Optical fibers and miniature silicon sensors designed for nuclear physics experiments are being sent underground. Their new mission is to protect drinking water supplies near upcoming commercial fusion power plants. The device is called the Subsurface Continuous Radioisotope Environmental Monitor. 

It watches over soil and groundwater in real time. Its main job is to find any leaks of tritium. Tritium is a rare radioactive isotope of hydrogen used to fuel fusion reactions. If it escapes, it can seep directly into local aquifers.

The system spots radiation using a two-step detection chain. Thin scintillator fibers are buried in the ground around the plant site. Radioactive beta particles strike the fibers and trigger tiny pulses of light. 

These photons travel along the strands to compact silicon photomultipliers, or SiPMs. “SiPMs convert photons into electric signals,” noted the US Department of Energy’s (DOE) Thomas Jefferson National Accelerator Facility. Connected computers then process the pulses to detect and track underground radiation levels instantly.

“Jefferson Lab was the first large-scale user of SiPMs on the planet,” said Brian Kross, a principal technician in the lab’s Detector Development Group. “They’re a type of light sensor, but the reason we needed these for experiments is because they can operate in magnetic fields.”

Conventional radiation monitors have long relied on traditional glass vacuum tubes. Those bulky glass instruments break easily and need high electrical voltage to run. They also malfunction whenever they encounter stray electromagnetic fields. Solid-state SiPMs eliminate these limitations completely. They need very little electrical power. 

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