by: Chris Patrick, Jefferson Lab
A newly invented detector is allowing physicists at the US Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Lab, CSA CSM) to “see” neutrons like never before. Fresh insight from these devices has improved operation of the lab’s powerful electron accelerator, which is used in nuclear physics studies of the atom’s nucleus.
The path toward this new invention began in 2012. The lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility, was in the midst of an energy upgrade. Among other tasks, this upgrade included installation of additional cryomodules, the components of the machine that actually accelerate electrons. These redesigned, state-of the-art cryomodules could accelerate electrons to higher energies than before, but this came with a tradeoff: They could also produce more radiation than their predecessors.
“That means that we had to be careful. We must measure that, we have to keep people away from it, and we have to understand what’s going on,” said Pavel Degtiarenko, a scientist in the Radiation Control (RadCon) department, the group overseeing the protection of people and equipment from radiation at Jefferson Lab.
To minimize the radiation, RadCon first needed a way to monitor it. However, none of the usual radiation detectors were both sensitive enough to provide meaningful measurements of the radiation and hardy enough to work long-term.
One way to monitor radiation is to measure neutrons, a component of radiation produced at higher energies. Many neutron detectors use a rare form of helium, helium-3, which lacks one neutron in its nucleus compared to the most abundant form, helium-4. When a neutron is near helium-3, the neutron-lacking nucleus captures it, producing two particles.
“They are charged, and you can detect them with various means,” Degtiarenko said. “There are different detectors based on that.”
However, in CEBAF’s tunnel, X-rays and gamma rays are also produced from the same process that makes the neutrons. These plentiful high-energy photons tend to block out the neutron signal in standard neutron detectors.
A New Device
In 2016, Degtiarenko succeeded in designing a new type of neutron detector for CEBAF to overcome these challenges. The device is called the Neutron Dose Rate Meter with Extended Capabilities (NDX). The technology underlying the device was issued patent # 10,281,600 in May 2019.*
The NDX contains helium-3 in an ionization chamber, which absorbs neutrons and converts them into a tiny but measurable electric current. To combat signal clouding from photons, it also features a second ionization chamber that contains helium-4. Whereas the neutrons mostly interact with helium-3, the photons interact with both types of helium equally. The electric current from the helium-4 ionization chamber measures the photon radiation. The difference between the helium-3 and helium-4 currents is proportional to the neutron signal.
“It distinguishes between them and quantifies them pretty reliably, so we know how much of each type of radiation is there,” Degtiarenko said.
The NDX detectors remain stable in high levels of radiation. Twenty-one of these detectors were installed around CEBAF’s cryomodules in July. Each detector reports a reading every second.
Improving CEBAF Operations
Jay Benesch is a staff scientist in the Accelerator Operations, Research and Development Division. He’s been tracking field emission, the process that produces radiation in the cryomodules, since 1995. He has been using the neutron signal from the NDX detectors to reduce field emission in the cryomodules.
In field emission, the electric field inside cryomodules accelerates electrons with so much energy that they hit the atomic nuclei of nearby materials and knock out particles including neutrons. Some of these resulting nuclei are radioactive and produce gamma radiation when they decay.
“Field emission can be an issue when people have to work on that equipment because of the residual radiation present,” Benesch said. “What some of us feared long term was that the activity would get so large that one couldn’t even walk past the cryomodules, and that would make maintenance work very difficult.”
Field emission depends on the accelerating gradient of a cryomodule. This gradient is a measure of how much acceleration each of the eight components of the cryomodules, called cavities, is providing for the electrons in the electron beam. In CEBAF, this parameter can be adjusted.
“However, when the cryomodule is in the tunnel, you don’t see it, you don’t know how much field emission there is,” Degtiarenko said. “Right now, this detector is the only means to see that. It’s like right now, they’re operating with open eyes.”
Benesch used the NDX detector readings to tweak the cavities and reduce field emission.
“What I did was go through the region, cavity by cavity, lowering and raising gradients to try to actually increase the energy gain while lowering the total neutron dose in the region,” Benesch said.
He was eventually able to reduce neutron radiation in the cryomodules 45-78%, depending on the region. This reduction also came with a 2% increase in available energy in each of CEBAF’s two straight sections. As new field emission appears, he continues to adjust the accelerating gradient to mitigate its effects.
Benesch also noticed that if CEBAF’s electron beam is not properly focused or is mislocated, the neutron signal on some of the NDX detectors goes up. This rise in neutron signal only appears when the beam is on, allowing him to see what fraction of the neutron signal originates from beam issues.
“That’s something we didn’t know prior to the advent of these detectors,” Benesch said. “We’ve known for a decade that there’s more activation in some of these cryomodules, but we didn’t know why. Now we know we have beam loss in that region. We’re just in the first steps of trying to figure out how to deal with that.”
NDX for the Win-Win
While these detectors aren’t cheap, they improve accelerator operations at less than one-tenth the cost of a cryomodule refurbishment.
“Pavel saw the need and used his considerable accelerator radiation and detector knowledge to address it,” said Keith Welch, head of the RadCon department. “Our primary mission is radiation safety. But it’s great to contribute directly to improving the operation of the accelerator.”
The detectors also work toward the goal of radiation safety. Reducing field emission in the cryomodules reduces radiation, resulting in less damage to equipment and less residual radiation in the accelerator tunnel once CEBAF is turned off.
“Those outcomes contribute to an overall reduction in radiation exposure to the workforce in the long run,” Welch said.
The first two NDX prototypes were built in 2017 with funding from the Experimental Nuclear Physics Division, then tested in Jefferson Lab’s experimental halls and the CEBAF accelerator tunnel.
“Some of the design, assembly and testing I did myself, but this project got a lot of help from other people,” Degtiarenko said.
The detectors currently in CEBAF’s tunnel were slightly redesigned and built with a team from the Accelerator Division. The two original prototype detectors are still in use, measuring radiation fields around sensitive electronics in one of the experimental halls. The Physics Division may eventually deploy them in other locations where radiation levels are elevated.
Another unique aspect of the NDX detector is its ability to convert a signal of neutron radiation into units of damage to human tissue. Higher-energy neutrons yield more damage.
“That means we can place the detector in an unknown neutron field and say, Okay this place produces this much damage,” Degtiarenko said.
The detector can measure higher levels of radiation than CEBAF’s cryomodules can produce and may find applications in different industries. Next, Degtiarenko hopes to build a lighter version of the detector.
“The prototype was designed just to show the principle,” he said. “It’s pretty bulky. For the Accelerator Division, that wasn’t an obstacle. The current modular design is still heavy, but it’s more convenient than the prototype.”
For now, the NDX detectors will continue reading neutron signals in CEBAF’s tunnel to optimize accelerator operations.
* NDX and related technologies are available for licensing. For more information about this and other Jefferson Lab technologies, contact Drew Weisenberger, Jefferson Lab chief technology officer.