In the mid-1960s, the Nobel Prize–winning physicist Luis Alvarez had a wild idea. He proposed using muons, highly penetrating subatomic particles created when cosmic rays strike Earth’s atmosphere, to search for hidden chambers within one of the pyramids of Giza.
These muon particles are heavyweight cousins of electrons that travel close to the speed of light. They can penetrate through many meters of solid rock, including the limestone and granite blocks used to build the pyramids. But some of the muons will be absorbed by this dense material, meaning that they can be used to essentially “X-ray” a pyramid, revealing its inner structure. So in 1968, Alvarez and his colleagues began making muon measurements from a chamber located at the base of the Pyramid of Khafre.
They didn’t find a hidden chamber, but they did confirm the feasibility of what has come to be called muon tomography. Physicists have since used the technique to discover hidden access shafts above tunnels, study magma chambers within volcanos, and even probe the damaged reactors at Fukushima. And, in 2017, muon measurements finally revealed a hidden chamber in one of the pyramids of Giza—just not the pyramid that Alvarez had chosen to explore.
You too can perform similar experiments with equipment that you can build yourself for only US $100 or so.
While some well-documented designs are available for low-cost muon detectors (in particular, the Cosmic Watch project from MIT), I decided to pursue a simpler—and slightly cheaper—approach. I purchased two Geiger-counter kits, each costing only $23. Although it’s called a “kit,” this board in fact comes fully assembled minus the key component: a Geiger-Müller (or GM) tube for detecting ionizing radiation. It also comes with no documentation.
The lack of documentation wasn’t a problem once I found a good source for information about this board—including a pointer to valuable instructions for how to set the tube’s anode voltage.
The muon detector uses two Geiger-Müller tubes [top], each inserted into a sensor board [bottom right]. Both boards are connected to an Arduino Nano microcontroller [bottom left].James Provost
For the GM tubes, I decided to buy what I understood to be good ones: Russian-made SBM-20 tubes. Many of these are listed on eBay by sellers in Ukraine, but I was able to obtain a pair of such tubes from a supplier in the United States for just $49.
“Why two kits and two tubes?” you might ask. It’s because GM tubes don’t react just to muons. Most of the time, they’re triggered by ionizing particles given off by radioactive substances in the environment, such as the daughter products of radon in the air.
Proving that the results reflected the flux of cosmic-ray muons wasn’t difficult.
To distinguish the high-energy cosmic-ray muons from the other, lower-energy particles isn’t hard, though. Just apply what physicists call the coincidence method: Detect only when two nearby tubes are triggered practically simultaneously, meaning one particle has barreled through both tubes. The two tubes in my device are separated by 25-millimeter spacers, making it unlikely that a particle coming from a nearby radioactive decay would be energetic enough to pass through both tubes. I reduced the likelihood even more by placing a layer of fishing-sinker lead between the tubes.
To turn the stacked pair of GM counters into a coincidence detector, I hooked up the output of each board (oddly labeled VIN, which usually means a pin for a voltage supply input!) to a spare Arduino Nano, programmed to record a hit only when one board registers a count within 1 millisecond of the other. Naturally this means the detector can recognize only muons with trajectories roughly aligned with the plane of the GM tubes so that the muons pass through both tubes.
Geiger-Müller tubes are activated by ionizing radiation, but unlike cosmic-ray muons [red particles], most terrestrial sources [green particles] are not powerful enough to travel through the detector’s two tubes. By registering only activations that occur almost simultaneously, we can plot the muon flux as a function of the angle from vertical of the detector, with the observed data following the predicted model closelyJames Provost
Proving to myself that the results indeed reflected the flux of cosmic-ray muons wasn’t difficult: I just measured the count rate as a function of how far away from vertical my detector was oriented. You see, the flux of cosmic-ray muons coming in vertically from the sky is greater than the flux of muons traveling horizontally. Between these extremes, the flux should have a cosine-squared dependence on the angle as the detector’s plane rotates from vertical to horizontal.
So I set about counting events with my device oriented at different angles from vertical, allowing at least 12 hours for each measurement. The results were pretty consistent with the expected cosine-squared variation. For example, when completely horizontal, the detector registered a value that was less than 10 percent of that obtained when vertical, but it wasn’t zero.
Getting nonzero muon counts even when horizontal isn’t so surprising. With only a 2.5-centimeter separation between the two 1-cm-diameter tubes, my detector’s angular resolution is pretty broad (±22 degrees). So even when I set the unit to sense horizontal flux, it was surely detecting muons coming in from as much as 22 degrees above the horizon.
With a working muon detector in hand, I set off to probe the Earth—or at least a small part of it—by visiting the Reed Gold Mine, in Midland, N.C., the first commercial gold mine in the United States. I spent about two and a half hours in the mine, making five 30-minute measurements. I easily detected the increasingly thick layer of rock above the mine’s main horizontal tunnel. My detector was even able to sense the presence of a vertical shaft at one spot, as the absence of rock allowed more muons to reach me than I measured nearby in the tunnel.
These measurements take a long time because you need to accumulate enough counts to provide reasonable statistical precision. So you’ll need patience. But it’s not a bad way to harness the power of the cosmos, even deep underground!
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