The Strike Zone – Amateur Astronomers Association

Last month’s column discussed the properties of neutrinos. Despite their tiny mass and elusive nature (they pass through everything), detecting them is important; that is the focus of this month’s column.

Consider a baseball’s strike zone: the imaginary rectangle, about 17 inches wide in front of the batter. In it, the batter tries to hit the pitched ball that’s moving at about 95 mph. Now imagine that a neutrino, moving close to the speed of light, is scaled up to the size of a baseball. In trying to detect the instant a neutrino collides with an atom; the strike zone would have to be scaled up to the width of one astronomical unit (AU): the distance between the Earth and the Sun.

_{Credit: Star Walk}

While daunting, several factors work in our favor. Unlike the solitary baseball, there are trillions of neutrinos available for detection. Second, generating neutrinos in proton accelerators is a well-established technology: allowing for extensive controlled study.

Neutrino detection from space is another matter. It is simple in theory. To begin with, neutrinos themselves are not actually being detected. Rather, an enormous chamber is set up that is filled with transparent liquid. As the trillions of neutrinos pass through the chamber, a tiny portion of them collide with the protons within an atom.

The resulting collision leads to the generation of new subatomic particles, some of which are electrically charged. These charged particles then set off detectors that signify that a collision took place. A simplified chemical illustration of this process could look like this: v_e +p→n+e^–

Here, an electron neutrino (the most abundant of the three “flavors” of neutrinos) collides with a proton. This leads to the generation of a neutron and an electron: a charged particle that can be detected.

The enormous size of these detectors is matched by their enormous budgets: billions. Governments subsidize them. Wikipedia lists some 50+ neutrino studies that have been conducted in dozens of installations. Why the intense interest?

Since before World War II, governments have been aware that neutrinos are at the heart of nuclear processes. Such knowledge yields a better understanding of nuclear reactions. Major world powers utilize this in nuclear weapons production. But this also helps governments better monitor the production of nuclear weapons in other countries. Currently, neutrino detectors are being proposed to monitor Iran’s nuclear capabilities.

More germane to our otherworldly interests, consider several odd neutrino “telescopes.” Unlike ordinary telescopes, they must be built deep underground in order to be protected from cosmic rays that could interfere with detecting neutrinos and other subatomic particles.

The design theory is fairly straightforward. Fill an enormous container with liquid. Wait for neutrinos to collide with an atom. Then record and analyze the collision. This was how, in the late 60s, Nobel winners Raymond Davis and John Bahcall studied solar neutrinos. They built a 100,000-gallon tank a mile underground, in a former gold mine in South Dakota. Then they filled it with cleaning fluid (yes, cleaning fluid).

^{Credit: U.S. Department of Energy}

The reaction they waited for was this:

{\displaystyle \mathrm {\nu _{e}+\ ^{37}Cl\longrightarrow \ ^{37}Ar^{+}+e^{-}.} }

An electron neutrino collides with a chlorine atom in the fluid. This generates a charged unstable Argon atom which decays into a stable Argon atom. This then releases an electron that is detected and analyzed.

Interestingly, the Davis/Bahcall experiment recorded only ⅓ of the collisions predicted. Numerous replications confirmed this. Now we know that this unanticipated result is due to electron neutrinos oscillating into the other two rarer flavors of neutrinos whose higher energies could not be detected.

A more recent detector, the Super-Kamiokande-Atlas (Super-K), located in Gifu, Japan (200 km NE of Osaka) is situated one kilometer underground. It is as big as a 15-story building. The interior of this vast container holds 55,000 tons of ultra-pure water – pure water is cheaper to produce than cleaning fluid and just as effective. About 13,000 photomultiplier tubes surround the interior of this enormous cistern.

^{Credit: Kamioka Observatory, ICRR, The Univ. of Tokyo}

When a neutrino collides with an electron it sets off a reaction that creates Cherenkov radiation, a bluish light, that is detected by one of the photomultipliers.

Among its numerous missions, the Super-K studies neutrinos emitted from supernovas. As noted, 99% of the energy released in a supernova is in the form of neutrinos. Because they are so weakly interacting with matter, they emerge from the dust cloud surrounding the supernova earlier than photons of light. So they reach Earth hours before light does. The Super-K is programmed so that when such neutrinos are detected, a worldwide alert is transmitted to all observatories, pinpointing the exact location of the neutrino source, giving astronomers valuable time to plan their observations.

The largest neutrino detector in the world is located under the South Pole, appropriately named Ice Cube (no relation to the rapper).

^{Credit: University of Wisconsin–Madison/National Science Foundation}

A cubic kilometer of pure ice (instead of pure water) is fitted with vertical arrays of 5100 photomultipliers embedded up to 2.5 kilometers under the ice. These can detect the Cherenkov radiation generated when a neutrino collides with an ice molecule.

Two of its missions are worth noting. The first involves trying to find low-energy “relic neutrinos” theoretically emitted at the time of the Big Bang. No luck yet. The second, more successful mission was identifying exceedingly rare and elusive tau neutrinos. Culling through 10 years of data, scientists were able to isolate seven high-energy tau neutrinos whose source is well beyond our own galaxy. This study provides a valuable window into astrophysical processes.

More ambitious and bigger detectors are being planned. The hope is that the largest of these instruments can unravel the mystery of the smallest of these particles. The ultimate purpose is to better understand how stars work and how our universe evolved.