The Underground Dry-Cleaning Tank
Raymond Davis Jr., the Homestake Mine, and the Missing Solar Neutrinos
Four thousand eight hundred and fifty feet beneath the pine ridges of Lead, South Dakota, in a blasted rock cavern at the bottom of the Homestake Gold Mine, a horizontal white steel cylinder twenty feet in diameter and forty-eight feet long sat submerged inside a flooded stone chamber. Filled between 1965 and 1967 by Brookhaven National Laboratory chemist Raymond Davis Jr. with 100,000 gallons (615 metric tons) of tetrachloroethylene ($\text{C}_2\text{Cl}_4$)—common commercial dry-cleaning fluid—the giant tank was built not to study geology, but to look directly into the thermonuclear core of the Sun fifteen million degrees Kelvin and ninety-three million miles away.
Every photon of visible sunlight striking the Earth was forged in the Sun's core roughly a hundred thousand years ago, scattering billions of times off dense plasma electrons on a slow random walk to the solar photosphere that erases all direct memory of the nuclear reactions at the center. Neutrinos—nearly massless, electrically neutral fermions emitted during the proton-proton fusion chain—fly straight out of the solar core at nearly the speed of light, passing through the overlying seven hundred thousand kilometers of solar plasma in two seconds and reaching Earth eight minutes later.
Detecting those solar messengers required overcoming the most lopsided cross-section in experimental physics. Because a neutrino interacts only via the subatomic weak nuclear force, its capture cross-section on a target atomic nucleus is roughly $10^{-45}\text{ square centimeters}$—meaning a typical solar neutrino can pass unimpeded through a solid wall of lead one hundred light-years thick. Following a 1946 radiochemical proposal by Bruno Pontecorvo, Davis exploited the inverse beta-decay reaction in which a solar electron neutrino ($\nu_e$) with energy above $0.814\text{ MeV}$ strikes a stable Chlorine-37 nucleus ($^{37}\text{Cl}$, which makes up twenty-four percent of natural chlorine), converting a neutron into a proton and emitting an electron to leave behind a single atom of radioactive Argon-37 ($^{37}\text{Ar}$, half-life 35.04 days).
Caltech theoretical astrophysicist John Bahcall calculated from his Standard Solar Model that out of the sextillions of neutrinos passing through Davis's 615 tons of dry-cleaning fluid every day, mainly those from Boron-8 decay in the Sun's core had enough energy to trigger the chlorine reaction—producing roughly 1.5 Argon-37 atoms per day across the entire 100,000-gallon tank. To find and count twenty individual atoms of argon swimming inside $2 \times 10^{30}$ molecules of cleaning fluid, Davis engineered a five-stage radiochemical extraction and shielding cascade:
- * The Forty-Eight-Hundred-Fifty-Foot Rock Overburden: If the tank had been placed on the surface of the Earth, cosmic-ray muons crashing through the atmosphere would have knocked protons out of nuclei to create thousands of false Argon-37 atoms per day; burying the tank nearly a mile underground inside Homestake cut cosmic-ray muon flux by a factor of a million.
- * The Subterranean Water Neutron Shield: Flooding the excavated rock cavity around the steel tank with several feet of water absorbed fast neutrons emitted by trace uranium and thorium decays in the surrounding Homestake schist wall rock.
- * The Four-Hundred-Thousand-Liter Helium Purge: Every sixty days, after Argon-37 production reached equilibrium with its thirty-five-day radioactive decay, Davis's team circulated 400,000 liters of pure helium gas through pairs of eductor nozzles inside the liquid $\text{C}_2\text{Cl}_4$, bubbling out more than ninety-five percent of the chemically inert Argon-37 atoms along with a micro-liter of carrier Argon-36 added to verify extraction efficiency.
- * Liquid-Nitrogen Charcoal Cryo-Trapping: The helium purge stream passed through a condenser at $-32^\circ\text{C}$ to strip out dry-cleaning vapors, a molecular sieve absorber, a hot titanium sponge getter at $900^\circ\text{C}$ to scrub reactive gases, and finally an activated charcoal trap immersed in liquid nitrogen at $-196^\circ\text{C}$ ($77\text{ K}$) that froze out the trace argon atoms while letting the helium recycle back into the tank.
- * The Half-Cubic-Centimeter Proportional Counter: Desorbed from the charcoal and purified via gas chromatography, the tiny sample of gas—containing the few dozen solar-forged Argon-37 atoms—was sealed inside a miniature lead-and-mercury-shielded proportional counter tube the size of a medicinal capsule to record the $2.82\text{ keV}$ Auger electrons emitted as each Argon-37 nucleus decayed back into Chlorine-37 over the next year.
Across twenty-five years and 108 extraction runs between 1970 and 1994, Davis captured a cumulative total of roughly 2,200 Argon-37 atoms. Every single year, his count was stubbornly, identically low: $2.56 \pm 0.23\text{ Solar Neutrino Units}$ (where $1\text{ SNU} = 10^{-36}$ captures per target atom per second), almost exactly one-third of Bahcall's predicted $7.6\text{ SNU}$. For three decades, critics suspected either that Davis's chemical plumbing was losing argon atoms—even after he injected known numbers of radioactive argon atoms and recovered ninety-six percent of them—or that Bahcall's equations of solar fusion were wrong. In June 2001, heavy-water detectors at the Sudbury Neutrino Observatory in Ontario proved that Davis's twenty-atom chemical extraction and Bahcall's solar fusion model were both exact: two-thirds of the Sun's electron neutrinos were oscillating into muon and tau flavors en route to Earth.