In the subterranean recesses of a world-class neutron beam facility within Oak Ridge National Lab (ORNL), there is a neutrino haven. A narrow basement hallway deemed “Neutrino Alley” is lined with detectors of various shapes and sizes. These detectors are metaphorical welcome mats, beckoning the teensy neutrino particles to kick up their feet, for the briefest of moments, and interact under scientists’ watchful eyes.
Trillions of neutrinos stream harmlessly through you every second of every day. Mostly, these invisible, non-charged particles are traveling to Earth from far-away sources, like our sun, or even a distant supernova. The stories they could tell from their travels are fantastical, but their lips are sealed very tight. And when they do talk, they whisper.
For particle physicists who seek to understand the attributes and affinities of neutrinos, they dream of building detectors in very “quiet” rooms — often underground — and next to powerful machines producing neutrinos with a frenzy. This combination increases the chance of a neutrino noticeably scattering off a nucleus inside of their controlled experiment. Collect enough data about enough scatters and the mysterious particle’s scroll unrolls a little more.
This dream is being fulfilled by the COHERENT Experiment, a modest yet mighty collaboration that operates the small-scale detectors in Neutrino Alley. In the experimental particle physics landscape, the trend has been towards grander and more expensive detectors. In some cases, thousands of people contribute within these enormous projects. By contrast, one of COHERENT’s most unique features is that detectors are usually managed by one to a few graduate students from start to finish. Project ownership at this level is not only rare, but wildly educational.
Kate Scholberg, Arts & Sciences Distinguished Professor of Physics and COHERENT collaborator, says the hands-on opportunities with Neutrino Alley detectors allow students to “sample the entire experimental process from design to prototype to construction to data-taking to data analysis.” Alongside mentors in the collaboration and staff at ORNL, students work through the inevitable hallmarks of research — “snags and bugs and bumps in the road along the way” — and “end up with deep expertise.”
One of COHERENT’s newest detectors exemplifies this student-first dynamic at a whole new scale. Christened “NaIvETe” (sodium iodide (NaI) neutrino (v) Experiment TonnE-scale), the detector will weigh over 45,000 pounds when completed. Think of it like a jewelry box filled with crystals. The crystals sparkle when incoming neutrinos from the beam above Neutrino Alley rattle the sodium and iodine nuclei inside each crystal. The sparkling light is collected by physicists that study rare processes.
The work is cross-generational. Hundreds of NaI crystals have to be “interviewed” for quality and performance in student-run test setups. Thousands of pounds of steel are machined at the in-house machine shop, a coordination between experienced machinists and qualified graduate students. A majority of the electronics are hand-crafted by undergrad and grad students, providing valuable soldering and fabrication experience. Likewise, even postdocs and faculty jump in on the project to feel the unique satisfaction of contributing to science with their own hands.
Now a postdoc at Lawrence Livermore National Lab, Samuel Hedges designed NaIvETe (and its predecessor NaIvE) when he was a graduate student at Duke. “I had very little experience starting out,” Hedges says. “The collaborators on COHERENT were a resource that make NaIvE possible.”
The COHERENT ecosystem is also exceptional in other ways. “We get the neutrinos for free, and most of the detector components came to us second-hand,” Hedges notes.
Indeed, 2.5 tonnes of NaI crystals found their way to institutions like Duke and ORNL as hand-me-downs. Their second life within NaIvETe facilitates a common cause for over a dozen apprenticing physicists. Everyone arrives to this project with something special to learn, and also to contribute. Without a shared desire to press into the unknown, a project like NaIvETe stays on paper.
The triumph of the last year is that the first portion of NaIvETe was successfully deployed at ORNL and is now taking data. This summer, students will return to Neutrino Alley to continue building the tonne-scale neutrino welcome mat. Now all that is needed are the whispering guests.
Adryanna Major is a Duke Physics PhD student in Kate Scholberg’s experimental neutrino group and a collaborator in the COHERENT Experiment.