Nav: Home

Neutron lifetime measurements take new shape for in situ detection

May 30, 2017

WASHINGTON, D.C., May 30, 2017 -- All matter making up the stars, our planet and life upon it came into existence 13.8 billion years ago as a result of the Big Bang. A millisecond after the Big Bang occurred, neutrons and protons formed and began to fuse into small atomic nuclei. This is known as the Big Bang Nucleosynthesis (BBN) era. During BBN, protons (hydrogen), the main building blocks of stars, combined with neutrons to form helium and other light elements. All of this happened within the first, approximately, 20 minutes of this new universe.

Neutrons, though, are inherently unstable (where lifetime, τ, is approximately 881 seconds) and don't last long outside an atomic nucleus. Because the neutron decays on a time scale similar to the period for BBN, accurate simulations of the BBN era require thorough knowledge of the neutron lifetime, the average time required for a neutron to decay, but this value is still not precisely known. This week in the journal Review of Scientific Instruments, from AIP Publishing, scientists at Los Alamos National Lab (LANL) report an exciting new method to measure it.

Measurements of the neutron lifetime and accurate simulations of BBN require old neutrons to be freed from their nuclear cages. Christopher Morris of LANL and author of the new study explained that neutrons have been essentially "fossilized" in the nuclei of atoms. Studying these "fossil particles," then, can provide a glimpse of the earliest moments of the universe's existence.

By the time BBN ended, most neutrons were locked up in the nuclei of helium atoms. Today, almost all matter in the universe is still close to the initial delicate ratio of helium to hydrogen. The ratio is important since it determines how fast our sun burns hydrogen, powering life on earth.

The number of neutrons on earth is a direct result of BBN and later processes that occurred in ancient stars. By 4.5 billion years ago, there were finally enough neutrons around to form rocky planets, like Earth, and elements like carbon and oxygen, essential for life.

Morris explained that there are two ways to measure the neutron lifetime: The first is counting the number of protons produced when cold neutrons in a beam decay. The second is trapping the neutrons in a metal bottle, with magnetic fields or even via gravity, similar to how water is "trapped" in a bathtub. The method his group has developed uses a magnetic-gravitational trap involving a combination of magnets and gravity.

The trapping approach uses very cold particles, so-called ultracold neutrons, or UCNs. Either the bottle container's walls or a magnetic field repel the neutral UCNs, causing them to hover in the device. According to standard physics, the only pathway these neutrons have for escape is through the decay into a proton and an electron.

The new device, assembled at LANL, involves a magnetic-gravitational trap with a shape designed specifically to stir the neutrons as they fill the trap. This avoids problems in earlier experiments where slow-moving neutrons filled parts of the trap unevenly, resulting in possibly false lifetime measurements.

Previous experiments with beams and containers appeared to give sharply different neutron lifetimes, the most precise measurement using a bottle trap differs by almost four standard deviations from that measured in a beam. In the results published this week, Morris and co-workers report a neutron lifetime of 878 seconds, very close to that found in material bottle traps but differing significantly from the neutron lifetime measured in beams.

The difference between the beam and bottle measurements may be due to a still-unidentified error. Morris suggests a more exotic explanation is that neutrons disappear from the beam without ever producing a proton. This raises the prospect that the somewhat controversial and still-mysterious notion of dark matter might be involved. Future studies will explore these intriguing possibilities.
The article, "A new method for measuring the neutron lifetime using an in situ neutron detector," is authored by Christopher L. Morris, et al. The article will appear in Review of Scientific Instruments May 30, 2017 [DOI: 10.1063/1.4983578]. After that date, it can be accessed at

For a video showing the researchers pouring ultra-cold neutrons, UCNs, from a stainless steel bottle into a Helium-3 counter, click here: (Credit: Robert W. Pattie Jr.)


Review of Scientific Instruments publishes original research and review articles on instruments in physics, chemistry, and the life sciences. The journal also includes sections on new instruments and new materials. See

American Institute of Physics

Related Hydrogen Articles:

World's fastest hydrogen sensor could pave the way for clean hydrogen energy
Hydrogen is a clean and renewable energy carrier that can power vehicles, with water as the only emission.
Chemical hydrogen storage system
Hydrogen is a highly attractive, but also highly explosive energy carrier, which requires safe, lightweight and cheap storage as well as transportation systems.
Observing hydrogen's effects in metal
Microscopy technique could help researchers design safer reactor vessels or hydrogen storage tanks.
The 'Batman' in hydrogen fuel cells
In a study published in Nature on Jan. 31, researchers at the University of Science and Technology of China (USTC) report advances in the development of hydrogen fuel cells that could increase its application in vehicles, especially in extreme temperatures like cold winters.
Paving the way for more efficient hydrogen cars
Hydrogen-powered vehicles emit only water vapor from their tailpipes, offering a cleaner alternative to fossil-fuel-based transportation.
More Hydrogen News and Hydrogen Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...