"Dustmaids Down a Drafty Hall..."

November 04, 2008

Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
     -- "Cosmic Gall," by John Updike
     from Telephone Poles and Other Poems (Knopf, 1960)

It's neutrino week at Twisted Physics! That's right, we're embarking on a week-long celebration of the humble "ghost particle," so named because they almost never interact with regular matter. Right this very moment, we are all being bombarded by trillions of neutrinos, but they just pass right through us with no effect whatsoever. To stop a beam of neutrinos, you'd need a wall more than 100 light-years thick; otherwise, they'd just breeze right through, as if nothing were there at all.

Most of the neutrinos that find their way to Earth come from deep inside our sun, and because they make that journey pretty much unimpeded -- no collisions with other particles along the way -- they can tell astrophysicists quite a bit about the inner workings in the solar core, i.e., more details about the mechanisms that make the stars shine. Oh, and there's the minor question of the nature of dark matter that scientists think holds our universe together, as well as the dark energy causing the expansion of the cosmos to accelerate. So you can see why they might be of interest.

First posited by Wolfgang Pauli and dubbed neutrinos by Enrico Fermi, these plucky ghost particles don't play well with others, primarily interacting via the weak nuclear force -- if, indeed, at all. This is the force that enables certain types of subatomic particles to exchange energy, mass and charge, thereby changing into one another. Say a neutron inside an atomic nucleus decays; in the process, it produces a proton, an electron and a neutrino. Now imagine that happening 200<36> times every second in the core of our sun (that's 200 followed by 36 more zeros), and you get an idea of just how many particles we're talking about here. They're the most abundant subatomic particle in the universe.

Given their plentitude, you'd think neutrinos would have been easier to detect, but that's the tricky thing about the weak force: it's very weak indeed. It only kicks in if neutrinos are so close as to be practically touching atomic nuclei, and even then, it's better if there's an actual collision. This would cause the atom to change into a different chemical element -- an easily measurable effect. Even then, the signal would be tiny, almost impossible to distinguish from the background radiation of cosmic rays constantly hitting our atmosphere. What to do?

One good strategy is to take your experiment deep underground to avoid as much interference as possible from other particles. So in the late 1960s, a physicist named Ray Davis, Jr. set up shop in the Homestake Mine, nestled in South Dakota's Black Hills. He set up an enormous tank filled with 600 tons of dry-cleaning fluid (primarily composed of chlorine), figuring that if enough neutrinos flowed through it, eventually a neutrino would strike an atom in the fluid and produce the telltale decay process. Basically, a chlorine atom would turn into an argon atom, emitting an electron which could be detected. (The effect is known as Cherenkov radiation.)

To Davis' delight, the experiment worked, making his team the first to detect neutrinos from the sun. (In 1956, Clyde Cowan and Frederick Reines observed the first neutrinos ever, but these were produced in a nuclear reactor in South Carolina.)

Ah, but the story doesn't end there. As is often the case in science, success led to even more questions. See, theorists had calculated that around 30 million neutrinos should pass through every cubic inch of Earth every second. Extrapolating from that calculation, Davis should have been detecting one neutrino collision per day. Instead, he detected one every three days. One-third of the expected solar neutrinos were missing, a mystery that would endure for the next three decades as the "solar neutrino problem."

Tune in tomorrow for our next installment, when scientists finally solve the Case of the Missing Solar Neutrinos....

Photo: Ray Davis Jr. takes a dip in the 300,000 gallons of water surrounding the perchloroethylene tank at the Homestake Mine's underground neutrino detector in 1971. Source: Brookhaven National Laboratory.