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New neutron lifetime measurement “key” to understanding early universe

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Scientists have refined their measurements of a particle that could be “key” to understanding the early universe.

A team of researchers led by the Johns Hopkins Applied Physics Laboratory (APL) USA, and including an academic from Durham University, last year became the first team to use spaced-based measurements to determine the lifespan of a neutron.

Building on that research the team has now used data from NASA’s 1998 Lunar Prospector mission to improve their calculations of how long neutrons can survive.

They estimate that neutrons are able to survive for 14 minutes 47 seconds with an uncertainty of 15 seconds.

Knowing the lifetime of neutrons is key to understanding the formation of elements after the Big Bang 13.8 billion years ago and could influence the standard model of physics that governs our understanding of the formation of the universe.

The findings are published in the Physical Review C journal.

Study lead author and APL nuclear physicist, Dr Jack Wilson, said: “The neutron lifetime is key to answering several big questions in cosmology and particle physics, for our understanding of the crucial early state of the universe and the behaviour of fundamental particles.

“It is important for our understanding of the fundamental structure of matter and it could also show if there are other fundamental particles yet to be discovered.”

Neutron lifetime is the easiest and most direct way of measuring the weak force, one of just four fundamental forces in nature.

The weak force governs certain types of radioactive decay, including the natural breakdown of lone neutrons into a proton, electron and anti-neutrino. It even kicks off the nuclear fusion reaction that powers the Sun and other stars.

The problem is nobody can agree on how long a free neutron can last. Since the early 1990s, researchers have tried two lab-based methods to determine the time: the so-called “bottle” method, which traps neutrons in a bottle and tracks how long they take to radioactively decay; and the “beam” method, which fires a beam of neutrons and scores the number of protons created by radioactive decay.

The bottle method says neutron lifetime is 14 minutes and 39 seconds. The beam method says the lifetime is nine seconds longer, at 14 minutes and 48 seconds. And although scientists suspect there’s a systematic error in one or both methods, nobody can deduce what that error is.

Dr Jacob Kegerreis, a member of The Institute for Computational Cosmology in the Department of Physics, at Durham University, said: “Space-based measurements offer an alternative, independent method.

“This method is entirely independent from the lab-based experiments which may be a way to resolve the current conflict on which method is the most reliable.”

Scientists have suggested various forms of space-based measurements to determine the neutron lifetime since 1959, but the Durham and APL team were the first to show it could be done.

Their method relies on neutrons released into space by cosmic rays colliding with atoms on a planet’s surface or in its atmosphere. The further the neutrons travel from the planet’s surface, the more time passes and the more neutrons decay. By collecting neutrons at various altitudes and comparing that data with a model of neutron production, transport and detection from that planetary body, scientists can estimate the neutron lifetime.

Last year’s results used the data from the NASA’s MESSENGER spacecraft, which were collected as it flew over Venus and Mercury showed a neutron lifetime of about 13 minutes.

However due to several systematic errors including the encounter being just 45 minutes long and the uncertainty about Venus’ and Mercury’s elemental composition, which is critical to knowing how many neutrons would decay before reaching the spacecraft, the team turned to Lunar Prospector neutron data that were collected during its first two days of operation in orbit around the Moon.

Dr Kegerreis added: “We know a lot more about the Moon compared to Venus and especially Mercury.

“Combined with the multiple orbits we could use of Lunar Prospector around the Moon compared with the one-off flyby we had for MESSENGER past Venus, that made this new result significantly more accurate and reliable.

“We’re delighted we were able to see an improvement on the results from last year by using the different data, especially as the neutron data were not actually collected during both the MESSENGER and Lunar Prospector missions but was in fact complied after the mission had been completed.

“A planned mission where measuring the neutron lifetime is a focus from the outset, amongst other things, would provide an even more accurate timespan of a neutron’s survival.”

Abigail Richman
Marketing Assistant
Abbie has worked in marketing for a number of years in a variety of different sectors and joins the team relatively new to the space sector.