Within the mid-Sixties, the Nobel Prize–successful physicist Luis Alvarez had a wild concept. He proposed utilizing muons, extremely penetrating subatomic particles created when cosmic rays strike Earth’s environment, to seek for hidden chambers inside one of many pyramids of Giza.
These muon particles are heavyweight cousins of electrons that journey near the pace of sunshine. They will penetrate via many meters of strong rock, together with the limestone and granite blocks used to construct the pyramids. However a few of the muons will probably be absorbed by this dense materials, which means that they can be utilized to basically “X-ray” a pyramid, revealing its internal construction. So in 1968, Alvarez and his colleagues started making muon measurements from a chamber positioned on the base of the Pyramid of Khafre.
They didn’t discover a hidden chamber, however they did verify the feasibility of what has come to be known as muon tomography. Physicists have since used the method to find hidden access shafts above tunnels, study magma chambers within volcanos, and even probe the damaged reactors at Fukushima. And, in 2017, muon measurements lastly revealed a hidden chamber in one of the pyramids of Giza—simply not the pyramid that Alvarez had chosen to discover.
You can also carry out comparable experiments with gear that you could construct your self for under US $100 or so.
Whereas some well-documented designs can be found for low-cost muon detectors (particularly, the Cosmic Watch mission from MIT), I made a decision to pursue a less complicated—and barely cheaper—method. I bought two Geiger-counter kits, every costing solely $23. Though it’s known as a “package,” this board the truth is comes totally assembled minus the important thing element: a Geiger-Müller (or GM) tube for detecting ionizing radiation. It additionally comes with no documentation.
The shortage of documentation wasn’t an issue as soon as I discovered a good source for information about this board—together with a pointer to valuable instructions for the way to set the tube’s anode voltage.
The muon detector makes use of two Geiger-Müller tubes [top], every inserted right into a sensor board [bottom right]. Each boards are linked to an Arduino Nano microcontroller [bottom left].James Provost
For the GM tubes, I made a decision to purchase what I understood to be good ones: Russian-made SBM-20 tubes. Many of those are listed on eBay by sellers in Ukraine, however I used to be capable of get hold of a pair of such tubes from a provider within the United States for simply $49.
“Why two kits and two tubes?” you would possibly ask. It’s as a result of GM tubes don’t react simply to muons. More often than not, they’re triggered by ionizing particles given off by radioactive substances within the atmosphere, such because the daughter merchandise of radon within the air.
Proving that the outcomes mirrored the flux of cosmic-ray muons wasn’t troublesome.
To tell apart the high-energy cosmic-ray muons from the opposite, lower-energy particles isn’t onerous, although. Simply apply what physicists name the coincidence method: Detect solely when two close by tubes are triggered virtually concurrently, which means one particle has barreled via each tubes. The 2 tubes in my system are separated by 25-millimeter spacers, making it unlikely {that a} particle coming from a close-by radioactive decay could be energetic sufficient to cross via each tubes. I diminished the chance much more by putting a layer of fishing-sinker lead between the tubes.
To show the stacked pair of GM counters right into a coincidence detector, I connected the output of every board (oddly labeled VIN, which normally means a pin for a voltage provide enter!) to a spare Arduino Nano, programmed to report successful solely when one board registers a rely inside 1 millisecond of the opposite. Naturally this implies the detector can acknowledge solely muons with trajectories roughly aligned with the aircraft of the GM tubes in order that the muons cross via each tubes.
Geiger-Müller tubes are activated by ionizing radiation, however in contrast to cosmic-ray muons [red particles], most terrestrial sources [green particles] will not be highly effective sufficient to journey via the detector’s two tubes. By registering solely activations that happen nearly concurrently, we will plot the muon flux as a operate of the angle from vertical of the detector, with the noticed knowledge following the expected mannequin carefullyJames Provost
Proving to myself that the outcomes certainly mirrored the flux of cosmic-ray muons wasn’t troublesome: I simply measured the rely price as a operate of how distant from vertical my detector was oriented. You see, the flux of cosmic-ray muons coming in vertically from the sky is bigger than the flux of muons touring horizontally. Between these extremes, the flux ought to have a cosine-squared dependence on the angle because the detector’s aircraft rotates from vertical to horizontal.
So I set about counting occasions with my system oriented at totally different angles from vertical, permitting no less than 12 hours for every measurement. The outcomes have been fairly in keeping with the anticipated cosine-squared variation. For instance, when fully horizontal, the detector registered a worth that was lower than 10 p.c of that obtained when vertical, nevertheless it wasn’t zero.
Getting nonzero muon counts even when horizontal isn’t so shocking. With solely a 2.5-centimeter separation between the 2 1-cm-diameter tubes, my detector’s angular decision is fairly broad (±22 levels). So even after I set the unit to sense horizontal flux, it was certainly detecting muons coming in from as a lot as 22 levels above the horizon.
With a working muon detector in hand, I set off to probe the Earth—or no less than a small a part of it—by visiting the Reed Gold Mine, in Midland, N.C., the primary industrial gold mine in america. I spent about two and a half hours within the mine, making 5 30-minute measurements. I simply detected the more and more thick layer of rock above the mine’s essential horizontal tunnel. My detector was even capable of sense the presence of a vertical shaft at one spot, because the absence of rock allowed extra muons to succeed in me than I measured close by within the tunnel.
These measurements take a very long time as a result of it’s worthwhile to accumulate sufficient counts to offer affordable statistical precision. So that you’ll want persistence. However it’s not a foul technique to harness the facility of the cosmos, even deep underground!
