Imagine a tiny particle, significantly less massive than an electron and carrying no electric charge. This, in essence, is a neutrino – likely one of the smallest particles in the known universe. Neutrinos may also be one of the most common, as they are produced by such significant and varying sources as stars, supernovae and even the nearest nuclear reactor.
So far, scientists have identified three distinct types of neutrino, each of which display unique interactions with normal atoms. But a relatively new theory proposes that a fourth type may exist — aptly named the sterile neutrino — which fails to interact with normal atoms at all. Enter the Precision Reactor Oscillation and Spectrum Experiment, or PROSPECT, which aims to find a conclusive answer to this pesky subatomic problem with the help of Drexel University assistant professors Michelle Dolinski and Russell Neilson.
The need for a theory such as this one came about when physicists gained enough data to realize that something wasn’t adding up. The detectors placed near nuclear reactors, which produce a great deal of neutrinos, consistently recorded fewer numbers of neutrinos than expected. This, scientists decided, could mean one of two things. One option is that the current understanding of nuclear physics as it occurs in nuclear reactors is insufficient. The other option is that there is another type of previously undiscovered neutrino coming through that simply cannot be detected — this would be the sterile neutrino.
Because neutrinos carry no charge of their own, the three known kinds are difficult to detect. Physicists must wait for one of the many millions of them to interact with an atom while passing through a detector, which will result in the deposit of a measurable amount of energy. The three known types of neutrinos react with normal atoms in three different ways. In other words, a fourth type of interaction has never been observed; if a fourth kind of neutrino does exist, that would mean it is incapable of interaction with normal atoms. This property would make the sterile neutrino very difficult, if not impossible, to observe with a regular neutrino detector. The PROSPECT detector is built specifically for the task.
The PROSPECT research team is based out of Yale University, but includes 68 scientists spread over ten universities. According to a May 31 release on the project’s website, the project, which has been in development for three years, received a federal grant totalling three million dollars to fund their search for the sterile neutrino. Dolinski and Neilson were chosen to lead the Drexel team, which also includes post-doctoral students Yung-Ruey Yen, PhD, and Jonathan Insler, PhD, as well as graduate student Kelley Commeford, and several undergraduate students.
“The challenge of this experiment is to operate a detector very close to a nuclear reactor core,” Dolinski explained in an email correspondence.
The PROSPECT detector will be situated just outside the pool wall of the High Flux Isotope Reactor at Oak Ridge National Laboratory. This means that naturally occurring radioactivity, neutrons from the reactor and other sources of background noise will compete with the neutrino interactions the team is looking for, but hopefully the detector’s design will prevail.
“The PROSPECT detector is designed to succeed in this high background environment by being able to distinguish our neutrino signals from all of our potential backgrounds,” Dolinski continued.
Neilson also commented on the uniqueness of PROSPECT’s design and functionality in an email interview.
“PROSPECT is bringing neutrino physics to the surface. This type of experiment has until now been [done] with detectors deep underground, shielded from cosmic rays that can mimic the signature of neutrino events. The ability to operate a precision neutrino detector at the surface opens up new applications, such as monitoring the fuel content of nuclear reactors,” he explained.
The real question is, should the PROSPECT team discover that sterile neutrinos actually do exist, what does that mean for the scientific world?
“They would be a totally new kind of particle that would open up a new experimental and theoretical physics program to understand their properties. Understanding the properties of sterile neutrinos would give us a new basis for understanding the basic building blocks of the universe,” Dolinski stated.
“PROSPECT is an exciting experiment to be a part of because of the potential to provide a definitive result on the question of sterile neutrinos in just a few years, with the possibility of discovering a new particle. Discovering new particles is the holy grail of particle physics, especially a totally new kind of particle like sterile neutrinos,” Neilson explained.
Whether or not the PROSPECT team confirms the existence of sterile neutrinos, they will hopefully find a conclusive answer to what’s going on in these nuclear reactors. Either way, it is clear that Dolinski and Neilson are a part of something big, even in the pursuit of something very, very small.