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Coffee cup-sized MIT machine can SEE actual ELECTRONS, boast boffins

Neutrino mass search takes another big step

A particle detector created by the Massachusetts Institute of Technology can detect the movements of individual electrons in a radioactive gas. The machine is designed to measure the mass of the neutrino, which is considered to be tiny even by the standards of subatomic particles.

In an article published in Physical Review Letters with the catchy title "Single-Electron Detection and Spectroscopy via Relativistic Cyclotron Radiation", boffins explain their contribution to efforts to measure the mass of the neutrino.

"It has been understood since 1897 that accelerating charges must emit electromagnetic radiation" the abstract begins, referring to the work of Irish physicist Joseph Larmor. "Although first derived in 1904, cyclotron radiation from a single electron orbiting in a magnetic field has never been observed directly."

The boffins demonstrate a means of single-electron detection in "a novel radio-frequency spectrometer," which, while not a direct observation, is an enormous development on previous means of observing the effects of neutrinos on the orbit of a lonely beta particle within a magnetic field.

A statement about the study begins: "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, detects the frequency of radiation and resultantly the electron’s energy, which, pending improvements in the mere matter of energy resolution, could lead to a new way to measure the neutrino mass.

The essential apparatus for the study is housed at the University of Washington in Seattle. It is essentially a small bottle, "about the size of an espresso cup," containing gaseous krypton-83 which itself is wrapped within a 1-tesla magnet. The gas is 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 25GHz", which is detected using "sensitive microwave amplifiers."

The radiated power from a single electron orbiting in a magnetic field was plotted (as shown above) as a function of time and the frequency of the electron's orbit. The bright, upwardly-angled streaks indicate the radiation emitted by a single electron. The jumps up in frequency that can be seen correspond to collisions with a residual atom or molecule in the gas cell. Though already theoretically established, the machine demonstrates how a circling electron continuously emits radiation.

"As a result, [the electron] gradually loses energy and orbits at a rate that increases linearly in time. The detected radiation streaks have the same predicted linear dependence, which is what allowed the researchers to associate them with a single electron."

The fundamental idea behind Project 8 is to "measure the energies of the electrons emitted in beta decay and compare them to the total energy of the decay. If the neutrino has mass, no electron can have an energy equal to the total energy, since some of this energy must be used to make the neutrino. The mass of the neutrino can therefore be determined by measuring the maximum energy of the emitted electrons."

Neutrinos, along with electrons, are produced in beta decay. Their existence, which was predicted by Wolfgang Pauli in 1930, is needed to ensure that beta decay conserves energy and angular momentum.

"Physicists have always assumed neutrinos were very light. In the standard model of particle physics, they are considered massless, largely because the model was developed before any experiments indicated neutrinos had mass. This picture changed when experiments showed that neutrinos, which come in three flavours (electron, muon, and tau), morph from one type into another – a quantum phenomenon known as neutrino oscillation that only occurs if neutrinos have mass."

These oscillations are the only non-theoretical evidence for the magnitude of the neutrino masses, although so far experiments have only managed to establish upper-bounds of what those masses may be.

The Karlsruhe Tritium Neutrino Experiment (KATRIN), a tritium decay experiment, will measure electron energies with a large spectrometer, "and is designed to measure a neutrino mass as small as 0.2eV/c2. With the technique KATRIN is using, however, the sensitivity to the neutrino mass can only be improved by building a larger spectrometer. And with KATRIN already the size of a building, a larger spectrometer is not realistic."

Project 8's energy resolution does not face such a limit in terms of size, but "by the design of its magnetic field and the precision with which the researchers can measure the electron’s cyclotron frequency."

"Both aspects can, in principle, be improved."

The Project 8 researchers are hoping to be able to detect the simultaneous emission from many electrons produced in tritium beta decays. Their current energy resolution of about 30eV, however, is "too low to measure the neutrino mass."

For comparison, KATRIN has an energy resolution of around 1eV. Future versions of the Project 8 spectrometer, using improvements in the design of its magnetic field and measurements, could have an energy resolution that improves upon that of KATRIN.

"In principle, this technique might have a sensitivity sufficient to reach the range of masses indicated by neutrino oscillation experiments, and thus may guarantee the measurement of neutrino mass. The main challenge will be adapting the technique so it can accommodate many electrons at once yet still detect their individual cyclotron emissions." ®

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