Measurement anomalies in nuclear reactors not related to a new particle

 By John Timmer 

January 11, 2023 6:19 PM

Diagram showing the placement of STEREO's array of detectors next to a nuclear reactor (left) (right).

                          The oddest known particle is probably a neutrino. They interact with other matter only through the weak force, which means they hardly ever interact with anything. They are much, much lighter than any other particle with mass. Neutrinos come in three different flavours or sorts, and no single particle has a known identity. It may instead be thought of as a quantum superposition of all three tastes, oscillating between them.

Not only that, but certain odd data have raised the possibility of a fourth form of neutrino that is undetectable since it doesn't even interact via the weak force. Although the existence of these "sterile neutrinos" makes them difficult to directly address, they may be able to explain both the existence of dark matter and the low masses of the other neutrinos.

Odd measurement findings in tests using additional flavours of neutrinos provide the clearest indications of their existence. However, a recent research disqualifies sterile neutrinos as the cause of one of these anomalies—while also establishing the veracity of the aberrant findings.

Spotting the undetectable

There are two ways that particles can interact with other matter: directly, or by decomposing into one or more other particles that do. Sterile neutrinos are therefore undetectable for this reason. They should not degrade into anything since they are fundamental particles. Additionally, because of their tiny masses, they can only interact with other matter gravitationally, making direct detection impossible.

Instead, we might be able to find them by observing neutrino oscillations. Then, you may try to detect those neutrinos using an experiment that generates a certain kind of neutrinos at a known rate. Some of the neutrinos you generated will oscillate into the identity of sterile neutrinos if there are any, making them undetectable. Thus, you find that you measure fewer neutrinos than you anticipated.

Nuclear reactors have experienced just that. Nuclear reactors create large quantities of neutrinos as one of the by-products of radioactive decay (powered by the weak force). However, measurements using adjacent detectors found roughly 6% fewer neutrinos than anticipated. That disparity might be explained by a quick oscillation into sterile neutrinos.

However, these experiments are quite challenging. Only a small percentage of the total number of neutrinos generated contact with detectors, which is extremely unusual. A further extremely complicated environment is a nuclear reactor. Even when you begin with a pure sample of a single radioactive isotope, decays swiftly produce a complex mixture of new elements, some of which are radioactive and others not. The reactor apparatus can produce new radioactive isotopes as a result of the neutrons emitted. It is therefore difficult to determine how many neutrinos you first make and what portion of those neutrinos will be detected by your detector.

It is difficult to know for sure whether any irregularities in neutrino observations are true for all the aforementioned reasons. Physicists frequently adopt a "wait and see" approach when there are signs that something unexpected is happening.

Actively waiting 

Physicists aren't all wait-and-see, though. Some of them set out to construct an experiment to see if the unusual readings were indicative of sterile neutrinos. The STEREO experiment is located next to a research reactor that uses a highly pure version of a single isotope of uranium to operate, greatly reducing the uncertainty associated with neutrino generation.

STEREO consists of a line of detectors constructed in an array along which the reactor is being moved away from. Neutrino oscillations have a predictable frequency over time and are probabilistic in nature. Additionally, because the neutrinos from the reactor are travelling at high speeds, the likelihood of an oscillation changes with distance (i.e., time means distance for a moving object). Therefore, the amount of oscillation you observe should vary depending on whatever detector in the array you are looking at.

As a result, the percentage of oscillating neutrinos in the whole population should change from the percentage at the fifth detector by the time they reach the first detector. Both detectors should show an abnormality, but it will seem different in each.

(Real enthusiasts of neutrinos will point out that there are certain oscillations that last long enough for all the neutrinos to have long since passed a detector that is so near the source. This is real. However, the particular oscillation in this case, from the electron antineutrino to the sterile neutrino, should happen swiftly. Additionally, all of the earlier aberrant data originated from detectors situated close to reactors.)

Still odd, but not sterile

By turning a proton into a neutron and releasing a positron, which can be detected by STEREO, neutrinos are able to be detected. The researchers utilised data from the reactor's unfueled state to acquire a feel of the background signal before subtracting it from their own data since other events, mostly caused by cosmic rays, might be mistaken for the positron's signal. The STEREO team could obtain approximately 400 neutrino detections every day when the reactor was operating, and they used data from more than 100,000 occurrences to produce the new report.

The measurements show that the abnormality is unquestionably there. STEREO detects less neutrinos than our reactor models predict should be generated. Nevertheless, a smooth probabilistic oscillation is not able to account for the variation in the anomaly from detector to detector. Therefore, you cannot support the existence of sterile neutrinos using the strange measurement data.

So why is there even a measurement anomaly? The researchers provide other evidence that implies we could be misinterpreting the specifics of the uranium fission processes, which could betray our expectations on the number of neutrinos generated. In other words, our current models of neutrino creation and behaviour could be adequate, but we still need to improve our understanding of the more commonplace process of fission.

Neutrinos that have been rendered infertile are still alive. The difficulties of dealing with neutrinos means that there will probably always be another experimental anomaly that sterile neutrinos may explain, even though this experiment excludes them as the cause of this particular anomaly.