* Astronomy

Researchers from the Commerce Department's National Institute of Standards and Technology (NIST) and four universities have made the first experimental observation of rare particles of light emitted during the radioactive decay of the neutron, a key building block of matter. This work confirms theoretical predictions of this type of decay of the neutron and sets the stage for a new class of tests of basic theories in particle physics.
The experiments, reported in the Dec. 21, 2006, issue of Nature, were performed at the NIST Centre for Neutron Research (NCNR) in Gaithersburg, Md., because of the unique instruments and expertise at the facility. The authors include researchers from the NIST Physics Laboratory, Tulane University, the University of Michigan, the University of Maryland and the University of Sussex (Brighton, England).
The team determined that slightly more than three out of 1,000 neutron decays on average (3.13 ± 0.34 x 10^-3 to be precise), produce a photon (a particle of light) above an energy level that is relatively low but still observable. The measured value has only about 10 percent uncertainty, which is considered remarkable given that this decay had never been observed before.

"This measurement is difficult because the neutron lifetime is very long, so very few neutrons decay at one time, and of those that decay, very few emit a photon. To make it even worse, the background radiation is very large" - NIST physicist Jeffrey Nico, lead author of the paper.

The neutron is stable only when confined in the nucleus of a stable atom; a free neutron decays into other particles--a proton, electron and anti-neutrino--within about 15 minutes. This process, known as "neutron beta-decay," has been studied for decades without proof of the occasional photon emissions predicted by theory. These photons can be easily missed amid the intense background radiation associated with a neutron beam. The researchers adapted existing instruments and techniques to minimize uncertainties and unwanted background effects, and designed a clever experiment to prove that they had indeed found the elusive photons. They also used a novel hybrid photon detector that could operate in the high magnetic field and at the extremely cold temperatures of the experiment.

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