CNL is about to begin testing of a prototype special nuclear materials (SNM) detector. Dubbed ALARM (A Liquid Argon Radiation Monitor), this detector is based on a liquid-argon system initially developed to help detect the presence of dark matter.
But let’s back up a little. Special nuclear material, or SNM is a bit of a catch all term for radioactive fissile materials which could present a concern from the perspective of proliferation or weapons development. This includes elements such as uranium-233, uranium-235 and plutonium-239. While these elements may have peaceful applications, it is critical that they are closely monitored to ensure they aren’t diverted for more nefarious purposes. Many international borders deploy a range of technologies and measures to identify and prevent the undocumented movement of SNM; while the current solutions are effective, it is critical that they continue to evolve and improve.
With this goal in mind, CNL, enabled by AECL’s Federal Nuclear Science & Technology Work Plan (FNST), have been working to develop disruptive technologies for applications in non-proliferation and nuclear counter terrorism. The planned commissioning and testing of the ALARM detector is the current phase in the evolution of several projects over the past four years.
“This whole endeavour started in 2016 with an assessment of detector technologies that can potentially be of interest to nuclear security and safeguards applications,” explains Oleg Kamaev, an R&D Scientist and Head of Simulations & Measurements for the Advanced Radiation Technologies section in CNL’s Applied Physics Branch. “Together with Victor Golovko, we identified the liquid argon (LAr) detector technology developed by DEAP as a strong candidate for SNM detection in border security applications. This in turn led to the signing of an NDA and formalization of a collaborative approach to the advancement of this LAr technology.”
But what is DEAP and why would they have a need for an SNM detector technology? Short answer is, they don’t. They aren’t looking for SNM at all.
The DEAP Collaboration, is a group of over 60 researchers (including Nobel Laureate and Chalk River alum. Dr. Art McDonald) from institutions in Canada, the UK, Mexico, Germany, Spain, and Russia who have developed the DEAP-3600 detector – one of the world’s most sensitive experiments for the direct detection of dark matter. Dark matter, which has as yet remained undetected, is believed to far outweigh visible matter, making up about 25% of our universe. Its existence, so far, is inferred by its gravitational effects on stars and galaxies. The goal of the DEAP experiment is to directly observe and identify this dark matter component of the universe. This will be achieved by observing faint light pulses due to the elastic scattering of dark matter particles from liquid argon nuclei.
Because dark matter is so difficult (currently impossible) to directly detect, the DEAP 3600 is buried over two kilometres underground at SNOLAB near Sudbury. The ground above provides protection and filtering from cosmic ray muons which would otherwise compromise the detectors effectiveness. At the heart of the DEAP-3600 lies an ultraclean acrylic vessel loaded with ~3.3 tons of liquid argon.
“DEAP’s connection to nuclear security is that their technology is effectively one of the most sensitive neutron detectors on the planet,” explains Andrew Erlandson, Applied Physicist at CNL. “Though DEAP is not searching for neutrons per se, they’ve effectively laid all the ground work for demonstrating the efficacy of liquid argon as a detection medium for radiation.”
Our work here at CNL is intended to demonstrate significant improvements in detecting neutron and gamma radiation as compared to existing technology like RPMs which consist of plastic scintillator and helium-3 (or similar) tubes for neutron detection. Liquid argon is able to simultaneously detect both gamma and neutron radiation, and discriminate between the two with high precision through pulse-shape discrimination. In addition, LAr detectors can be easily scaled to arbitrary sizes (kg to tonnes) and form factors which can be customized to specific applications since LAr is a liquid and argon is very low cost.
The ALARM detector will function in much the same way as DEAP-3600 albeit in a somewhat simplified way. The liquid argon at the centre of the device is insulated from the environment and kept at very cold temperatures (87 K or -186 C). When struck by a neutron (or any other ionizing radiation), the interaction creates a large flash of ultraviolet light. That light is wavelength shifted to the visible spectrum, captured by photomultiplier tubes, amplified, and measured. The amount of detected light is proportional to the energy of the radiation. The timing characteristics of when the light was detected determines whether it was from a neutron or beta/gamma interaction.
Over the past few years, Evan Rand and the team in Applied Physics, supported by a Postdoctoral Fellow, Badamsambuu Jigmeddorj worked to further design, procure, assemble and prepare to operate the detector. ALARM is in the final stages of assembly and commissioning of the hardware is expected this year.
“It is an exciting project,” adds Oleg. “It draws together two of Canada’s “big science” teams in SNOLAB and CNL, is cutting edge technology, and we are optimistic that this new LAr prototype will lead to further developments in passive detection techniques which could be deployed in Canadian radiation portal monitors at the ports of entry, ultimately making a difference in global security.”
In addition, through the collaboration with DEAP, the team at CRL is able to provide support and share knowledge as the DEAP project matures and evolves with new projects on the horizon helping to solve one of the greatest physics mysteries of our time.
Andrew adds: “Towards the DEAP efforts, CNL is contributing our expertise in particle physics (detector development, data analysis/simulation, etc.) to the on-going search for dark matter as well as R&D towards next generation liquid argon dark matter detectors (DarkSide-20k, ARGO) which will be coming on line in the near future. While the problems we’re trying to solve at CNL are more terrestrial in nature than dark matter, one never knows which “out there” idea in fundamental science is the key to making the world a safer and more secure place. ”