Tabletop particle detector may help measure neutrino mass

Chuck Bednar for – @BednarChuck

Physicists from the Massachusetts Institute of Technology (MIT) are one step closer to measuring the mass of the elusive neutrino, developing a new tabletop particle detector capable of identifying single electrons in a radioactive gas by trapping them in a magnetic bottle.

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A new tabletop particle detector (shown here) is able to identify single electrons in a radioactive gas. (Credit: MIT)

As they explained in research published earlier this week in the journal Physical Review Letters, the detector uses a magnet to trap those electrons in the bottle, then uses a radio antenna to pick up extremely weak signals they emit to map their precise activity for several milliseconds.

The work was completed along with colleagues from the Pacific Northwest National Laboratory, the University of Washington, the University of California at Santa Barbara (UCSB) as part of an experiment known as the Project 8 collaboration. The device has been used to record the activity of more than 100,000 individual electrons in krypton gas, the Institute noted in a statement.

Using electron measurements to hunt for neutrino mass

According to the researchers, as the radioactive krypton gas decayed, it emitted electronics that vibrated as a baseline frequency before fading. That frequency increased again when one of the electrons hit an atom of radioactive gas, and as an electron collided with multiple atoms in their detector, its energy appeared to spike in what a step-like pattern.

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Shown here is "event zero," the first detection of a trapped electron in the MIT physicists' instrument. The color indicates the electron's detected power as a function of frequency and time. The sudden “jumps” in frequency indicate an electron collision with the residual hydrogen gas in the cell. (Credit: MIT)

Joe Formaggio, an associate professor of physics at MIT and a member of the Project 8 group, said that they are able to image the frequency of the electron, and that since they can be detected using the radio antenna, they are “chirping” in radio waves. Their findings are a big step forward in ongoing efforts to measure the mass of the mysterious particles known as neutrinos.

Billions of neutrinos pass through our body’s cells every second, but little is known about these elementary particles, which don’t appear to interact with ordinary matter and tend are notoriously difficult to detect, the researchers explained. While scientists have established theoretical limits on neutrino mass, they have yet to precisely detect it. However, Formaggio noted that measuring the energy of an electron, it should be possible to learn more about the neutrino.

When a radioactive atom decays, it turns into a helium isotope and releases both an electron and a neutrino. The sum of the released particles’ energy equals that of the original parent neutron, so that measuring the electron’s energy can reveal the energy and mass of the neutrino. By using the radioactive hydrogen isotope tritium, which has a decay rate through which the byproducts of its electrons can be easily observed by scientists, they can obtain precise measurements.

Five-year effort pays off with early successful results

The Project 8 team’s detector is based on a phenomenon known as cyclotron radiation, in which electrons or other charged particles emit radio waves in a magnetic field. The team found that the electrons emit radiation at a frequency of 26 gigahertz, similar to that used by the military for its radio communications. This baseline frequency changes only slightly if the electron has energy, they explained, so they opted to look directly at the radiation emitted by electrons.

Formaggio and his colleagues proposed that by tuning into this baseline frequency, they would be able to catch the electrons as they were emitted by a decaying radioactive gas and measure the energy in a magnetic field. By doing so, they believe that they would be able to conduct far more accurate measurements than is possible through other methods, but it requires observing a rather weak signal over a long period of time, so it had not been attempted previously.

It took them five years of work before they were finally able to build a particle detector that was up to the task, and once the researchers switched it on, they said that they were able to record the individual electrons within the first 100 milliseconds of the experiment. It took longer to analyze the results, but their hard work ultimately paid off with precise krypton gas measurements.

On the heels of their success with krypton, the researchers believe that they may be able to move on to tritium within the next year or two. If they can accomplish that, Formaggio explained, they could be well on their way toward eventually measuring the mass of a neutrino.

A look inside the research with MIT’s Noah Oblath

We had a chance to get chat via email with Noah Oblath, a postdoctoral fellow at MIT who was involved in the project. He explained that the research was “significant on several levels. By itself, we have made a direct observation of a natural phenomenon, single-electron cyclotron radiation, that previously had only been seen indirectly. We’ve also shown that it’s possible to use electron cyclotron radiation as a tool to perform spectroscopy.”

While neutrino physics was “the original motivation for the experiment,” he explained that the technique of Cyclotron Radiation Emission Spectroscopy could be applicable to other scientific issues, some of which the team was looking into. Oblath added that their work demonstrates that there is “hope for using tritium beta decay to probe even lower neutrino masses.”

That’s not to say that the work was easy, as evidenced by the five-year period it took to build the detector. “We knew from the beginning that the principle behind Project 8 was straight-forward, but that the implementation would be challenging,” he told redOrbit. Since they were attempting to observe “a very weak radio-frequency signal,” they knew that it would “require some care.”

“Initially we did not have any funding specifically for Project 8, so we tried to re-use equipment where we could find it; our initial attempts were not sensitive enough to detect the cyclotron radiation,” Oblath said. “Piece by piece we managed to improve our equipment and how it was put together, and eventually we were able to demonstrate that this technique does, in fact, work.”

What would it mean to the team to be able to use the tabletop detector to measure the mass of a neutrino? Oblath said that it would be “a remarkable achievement” to directly measure neutrino mass “with any experiment.” He added that in the field of neutrino physics, “we often focus on a particular set of techniques that have been shown to work well, and improve on them gradually. In the case of Project 8 we have started with a brand-new technique and have shown that we will be able to apply it to the quest to measure the mass of the neutrino.”

“That uniqueness makes it an extremely exciting project to work on,” he concluded.


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