Within the mid-Sixties, the Nobel Prize–profitable physicist Luis Alvarez had a wild thought. 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’ll penetrate by means of many meters of stable rock, together with the limestone and granite blocks used to construct the pyramids. However among the muons will probably be absorbed by this dense materials, that means that they can be utilized to primarily “X-ray” a pyramid, revealing its interior construction. So in 1968, Alvarez and his colleagues started making muon measurements from a chamber situated on the base of the Pyramid of Khafre.
They didn’t discover a hidden chamber, however they did affirm the feasibility of what has come to be referred to as muon tomography. Physicists have since used the approach to find hidden entry shafts above tunnels, examine magma chambers inside volcanos, and even probe the broken reactors at Fukushima. And, in 2017, muon measurements lastly revealed a hidden chamber in one of many pyramids of Giza—simply not the pyramid that Alvarez had chosen to discover.
You can also carry out comparable experiments with tools which you could construct your self for under US $100 or so.
Whereas some well-documented designs can be found for low-cost muon detectors (specifically, the Cosmic Watch mission from MIT), I made a decision to pursue a less complicated—and barely cheaper—strategy. I bought two Geiger-counter kits, every costing solely $23. Though it’s referred to as a “equipment,” this board in truth comes absolutely assembled minus the important thing element: a Geiger-Müller (or GM) tube for detecting ionizing radiation. It additionally comes with no documentation.
The dearth of documentation wasn’t an issue as soon as I discovered an excellent supply for details about this board—together with a pointer to precious directions for find out how 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 in a position to receive a pair of such tubes from a provider within the United States for simply $49.
“Why two kits and two tubes?” you may 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 setting, 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 differentiate the high-energy cosmic-ray muons from the opposite, lower-energy particles isn’t onerous, although. Simply apply what physicists name the coincidence methodology: Detect solely when two close by tubes are triggered virtually concurrently, that means one particle has barreled by means of each tubes. The 2 tubes in my gadget are separated by 25-millimeter spacers, making it unlikely {that a} particle coming from a close-by radioactive decay can be energetic sufficient to cross by means of each tubes. I diminished the probability 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 file a success 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 by means of each tubes.
Geiger-Müller tubes are activated by ionizing radiation, however in contrast to cosmic-ray muons [red particles], most terrestrial sources [green particles] aren’t highly effective sufficient to journey by means of the detector’s two tubes. By registering solely activations that happen virtually concurrently, we are able to 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 gadget oriented at totally different angles from vertical, permitting at the very least 12 hours for every measurement. The outcomes had been fairly in keeping with the anticipated cosine-squared variation. For instance, when fully horizontal, the detector registered a price that was lower than 10 p.c of that obtained when vertical, however 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 once 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 at the very least a small a part of it—by visiting the Reed Gold Mine, in Midland, N.C., the primary business gold mine in the USA. 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 predominant horizontal tunnel. My detector was even in a position to sense the presence of a vertical shaft at one spot, because the absence of rock allowed extra muons to achieve me than I measured close by within the tunnel.
These measurements take a very long time as a result of you’ll want to accumulate sufficient counts to offer cheap statistical precision. So that you’ll want endurance. However it’s not a nasty method to harness the ability of the cosmos, even deep underground!
