*New tabletop detector 'sees' single electrons

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*New tabletop detector 'sees' single electrons

Post by Cr6 on Wed Apr 22, 2015 12:44 am

April 21, 2015

New tabletop detector 'sees' single electrons

by Jennifer Chu


(more at link...)

Formaggio says the group's results, published in Physical Review Letters, are a big step toward a more elusive goal: measuring the mass of a neutrino.

A ghostly particle

Neutrinos are among the more mysterious elementary particles in the universe: Billions of them pass through every cell of our bodies each second, and yet these ghostly particles are incredibly difficult to detect, as they don't appear to interact with ordinary matter. Scientists have set theoretical limits on neutrino mass, but researchers have yet to precisely detect it.

"We have [the mass] cornered, but haven't measured it yet," Formaggio says. "The name of the game is to measure the energy of an electron—that's your signature that tells you about the neutrino."

As Formaggio explains it, when a radioactive atom such as tritium decays, it turns into an isotope of helium and, in the process, also releases an electron and a neutrino. The energy of all particles released adds up to the original energy of the parent neutron. Measuring the energy of the electron, therefore, can illuminate the energy—and consequently, the mass—of the neutrino.

Scientists agree that tritium, a radioactive isotope of hydrogen, is key to obtaining a precise measurement: As a gas, tritium decays at such a rate that scientists can relatively easily observe its electron byproducts.
Researchers in Karlsruhe, Germany, hope to measure electrons in tritium using a massive spectrometer as part of an experiment named KATRIN (Karlsruhe Tritium Neutrino Experiment). Electrons, produced from the decay of tritium, pass through the spectrometer, which filters them according to their different energy levels. The experiment, which is just getting under way, may obtain measurements of single electrons, but at a cost.

"In KATRIN, the electrons are detected in a silicon detector, which means the electrons smash into the crystal, and a lot of random things happen, essentially destroying the electrons," says Daniel Furse, a graduate student in physics, and a co-author on the paper. "We still want to measure the energy of electrons, but we do it in a nondestructive way."
The group's setup has an additional advantage: size. The detector essentially fits on a tabletop, and the space in which electrons are detected is smaller than a postage stamp. In contrast, KATRIN's spectrometer, when delivered to Karlsruhe, barely fit through the city's streets.

Tuning in

Furse and Formaggio's detector—an experiment called "Project 8"—is based on a decades-old phenomenon known as cyclotron radiation, in which charged particles such as electrons emit radio waves in a magnetic field. It turns out electrons emit this radiation at a frequency similar to that of military radio communications.

"It's the same frequency that the military uses—26 gigahertz," Formaggio says. "And it turns out the baseline frequency changes very slightly if the electron has energy. So we said, 'Why not look at the radiation [electrons] emit directly?'"
Formaggio and former postdoc Benjamin Monreal, now an assistant professor of physics at UCSB, reasoned that if they could tune into this baseline frequency, they could catch electrons as they shot out of a decaying radioactive gas, and measure their energy in a magnetic field.


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