BYLINE: Greg Cunningham, with contributions by Chris Driver and Dawn Levy
Newswise — David Radford has built his career on helping his fellow scientists see what would otherwise remain invisible. A nuclear physicist at the U.S. Department of Energy’s Oak Ridge National Laboratory, Radford specializes in creating exquisitely sensitive detectors to peer into the mysteries of the atomic nucleus, thereby illuminating the nature of matter.
In a fitting capstone to a long career seeking scientific truth, Radford’s detectors will lie at the heart of the massive LEGEND-1000 project, which will seek out an elusive nuclear decay process known as neutrinoless double beta decay.
You’d have never bet on that outcome if you looked at his family, though.
Born in New Zealand to a musician father with two brothers who became an engineer and a preacher, respectively, Radford was the only one in his family to pursue a career as a scientist. But if you look closely enough, you can detect the throughline.
“My wife is an artist, and I see what I do in the same light as what she does,” he said. “Humanity has a basic need to see beauty and to visualize the world in new and different ways. You can help them do that through art. But you can also do that as a scientist.”
Building out the scientific toolkit
The last half of Radford’s 40-year career has been consumed by a very particular quest for scientific understanding of the world around us. To gain that understanding, he has built tools to delve into the mysteries of the atomic nucleus and the neutrino—one of the most abundant particles in the universe and perhaps the least understood.
Radford and his collaborators have designed and built a new type of enriched germanium detector. Hundreds of the detectors will lie at the heart of the massive LEGEND-1000 experiment to search for a long theorized but never-before-seen nuclear process. Radford leads the international Consolidated Project Office for LEGEND-1000, where he coordinates activities across all the institutions contributing to the project, which will seek out a ghostly signal known as neutrinoless double beta decay that could shed light on one of the biggest mysteries currently perplexing physicists.
Germanium, which was originally used in diodes and as a semiconductor in transistors, has many special properties that make it ideal for pursuing hard-to-see signals. Most important is the fact that it has high efficiency and the best energy resolution for detecting gamma rays. When a nucleus decays, it often releases a gamma ray which can be observed and measured to provide clues to what is going on in the nucleus when it breaks down.
A physicist at heart, Radford was drawn to germanium detector design and construction because he needed capabilities from detectors that did not yet exist in order to pursue the science that drove him.
“When it comes to detectors, I like to think of myself not as the captain or a rower, but actually as the ship builder,” Radford said. “I like to build the tools that we need. But it’s really for the science. I build them partly so that I can use them myself.”
For much of his career, Radford worked in nuclear structure physics, using gamma ray detection and other techniques to study how protons and neutrons inside atomic nuclei give rise to the many complex behaviors observed by researchers. He developed tools to observe and study unstable neutron-rich nuclei, created software still used around the world and helped advance the detector and data analysis systems behind arrays such as GRETINA and its successor GRETA. He also invented the inverted-coaxial point-contact high-purity germanium detector design, an innovation later adopted for LEGEND because it allows larger detectors and reduces background signal.
In 1997, Radford joined ORNL, which is where his work with neutrinos began. Neutrinos are the second most common particles in the universe (after photons), but perhaps the least understood. They barely interact with matter, having no electric charge and a tiny amount of mass. Indeed, their true mass is unknown, and the fact they have any at all puts them in conflict with the Standard Model of Physics.
The Standard Model has been extraordinarily successful, but it does not fully account for neutrinos as we know them today. For Radford, that gap is exactly what makes the problem irresistible. “You always want to prove that what we understand about the universe is incomplete,” he said. “That’s the ultimate goal — to rewrite a piece of the scientific study.”
Neutrinos — and more specifically double beta decay with no emitted neutrinos — are at the heart of LEGEND’s quest to rewrite our understanding of the nature of matter and the formation of the universe.
A rare atomic decay is LEGEND’s quest
In ordinary double beta decay, two neutrons in a nucleus transform into two protons and emit two electrons and two antineutrinos. In the neutrinoless version, theory holds that the two antineutrinos would effectively cancel each other out, something that can happen only if the neutrino is its own antiparticle — what physicists call a Majorana particle. For researchers, the signature would be subtle but unmistakable. Instead of some energy being carried away by neutrinos, all the decay energy would show up in the two electrons. That would appear as a sharp energy peak, and germanium detectors are exceptionally good at measuring exactly that. Neutrinoless double-beta decay only occurs in a handful of isotopes, and LEGEND is searching for it in the germanium-76 isotope.
If LEGEND-1000 finds that energy peak, the implications would be enormous. It would show that lepton number — one of the bookkeeping rules physicists use to describe the number and type of particles involved in nuclear decay — is not always conserved in decays. It would demonstrate that the neutrino is fundamentally different from most other particles we know. And it could open a path toward understanding one of the biggest unsolved questions in cosmology: why does the universe contain so much more matter than antimatter?
According to current theory, the early universe should have produced matter and antimatter in nearly equal amounts. If that had happened perfectly, those two would have cancelled each other out, leaving behind a universe filled with formless energy rather than stars, planets and people. But clearly matter survived.
Radford is careful not to overstate the connection; observing neutrinoless double beta decay would not instantly solve the matter-antimatter asymmetry. What it would do is reveal a crucial missing piece. “I do know proving that neutrinoless double beta decay exists will give us a path forward,” he said. “And for the neutrino to explain the matter-antimatter imbalance, it needs to be a Majorana particle.”
Overcoming obstacles to change the path of science
The challenge is one of scale and the rarity of the signal being sought. Radford likes to explain it in terms that make even physicists pause: a metric ton of germanium contains about 1028 (1 followed by 28 zeroes) atoms. But even with so many candidate atoms, theory holds that scientists may be looking for only a few decay events over ten years of observation. That is why LEGEND-1000 must be enormous, use exquisitely pure materials and be buried deep underground at Italy’s Gran Sasso National Laboratory, where rock overhead helps block cosmic rays and other interference.
The experiment will also use shielding and clever background rejection techniques to eliminate ordinary radiation from dust, rock and the materials of the apparatus itself. Radford’s detector design helps here too. By making detectors larger, LEGEND can use fewer of them, reducing the amount of surrounding material that could generate unwanted signals. In a search for an event this rare, success depends not only on seeing the needle in the haystack, but on removing almost all of the haystack first.
If LEGEND is successful in its intended purpose, it would serve as a fitting capstone to a career spent pursuing scientific discovery and the tools needed to facilitate it. For Radford, the attraction is as simple now as it was when he was young: the chance to follow curiosity to the edge of what is known — and perhaps a little beyond it.
UT-Battelle manages ORNL for the Department of Energy’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.
