Astronomers Have Obtained The First Space-Based Measurement Of A Neutron’s Lifetime

On its journey to Mercury, NASA's MESSENGER spacecraft collected neutron data from Venus that enabled this new measurement to be calculated. NASA/Johns Hopkins APL/Carnegie Institution of Washington 

Katy Pallister 12 Jun 2020, 17:25

Lone neutrons don’t have much of a life. Outside of an atom’s nucleus, they quickly decay to other subatomic particles. However, the exact survival time of a free neutron before its disintegration is not precisely known despite decades of research.

In a bid to end this stalemate, researchers have turned from the lab to the sky for their next set of measurements. Using data gathered by NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, the team estimated a neutron’s lifetime to be around 780 seconds, with an error of 60 seconds on either side. Whilst this figure itself does not offer huge clarity, its generation has demonstrated the viability of this new spaceward approach.

“This is the first time anyone has ever measured the neutron lifetime from space,” lead author Jack Wilson, a scientist at Johns Hopkins Applied Physics Laboratory (APL), US, said in a statement. “It proves the feasibility of this method, which could one day be the way to resolve this anomaly.”

The two methods previously employed by researchers have produced conflicting results. In the “bottle” technique, neutrons trapped by magnetic fields in a “bottle” are periodically counted to track how long they last. The “beam” experiment, on the other hand, tallies one of the particle’s decay products (protons) that emerges from a beam of neutrons. Coming in at 879 seconds and 888 seconds, respectively, this discrepancy is significant when compared to the measurements’ uncertainties.

However, scientists have been looking since 1990 to a third method – space. Although no specific mission has ever been launched to measure a neutron’s lifetime, a neutron spectrometer onboard the MESSENGER spacecraft offered up the right tools for the job. On its way to its destination in the late 2000’s, MESSENGER was able to detect neutrons that had been ejected from Venus’ atmosphere by incoming cosmic rays. These neutrons were counted over a range of altitudes, as the higher the neutron traveled, the longer it had survived.

A schematic to demonstrate how MESSENGER provided data to estimate neutron lifetime. Johns Hopkins APL

“It’s like a large bottle experiment, but instead of using walls and magnetic fields, we use Venus’ gravity to confine neutrons for times comparable to their lifetime,” Wilson said.

Comparing MESSENGER’s measurements with computed models of different neutron lifetimes, yielded a best match of 780 seconds (13 minutes). When uncertainties and statistical errors are accounted for this estimate agrees with the values obtained from both the bottle and beam methods.

“This result shows that even using data from a mission designed to do something entirely different, it’s still possible to measure the neutron lifetime from space,” co-author Jacob Kegerreis, a researcher at Durham University, UK, said in a statement.

In their paper published in Physical Review Research, the authors argue that if a dedicated mission were launched, possibly to Venus where its thick atmosphere and large mass could effectively trap neutrons, they would be able to produce the most precise measurement on a neutron’s lifetime yet. Subsequently, this could help our understanding of how elements formed after the Big Bang.

“We ultimately want to design and build a spacecraft instrument that can make a high-precision measurement of the neutron lifetime,” Wilson said.

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