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The short life of a neutron

Although the neutron is a senior citizen of sorts, whose existence was predicted in 1920 and confirmed in 1932, it’s still not fully understood.

It was James Chadwick of Cambridge’s Cavendish Laboratory who, after nearly a decade of experimentation, devised a method to detect these particles, which have essentially the same mass as the proton but no electrical charge. Not only did Chadwick’s triumph earn him the Nobel Prize in Physics three years later, it also gave him and the physics community a new particle to play with.

The discovery of the neutron allowed scientists to conceive of the possibility of nuclear chain reactions and explain isotopes—versions of an element that contain the same number of protons but different numbers of neutrons. Neutrons also provided a versatile tool for exploring the structure and dynamics of materials, including soft matter.

Despite the attention it has received, however, the neutron is still in some ways a mystery. For instance, what is the half-life of an unbound neutron?

Like most social beings, neutrons don’t do well on their own. Put them inside a stable nucleus and they last indefinitely, but take them out and their time is limited. Attempts before World War II to determine the neutron’s half-life were hobbled by the lack of intense neutron sources.

Enter the Oak Ridge Graphite Reactor, the world’s first permanent nuclear reactor.

Soon after the war ended, Arthur Snell and Leonard Miller of ORNL’s predecessor, Clinton Laboratories, built a vessel that allowed them to focus a beam of neutrons from the Graphite Reactor. When a neutron decays, it emits a proton, an electron and an antineutrino, and the device was able to detect the proton and electron. The two researchers observed this neutron decay, and Snell presented their findings in April 1948 to a meeting of the American Physical Society in Washington. (Miller had died in a vehicle collision.)

Observing the decay was one thing, but measuring it was going to take a more refined instrument. After Miller’s passing, Snell had the help of Physics Division colleagues Frances Pleasonton and Rube McCord in perfecting a device to deliver more precise counts of decay products in the neutron beam.

By this time nuclear reactors were becoming more common, and British physicist John Robson tackled the same problem using the NRX reactor at Canada’s Chalk River Laboratories. In 1950 Robson’s and Snell’s teams published their work simultaneously in The Physical Review, with Snell’s team coming up with a neutron half-life of 10 to 30 minutes and Robson’s reporting a half-life of 9 to 25 minutes.

Physicists have been repeating their experiments for going on seven decades but have yet to land on a specific neutron decay time. Both Snell’s and Robson’s time ranges for neutron half-life span the currently estimated value, somewhere around 10.2 minutes, but the hunt continues. ORNL—through the Spallation Neutron Source's Fundamental Physics Beam Line—is one of more than a dozen institutions around the world working to pin down this elusive number.