ANN ARBOR—For the first time since scientists predicted 60 years ago that light is emitted during radioactive decay, a team of researchers has been able to observe this phenomenon during what is known as neutron decay.

The results are reported in the Dec. 21 issue of the journal Nature. The time it takes for a neutron to decay, which called the half-life, and the nature of radioactive decay ARE important to understanding how elements were created during the Big-Bang.

They are also important to understanding what controls the rate of energy production in the sun. Studying the light that is emitted as photons will lead to more precise understanding of neutron decay and the underlying particle physics. The success of this experiment makes it possible to design new tests of particle physics, and to search for undiscovered phenomena.

The team was led by former University of Michigan graduate student Jeffrey Nico, now of NIST (National Institute of Standards and Technology), and current U-M graduate student Rob Cooper. Cooper is supervised by U-M physics professor Tim Chupp.

The neutron is a subatomic particle typically found inside atoms, and these particles comprise more than half of the matter in the world. Within atoms, the neutron remains stable, or non-radioactive, but when freed from the nucleus, it becomes radioactive and decays, which means it emits electrons and protons.

In the experiment neutrons produced in the NIST reactor travel in a glass pipe to an apparatus in which electric and magnetic fields guide the decayed electrons and protons to a particle detector. The team, with collaborators from University of Maryland, Sussex University and Tulane, modified the apparatus to detect the light emitted in the form of photons.

“The most difficult aspect of detecting what we call the radiative neutron decay mode is that there are photons flying all over the place, produced by the reactor, and produced by other instruments in the experimental guide hall. The first challenge was to be sure the photons we detected had a high probability of coming from neutron decay,” Chupp said.

The team did this by simultaneously observing the proton and the electron from a decay, but they used a simple electrostatic mirror to reflect slower protons. This allowed them to choose which light emitting decay to observe, which in turn allowed them to verify that the photons came from neutron decay and not from other sources.

“Rob Cooper worked out in detail how the mirror affected what we observed, which provided the definitive confirmation that the photons came from neutron decay,” said Chupp.

The team now plans to improve the photon detection efficiency by a factor of ten. This will enable precision study of the predictions of theory and may even reveal unexpected signals of new physics.

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