Project Poltergeist

From 1943 to 1945, some of the world’s most brilliant scientists descended on Los Alamos, New Mexico, in support of the Manhattan Project—the U.S. government’s top-secret effort to create the world’s first atomic weapons. Eighteen of these scientists were or would become Nobel Laureates. But only one would be honored for work actually performed at what is today Los Alamos National Laboratory.

That one was Frederick Reines, who remained at the Lab following the end of World War II. Around 1947, Reines started to explore ideas that would culminate in his detection of the neutrino—and  a Nobel Prize in physics.

The neutrino is a neutral (non-charged) particle with a mass of less than one electronvolt. For comparison, an electron has a mass of 511,000 electronvolts. To give a different perspective, a neutrino is about 10 billion, billion, billion times smaller than a grain of sand.

In addition to being incredibly small, neutrinos are incredibly abundant. Every object emits neutrinos, but because of their neutrality and low mass, neutrinos very rarely interact with other particles. At any given moment, tens of trillions of neutrinos are passing through your body.

Austrian physicist Wolfgang Pauli first hypothesized in 1930 that such particles existed. In 1932, Nobel laureate and future Manhattan Project scientist Enrico Fermi coined the term neutrino (“little neutron” in Fermi’s native Italian). But it would be more than two decades before anyone had experimental proof the neutrino existed.

In the late 1940s, Reines took an interest in nuclear weapon effects and studied blast waves and radiation pollution. Reines suspected that a nuclear detonation might emit large quantities of neutrinos, but together with Los Alamos physicist Clyde Cowan decided the rapid timescale of a detonation would make detection too difficult. The pair then turned to nuclear reactors, believing a more controlled environment and longer timescale would make detection easier. 

Reines and Cowan hypothesized that a neutrino might interact with a proton to produce a neutron and a positron. The positron would in turn interact with any nearby electron, annihilating both itself and the electron and producing two detectable gamma rays. The probability of such an interaction is extremely low (one in one trillion trillion), so a large neutrino generator and a large source of protons would be necessary.

Because the Lab didn’t have a nuclear reactor to facilitate their experiment—dubbed Project Poltergeist—the two Los Alamos researchers traveled to Hanford, Washington, in 1953 and then to the Savannah River Site in South Carolina in 1955.

At Savannah River, two 200-liter tanks filled with water were placed near the reactor. Because of the weak bonds between hydrogen and oxygen in water molecules, the hydrogen atoms functionally serve as free protons. Neutrinos would interact with the hydrogen atoms, setting off the hypothesized chain of reactions that would produce gamma rays. The water tanks were surrounded by liquid scintillator material, which responds to gamma rays by giving off a flash of light.

In the end, after collecting months of data, Reines and Cowan had evidence of three neutrino interactions per hour. To ensure the resulting gamma rays weren’t being produced by some other reaction, Reines and Cowan switched off the reactor and saw the rate of detected events decrease significantly, an indication that the interactions were driven by the reactor’s beta decay and proof that the ghost particle existed. In 1956, they published their findings in Science magazine.

Reines would leave Los Alamos shortly after this discovery, and although he would dedicate much of his remaining career to neutrino research, it was Project Poltergeist that ultimately won him the Nobel Prize in 1995. He and Cowan shared credit for the discovery, but Reines alone received the Nobel Prize (the award isn’t bestowed posthumously, and Cowan had died in 1974). This accomplishment marked the first, and so far only, time that Nobel Prize–winning work was performed by a Los Alamos National Laboratory scientist.

Despite efforts to study neutrinos in the decades since Reines and Cowan’s discovery, the “little neutrons” remain fairly mysterious. Scientists hope they could one day provide insights into the early universe, stars, and dark matter. ★

Article by Ian Laird, National Security Science magazine writer