New Moon-Based Study Again Shows Spacecraft Could Answer Neutron Lifetime Mystery

Last year, a team of researchers from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, and Durham University in England proved that spacecraft could answer an outstanding physics question that has far-reaching implications on how elements formed after the Big Bang: How long can a neutron last outside an atom’s nucleus before it decays into a proton, an electron and an antineutrino?

Well, the team has done it again — and this time even more precisely than before.

Using neutron data from NASA’s Lunar Prospector mission, which launched more than 20 years ago in 1998, the team of nuclear physicists estimated the neutron lifetime to be 887 seconds, or 14 minutes and 47 seconds. That time closely matches accepted values from lab-based measurements and is a tenfold improvement over the team’s estimates last year using data from NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission.

“We’re very happy and surprised,” said APL nuclear physicist and study lead author Jack Wilson. “It’s unusual that you can get such a big improvement in just a year.”

Wilson will present the findings virtually Oct. 13 at the 2021 Fall Meeting of the American Physical Society’s Division of Nuclear Physics. The paper is expected to be published later this week in the journal Physical Review C.

Knowing how long a free neutron can last outside a nucleus is important to our understanding of the fundamental structure of matter. It could also show if there are other fundamental particles yet to be discovered.

“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 behavior of fundamental particles,” Wilson said.

The 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 antineutrino. 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 the 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.

Space-based measurements offer an alternative, independent method from lab-based experiments, said Jacob Kegerreis, a physicist at Durham University and a study co-author. “[It] 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 Wilson, Kegerreis and the 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 farther 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 from MESSENGER data at Venus and Mercury showed a neutron lifetime of about 13 minutes.

Because of several systematic errors, however (such as the encounter being just 45 minutes along a quirky orbit and the uncertainty about Venus’ and Mercury’s elemental composition, which is critical to knowing how many neutrons would decay before reaching the spacecraft), that data was limited. So, the group turned to Lunar Prospector neutron data that was collected during its first two days of operation in orbit around the Moon.

“There’s a lot that we know about the Moon compared with Venus and especially Mercury,” Kegerreis said. “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.”

Wilson noted that the Lunar Prospector data still has a combined error of about 15 seconds, much longer than the less-than-one-second error in the lab-based methods. So, there’s still plenty of room for improvement.

Although researchers could continue mining spacecraft data already retrieved by previous missions, Kegerreis noted their orbital paths and measurement design are rarely conducive to making the measurements needed to test the neutron lifetime. For the Lunar Prospector measurements used in this analysis, the spacecraft followed simple, elliptical orbits. A better strategy, he said, would be to make the neutron lifetime measurement one of a future mission’s scientific goals.

“With a mission planned from its outset to study the lifetime alongside other measurements, we could jump to far better results than we’ve been able to get by scrounging these unintentional datasets,” he said.

The success of the Lunar Prospector data underscores that the Moon is a viable and logical destination for such a mission. And with increasing efforts for robotic and human exploration of the Moon, Wilson sees an opportunity to capitalize on the excitement. “Our hope would be to make a measurement there in situ as a stepping stone and then later get something on the lunar surface,” he said.