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NASA’s MESSENGER

Mission Carries Out Space-Based Measurements of Neutron’s Lifetime

Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASA’s MESSENGER spacecraft, researchers found the neutron lifetime to be 780 seconds.

Cosmic rays striking Venus’ atmosphere eject neutrons that gradually fly into space. As neutrons move to higher altitudes, more time passes, and more neutrons radioactively decay. MESSENGER counted the number of neutrons seen at various altitudes, allowing researchers to home in on where neutron numbers start falling off. Using models, researchers can then estimate the neutron lifetime. Image credit: Johns Hopkins Applied Physics Laboratory.

Cosmic rays striking Venus’ atmosphere eject neutrons that gradually fly into space. As neutrons move to higher altitudes, more time passes, and more neutrons radioactively decay. MESSENGER counted the number of neutrons seen at various altitudes, allowing researchers to home in on where neutron numbers start falling off. Using models, researchers can then estimate the neutron lifetime. Image credit: Johns Hopkins Applied Physics Laboratory.

“The lifetime of free neutrons provides a key test of the Standard Model of particle physics, and it also affects the relative abundances of hydrogen and helium formed in the early Universe just minutes after the Big Bang, so it has wide-ranging implications,” said Dr. Vincent Eke, a scientist in the Institute for Computational Cosmology at Durham University.

“Space-based methods offer the possibility of breaking the impasse between the two competing Earth-based measurement techniques.”

Neutrons are normally found in the nucleus of an atom but quickly disintegrate into electrons and protons when outside the atom.

Physicists have previously used two lab-based methods — the ‘bottle method’ and ‘beam’ technique — to try and determine the lifetime of neutrons.

The bottle method traps neutrons in a bottle and measures how long it takes for their radioactivity to decay. It suggests neutrons can survive on average for 879 seconds.

The beam technique, which fires a beam of neutrons and counts the number of protons created by radioactive decay, gives about 888 seconds.

While this might seem a small difference, scientists say the gap could be enormous.

As the Standard Model of particle physics requires the neutron lifetime to be about 879 seconds, any deviation from this would provoke a fundamental change in our understanding of this model.

MESSENGER carried a neutron spectrometer to detect neutrons set loose into space by cosmic rays colliding with atoms on Mercury’s surface as part of research to determine the existence of water on the planet.

On its way the spacecraft first flew by Venus, where it collected neutron measurements for the first time ever.

“Even though MESSENGER was designed for other purposes, we were still able to use the data to estimate the neutron lifetime,” said Dr. Jacob Kegerreis, in the Institute for Computational Cosmology, at Durham University, said:

“The spacecraft made observations over a large range of heights above the surfaces of Venus and Mercury, which allowed us to measure how the neutron flux changes with distance from the planets.”

Using models, the researchers estimated the number of neutrons MESSENGER should count at its altitude above Venus for neutron lifetimes would be between 600 and 1,020 seconds. For the shorter lifetimes, fewer neutrons survive long enough to reach MESSENGER’s neutron detector.

They found the neutron lifetime to be 780 seconds, with an uncertainty of about 130 seconds from statistical and other uncertainties, like whether the number of neutrons changes during the day and uncertainty about the chemical make-up of Mercury’s surface.

“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,” said Dr. Jack Wilson, a researcher at the Johns Hopkins Applied Physics Laboratory.

The findings were published in the journal Physical Review Research.

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Jack T. Wilson et al. 2020. Space-based measurement of the neutron lifetime using data from the neutron spectrometer on NASA’s MESSENGER mission. Phys. Rev. Research 2 (2): 023316; doi: 10.1103/PhysRevResearch.2.023316

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