DARPA Muon Scanner to Search for Tunnels or Monitor Volcanoes

The Defense Department and other federal agencies have sought advanced sources that generate gamma rays, X-rays, neutrons, protons, and electrons to enable a variety of scientific, commercial, and defense applications – from medical diagnostics, to scans of cargo containers for dangerous materials, to non-destructive testing of aircraft and their parts to see internal defects. But none of these sources can image through concrete walls several meters thick, map the core of a volcano from the outside, or peer deep underground to locate chambers and tunnels. For such imaging capabilities, a more powerful particle is needed.

DARPA’s Muons for Science & Security program (MuS2 – pronounced Mew-S-2) aims to create a compact source of deeply penetrating subatomic particles known as muons. Muons are similar to electrons but about 200 times heavier. At high energy, muons can travel easily through dozens to hundreds of meters of water, solid rock, or soil. Producing muons, however, is a challenge, because it requires a very high-energy, giga-electronvolt (GeV) particle source. Currently, two primary sources for muons exist. Cosmic ray interactions in the upper atmosphere naturally generate muons as they descend to Earth in created particle showers. Harnessing these muons for imaging is tedious and not very practical. Cosmic muons have played a role in special projects, such as when scientists used them to image interior chambers of the great pyramids in Egypt. Given the small number of muons that reach the Earth’s surface and the divergent paths they travel through the atmosphere, it can take days to months to capture enough muon data to produce meaningful results. Muons can also be generated terrestrially. But making muons requires such high-energy particles that production is limited to large physics research facilities such as the United States’ Fermilab national particle accelerator in Illinois and the European CERN accelerator in Switzerland.

“Our goal is to develop a new, terrestrial muon source that doesn’t require large accelerators and allows us to create directional beams of muons at relevant energies, from 10s to 100s of GeVs – to either image or characterize materials,” said Mark Wrobel, MuS2 program manager in DARPA’s Defense Sciences Office. “Enabling this program is high-peak-power laser technology that has been steadily advancing and can potentially create the conditions for muon production in a compact form factor. MuS2 will lay the ground work needed to examine the feasibility of developing compact and transportable muon sources.”

MuS2 aims to employ what’s called laser-plasma acceleration (LPA) to initially create 10 GeV particles in the space of tens of centimeters compared to hundreds of meters needed for state-of-the art linear accelerators. Ultimately, MuS2 seeks to develop scalable and practical processes to produce conditions that can create muons exceeding 100 GeV through innovations in LPA, target design, and compact laser driver technology.

Muons are sensitive to density variation as they penetrate materials, which makes them particularly advantageous for locating voids in solid structures. If MuS2 and any follow-on efforts are successful, whole buildings could be scanned from the outside to characterize internal structures and detect the presence of threat materials such as special nuclear materials. Other potential applications include rapidly mapping the location of underground tunnels and chambers hundreds of meters below the Earth’s surface.

MuS2 is a four-year program divided into two phases. During the 24-month first phase, teams will conduct initial modeling and scaling studies and use experiments to validate models as well as attempt to produce 10 GeV muons. In the second 24-month phase, teams will aim to develop scalable accelerator designs for 100 GeV or greater and produce relevant numbers of muons for practical applications.

People will be able to use 1-2 meter long laser accelerators to generate muons instead of waiting months for cosmic ray generated muons to form useful detecctions. Several muon generators and detectors would be able to map out buildings, volanoes or scan underground up to kilometers to detect voids or different density deposits.

Sandia Labs has a publication discussing the use of Muons to detect sub-meter diameter underground tunnels.

Muon scattering tomography can be used to distinguish between materials of different densities, provided there is sufficient density contrast. Results from these experiments using the analyses discussed herein are inconclusive. However, rock density does show a linear relationship with muon scattering density per rock volume for these samples when this ratio is greater than 0.10.

You need to have your detector on the other side of the muon generator source to detect the deflections caused by a denser object or the void of a tunnel.

Muon Scanning Using Natural Cosmic Rays Takes Months

The energy of cosmic rays is usually measured in units of MeV, for mega-electron volts, or GeV, for giga-electron volts. (One electron volt is the energy gained when an electron is accelerated through a potential difference of 1 volt). Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light). Muons are 200 times heavier than electrons and Protons are 8 times heavier than Muons.

Higher energy muons are faster and would enable deeper or further scanning.

For years, these scientists explored every corner of the Great Pyramid using muography, a non-invasive imaging technique that uses infrared cameras, 3D scanners, and cosmological particle detectors to see inside.