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.
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.
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So.
Step 1: Build a device to quickly spot underground secret tunnels.
Step 2: Build a scrambler, or jammer, against such a device.
Step 3: Build a device providing an active stealth capability against such a device.
Step 4: Develop a passive stealth capability against such a device (if possible).
Step 5: Build a better device to quickly spot underground secret tunnels.
Job security.
The concept of a particle with the charge of an electron and bearing 13% of the mass of a proton being “highly penetrating” is contrary to basic radiation transport theory.
Additionally, using arrays of ion chambers, which are quite macroscopic and have poor energy resolution (relative to semi-conductor detectors), to digitize space is bordering on “not even wrong” or quasi-science in my humble opinion as someone trained years ago on these topics.
Attempting to piece together an image from magic particles that pass through everything but somehow intercept the detector. It is remarkable how this field of study has deteriorated/stalled since the 1930s.
“oh, we’ve got 8 counts on our detectors – that must be a mine-shaft!”
To heck with statistical methods! We’ve got 8 counts, and they’re either muons or neutrinos (or tramp uranium in the ion chambers).
“Highly penetrating” compared to electrons, certainly. I’m not quite clear on why they’d be more penetrating than protons of a comparable energy.
Proton would be least penetrating due to qty=mass/charge, followed by the supposed muon, with the light electron being the most penetrating. We weld with electrons.
Perhaps things get ‘strange’ at 99.999c… but they would be similarly strange for protons and electrons at those speeds. Considering mass goes to infinity as tending towards c, perhaps the muon is simply an electron, and the additional mass is not rest mass. Or maybe scientists are over-extending their interpretations of bubble chamber tracks.
The only way to suppose the muon is more penetrating than the electron is to assign magic properties to it, or to not understand what the observed species is. Where there are neutrinos involved, I become highly suspicious of caca de vaca.
We weld with electrons, sure, but that actually requires electrons to be efficiently stopped, not penetrating.
E=(MV^2)/2
momentum=MV
So lighter particles carry less momentum at any given energy. Until you get into the relativistic regime where most of the mass IS kinetic energy, anyway.
So, for a given energy, you’d expect a heavier particle to be more penetrating, because the higher momentum to charge means its course is less effected by interactions with charges inside the matter it’s passing through.
I’d assume on this basis that protons would actually be more penetrating than muons at any given energy.
But, of course, muons derived from cosmic rays are very high energy indeed, and thus very penetrating due to their energy. But it seems to me that for the same energy investment, you’d be better off just accelerating a beam of protons, if you want penetration.
Could it be that muons are being looked at for this purpose because of some undesired effect that using protons would cause that muons would not cause? I don’t know what undesired effect it might be — I’m just trying to imagine a reason why muons would be preferred if protons would penetrate farther.
People tend to get fixated on examples, rather than the basic principles they’re examples of. For instance, the way it’s been proposed to geoengineer by lofting sulfate particles into the high atmosphere, just because that’s what volcanos put up there, when some other sort of particle might be more effective, and have less harmful side effects.
We’re using muons for this purpose now, just because cosmic rays produce a continual shower of them at very high energies. So the proposal is to produce the muons artificially. Without evaluating whether some other particle makes more sense.
Sometimes humanity causes me to despair. It really does.
It is highly penetrating because it is losing energy so slowly with each interaction.
By far the largest cross section at >10 TeV is electron-positron pair production at a relative energy loss of between 0.1% and 1% per interaction. The other crossections (including pair production and a relative energy loss of >>1%) are small in comparison. So it’s not interacting *rarely* with matter, many times before it comes to a stop.
Wouldn’t this technology also be useful for Anti-Submarine Warfare (ASW)?
But submarines are equal in density to water, on average, which is why they’re capable of neutral buoyancy. You’d need a high enough resolution muon scan to resolve internal details of the submarine to detect it using this technique.
Given the likely power consumption and other requirements, then, you could only muon scan a submarine if you already knew its location. Or had a fixed location you knew submarines were likely to pass through.
I suppose there might be some potential utility in building a muon source into a torpedo, with the sensor end in a submarine, so that once you knew another submarine was present nearby you might get a look at internal details.
But as an active scanning technique, the moment you use it, the target could be aware they were being looked at, and quite likely from which direction. Similar to sonar in that regard.
Not sure if you’re recommending using a torpedo as a drone here – but that would be a very bad idea. Once you have a torpedo in the water it’s ‘Weapons Free’ for every one who can hear the cavitation.
Maybe droppable Muonbuoys, interlaced with the sonobuoys for an active/passive detection pattern.
Offtopic: compact muon source -> “cheap” muon -> affordable muon-catalysed fusion?
Ah, not really. First, muon catalyzed fusion turns out not to be feasible because the muon doesn’t survive long enough to catalyze enough reactions to pay back the energy cost of creating it.
But also because this is supposed to be a source of particularly energetic muons, and you catalyze fusion with particularly “cold” muons.
Muons for muon catalyzed fusion are created at 5 GeV, which is a few factors slower than the 10s of GeV minimum for DARPA muon “underground bunker” detector. Making the same amount of muons with a fraction of the power would go a long way towards making muon catalyzed fusion a practical source of power.
The rest mass of a muon is about 106 MeV. Yes, they’re created at about 5GeV, which is to say that, for fusion purposes, you’ve already thrown away 99.5% of the energy investment just making them, because catalytic fusion requires muons at low enough energies to displace electrons in an atom’s orbitals, and all that excess energy ends up as heat.
The problem is that they’re unstable particles, and don’t last long enough to catalyze enough reactions to pay back the energy cost of creating them. Otherwise, great idea.
The same source says one muon will produce, on average 200 to 300 fusion events. 5 GeV is about 50× more than the 106 MeV rest mass of a muon. So, for D-T reactions, it’s about break even barring inefficiencies in muon production which this DARPA project is working on.