Viewpoint Cyclotron Radiation from One Electron

Noli turbare circulos meos—do not disturb my circles—was the final wish of Archimedes, who demanded that Roman soldiers not destroy the perfect circular forms he had drawn on the ground. The literal opposite is true for a team of researchers who have designed a new way to accurately measure the tiny shifts in an electron’s circular orbit as the particle moves in a magnetic field. The experiment, known as Project 8, is run by a collaboration of 27 scientists from six institutions in the U.S. and Germany and it detects the frequency of radiation emitted by an individual, orbiting electron [1]. This frequency, and in turn, the electron’s energy, can be measured very accurately. If the energy resolution can be further improved, their experiment could lead to a new way to measure the neutrino mass. Electric charges radiate when moving in circular orbits in a magnetic field, an effect that was first predicted by Oliver Heaviside in 1904. Cyclotron radiation has been observed in astrophysical radio sources and is the basis for generating x rays at synchrotrons. Radiation emitted by these sources comes from the motion of many electrons, but Project 8 is the first experiment to detect cyclotron radiation from a single electron. The core of their apparatus (Fig. 1), which is located at the University of Washington in Seattle, is a small low-pressure gas cell (about the size of an espresso cup) inside a 1-tesla magnet. The cell contains a small amount of gaseous krypton-83, a radioactive isotope that produces electrons as its nuclei undergo beta decay. These electrons are forced into a circular orbit by the magnetic field and emit cyclotron radiation with a frequency of around 25 gigahertz (GHz), which is detected using sensitive microwave amplifiers. The researchers plotted the detected radiation power as a function of time and frequency (Fig. 2). The bright, upward-angled streaks of radiation indicate the radiation emitted by a single electron. It is well known theoretiFIG. 1: A schematic of the Project 8 experimental apparatus. A small cell containing krypton-83 at low pressure is surrounded by a magnet coil. Electrons are emitted by the krypton nuclei when they undergo beta decay. The microwaves emitted by electrons in cyclotron orbit are channeled up a waveguide to the detection system. (APS/Alan Stonebraker, based on a drawing by Luiz de Viveiros/UCSB)