Spotting neutrinos is a thrilling scientific endeavor in and of itself, but it may also be a matter of national security.
Neutrinos — specifically, their corresponding partner, antineutrinos — are elusive, elementary particles that pass through most of the universe without being affected.
One way that scientists have attempted to understand neutrinos is by devising sophisticated experiments to detect neutrinos near nuclear reactors. This is because neutrinos are known to be produced when radioactive material decays during a nuclear reaction.
Initially, detectors had to be placed deep underground to reduce interference from cosmic radiation. Later, scientists developed detectors that could operate effectively above ground.
As part of the neutrino detection and measurement process, physicists are able to trace the signature of certain neutrinos —which may also provide information valuable to nuclear safety, containment, and disposal. Refining this information, scientists say, might tell us about such things as whether a nuclear reactor is producing weapons-grade plutonium, where such production is occurring, and whether stored plutonium is being destroyed in accordance with a treaty.
Today, as policymakers in the United States discuss the possibility of re-engaging in nuclear treaty talks with Iran and North Korea, one of the tools for monitoring nuclear activity could come from neutrino detector experiments — including one with some important Yale connections.
Yale is a key partner in the Precision Reactor Oscillation and Spectrum (PROSPECT) experiment, a prominent neutrino detection collaboration located at the High Flux Isotope Reactor (HFIR) at the U.S. Department of Energy’s Oak Ridge National Laboratory in Tennessee.
PROSPECT studies electron antineutrinos being emitted from nuclear decays within the reactor. The goal is to detect a new form of matter — so-called “sterile antineutrinos.” More than 60 scientists from 10 universities and four national laboratories collaborate on PROSPECT.
YaleNews spoke with Karsten Heeger, professor and chair of physics, director of the Wright Lab, and principal investigator for PROSPECT, about how the experiment may be able to provide a new tool for safeguarding nuclear reactors.
This conversation has been edited and condensed.
What is the scientific connection between neutrino detection and the monitoring of another country’s nuclear program?
What we have shown is we can not only detect that neutrinos are coming out of a reactor, but also what isotope the neutrinos are coming from. When nuclear reactors burn fuel, the isotopic composition changes. We can see the resulting change in the energy distribution of neutrinos coming out of the reactor. By making a precise measurement of the neutrino spectrum from a reactor, we can get a fingerprint of the isotopic composition inside the reactor. This of course is the key feature because then it allows you to ask the question, “Can you detect whether highly enriched fuel is being diverted for other purposes?” You can essentially study the burnup of nuclear fuel and the enrichment in a reactor.
When did the nuclear safety aspect of neutrino detection emerge as a serious consideration?
This is being looked at closely right now, but people have had the idea for quite a while. People have suggested placing the detector close to a reactor, essentially right next to the facility, or even inside the facility. But it was never clear whether the technology could be advanced enough to learn something meaningful from the reactors. Now we have shown this is possible and potential use cases are being studied.
Why was it so difficult to gather meaningful information?
Historically, neutrino detectors require a lot of shielding — what we call “overburden” — because neutrinos create such a faint signal and there is a lot of environmental background interference, including cosmic rays that come from the atmosphere. This has always been a limitation. When you want to make a measurement close to a reactor or next to a reactor, you’re bombarded with all of this environmental background noise.
What the PROSPECT detector has demonstrated is that we can make a precise measurement of the neutrinos and energy that comes out of the reactor, even in the presence of these backgrounds. We have designed a detector with a novel detection liquid [a lithium-doped liquid scintillator] that allows us to pull out the neutrino signature with good confidence. Results from PROSPECT were recently published in the journal Physical Review D as an editor’s suggestion. Another important feature of PROSPECT is that it is room sized. It fits inside the reactor building. We’ve extrapolated that we could even put it on a truck and drive it up to a reactor.
What is the likelihood that neutrino detection will play a role in nuclear monitoring in the short term?
There have been some experimental efforts to pursue this and a current study is now trying to sort through these applications. But one thing we know is that organizations like the International Atomic Energy Agency (IAEA) already have established tools for monitoring inside a reactor facility that are less expensive than neutrino detectors.
So why pursue this avenue?
I think it gives you the possibility of scientific engagement with other countries, which is something you can’t put a price tag on. So often, we wonder, “How do you engage with countries like North Korea?” There is monitoring you can do, but what you really want is to engage the technical and scientific community there. We can build a detector there for a scientific purpose next to a reactor facility and you actually start working with the scientific personnel there and build relationships. That gives you much more than what you learn from monitoring alone.
Are you at all surprised that neutrino science has progressed to the point where it is being discussed in terms of national security?
It took us 25 years to make the first measurement of a neutrino. People would have never thought even 20 years ago that it would be possible to make a precision measurement with a detector that isn’t deep underground. But we’ve seen amazing advancement. I think it’s accurate to say that neutrino physics has now reached a point of intersection between policy, societal impact, and fundamental science.