The enigma of the nuclear reactor has been solved, and there is no longer a need for additional particles
A long-standing enigma in the field of physics has finally been solved, and the answer is almost as stunning as "the butler did it." For the last ten years, physicists have been puzzling over the question of why nuclear reactors produce a lower number of particles called neutrinos than was anticipated. Some people have hypothesized that the mysterious particles of matter could be transforming into neutrinos that are more stranger and cannot be detected. Instead, the new findings provide concrete evidence for what previous research had hinted at, namely, that theorists underestimated the number of neutrinos that a reactor ought to create.
Georgia Karagiorgi, a particle physicist at Columbia University who was not involved in the investigation, described the findings as "not a surprise outcome, but an important one." "It's still something that I had an interest in seeing," she said.
Depending on the process by which they are generated, neutrinos may be classified as electron, muon, or tau neutrinos. Electron neutrinos stream from the Sun, muon neutrinos fall from the sky when cosmic rays contact the atmosphere, and tau neutrinos arise through the decay of tau particles, which may be generated using atom smashers. All three types of neutrinos can be found in the universe. Because the almost massless particles transform into other varieties as they travel, an electron neutrino that originates from the Sun may become a different sort before it reaches Earth.
A few investigations have raised the possibility that there is a fourth neutrino that is sterile. It is impossible for it to interact with regular matter, hence the only way it could come into being is if an ordinary neutrino were to mutate into it. Some strange findings suggested the existence of a sterile neutrino with a mass of around 1 electron volt (eV), which is at least ten times more than the weight of regular neutrinos.
These clues were given unanticipated confirmation in 2011, when two groups of theorists determined that nuclear reactors generated 6% less electron neutrinos, which are technically known as electron antineutrinos, than the theory predicted. The results revealed that the neutrinos that had been missing were being converted into sterile ones.
It was a difficult debate to follow. In the core of a reactor, a chain reaction causes uranium and plutonium nuclei to break apart, and antineutrinos are produced as a byproduct of the radioactive "beta decay" of the lighter nuclei that are left behind. During this kind of decay, a neutron that is present in a nucleus transforms into a proton while also giving out an electron and an electron antineutrino. In order for physicists to accurately forecast the overall flow of antineutrinos, they had to take into consideration the quantities and decay rates of a vast variety of nuclei.
The accounting suggested that there was a deficit, but in 2017, researchers working on the Daya Bay Reactor Neutrino Experiment in China cast doubt on the validity of that assertion. They looked at antineutrinos produced by six different commercial reactors, each of which burned fuel containing either 4% uranium-235 atoms, which are capable of keeping a chain reaction going, or 96% uranium-238 atoms, which are incapable of doing so. Neutrons produced by the fission of uranium-235 transform uranium-238 into plutonium-239, which also contributes to the maintenance of a chain reaction. This process occurs as the uranium-235 is consumed. Researchers from Daya Bay discovered that the antineutrino deficit decreased as the quantity of uranium-235 decreased, which indicates that theorists underestimated the flow of antineutrinos that originated from uranium-235.
Now, scientists working at a modest research reactor in France have verified what was previously thought to be a possibility. Neutrons in large quantities are generated by the reactor at the Laue-Langevin Institute (ILL), which is used for research on various materials. In addition to this, the fuel it utilizes is composed of 93% uranium-235. Therefore, researchers working with a neutrino detector called STEREO were able to determine the flow of antineutrinos coming from uranium-235 all by themselves by analysing the antineutrinos that came from it.
The detector is made up of six identical oil-filled segments that are lined up like teeth and cover a distance of between nine and eleven meters away from the core of the reactor. Sometimes, a proton in the oil may take in an electron antineutrino and, as a result, decay into a neutron while simultaneously emitting a positron. This process is similar to the opposite of beta decay. Light is produced by the positron as it travels through the oil, and the amount of light produced is proportional to the energy of the neutrino that was initially there.
Researchers working on the STEREO project demonstrated that the spectrum of energy possessed by electron antineutrinos remained unchanged regardless of the distance from the core. This discovery is in conflict with the theory that some neutrinos are transforming into sterile neutrinos. Lower energy neutrinos should transform more quickly than higher energy neutrinos, which would result in a shift in the spectrum as the neutrinos progress. As reported in today's issue of Nature, STEREO researchers demonstrated that the overall flow of antineutrinos emanating from uranium-235 was far lower than the value that was utilized in theoretical models.
According to David Lhuillier, a neutrino physicist at France's Atomic Energy Commission and spokesperson for the 26-member STEREO team, the observations, when taken as a whole, put an end to the reactor antineutrino deficit as evidence for a 1-eV sterile neutrino. In other words, the deficit no longer serves as evidence. "Could it be explained by a neutrino with no charge and a mass of around one electron volt?" The correct response is "no."
According to Lhuillier, the results of other tests, such as the one at Oak Ridge National Laboratory known as PROSPECT, had arrived at the same findings. According to Bryce Littlejohn, a neutrino physicist at the Illinois Institute of Technology and a PROSPECT coauthor, the new STEREO report has reduced uncertainties and neatly puts up the case in one neat package. "Rather than being a turning point in our lives, I view it more as a great summary of everything we've picked up along the way," she said.
The physicists have a strong hunch that they overstated the flow of antineutrinos that originate from uranium-235, but they don't know precisely how they did this. Neutrons from the reactor were used by researchers at ILL in the late 1980s to cause atoms to split by exposing foils containing plutonium-239 and uranium-235 to the neutrons. After that, they studied the energy spectrum of the electrons and counted the electrons that were produced as a result of the beta decay of the fission pieces. Twenty years later, theorists utilized those data to extrapolate the spectrum of the antineutrinos that had to arise at the same time as the electrons.
However, according to Patrick Huber, a theoretical physicist at Virginia Polytechnic Institute and State University and one of the theorists who identified the so-called reactor antineutrino anomaly, multiple lines of evidence now suggest those experiments may have overestimated the total number of electrons coming from the uranium-235 sample. "The problem is simply that the input data that we have been using has been incorrect."
According to Zahra Tabrizi, a theoretical particle physicist at Northwestern University, the unraveling of the riddle surrounding reactor neutrinos does not definitively exclude the existence of the sterile neutrino with a mass of 1 eV. She claims that there are still additional mysteries in neutrino physics that scientists have not been able to solve. When all neutrino research is considered, however, Huber argues that the evidence for the sterile neutrino is not very compelling: It does not provide a satisfactory overall fit to the data.