.2-MeV signal produced when a neutron produced in an IBD captures hydrogen in water regardless of the energy of the incident antineutrino.
In the future, the study could inspire additional antineutrino searches and measurements using pure water Cherenkov detectors. Cherenkov detectors can distinguish coincidence signals from unrelated background signals because they provide an estimated direction for incident particles, which has not yet been achieved using scintillator detectors.
RELATED ARTICLE: Neutrinos Created by a Particle Collider at CERN for the First Time, Deepening Understanding of Their Role in the Universe
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