[Paper Review] Measurement of the Free Neutron Lifetime using the Neutron Spectrometer on NASA's Lunar Prospector Mission

Jack T. Wilson, D. J. Lawrence+3 more|arXiv (Cornell University)|Nov 13, 2020

Planetary Science and Exploration34 references17 citations

TL;DR

This study presents the second space-based measurement of the free neutron lifetime using data from the Lunar Prospector neutron spectrometer, achieving τn = 887 ± 14stat +7−3 syst s—within 1σ of the accepted value. By incorporating non-uniform lunar elemental composition into Monte Carlo simulations, the authors significantly reduce systematic uncertainty, advancing space-based neutron lifetime measurements toward laboratory-level precision.

ABSTRACT

We use data from the Lunar Prospector Neutron Spectrometer to make the second space-based measurement of the free neutron lifetime finding $ au_n=887 \pm 14_ ext{stat}{\:^{+7}_{-3\: ext{syst}}}$ s, which is within 1$\sigma$ of the accepted value. This measurement expands the range of planetary bodies where the neutron lifetime has been quantified from space, and by extending the modeling to account for non-uniform elemental composition, we mitigated a significant source of systematic uncertainty on the previous space-based lifetime measurement. This modeling moves space-based neutron lifetime measurement towards the ultimate goal of reducing the magnitude of the systematics on a future space-measurement to the level of those seen in laboratory-based experiments.

Motivation & Objective

• To measure the free neutron lifetime τn using space-based neutron flux data from a planetary body.
• To reduce systematic uncertainties in space-based τn measurements by accounting for non-uniform elemental composition of planetary surfaces.
• To validate and extend the feasibility of measuring τn from space, building on prior MESSENGER flyby results.
• To contribute to resolving the neutron lifetime puzzle by providing an independent measurement with improved systematics.

Proposed method

• Utilized thermal neutron flux data from the Lunar Prospector Neutron Spectrometer during highly elliptical lunar orbits.
• Measured neutron count rates using two 3He gas proportional counters—one shielded with Cd and one with Sn to differentiate thermal neutron absorption.
• Applied solid-angle scaling to correct for spacecraft-induced background, using high-altitude spectra as a reference.
• Performed Monte Carlo simulations of neutron transport through a lunar surface model with depth-dependent elemental composition.
• Minimized chi-squared between observed and modeled neutron count rates to infer τn.
• Incorporated detailed elemental abundances from lunar surface models (e.g., FHT, PAN, SPA, nPKT, PKT) into simulations to account for compositional heterogeneity.

Experimental results

Research questions

• RQ1 Can the free neutron lifetime τn be accurately measured from space using planetary neutron flux data?
• RQ2 How does non-uniform elemental composition of a planetary surface affect systematic uncertainty in space-based τn measurements?
• RQ3 To what extent can Monte Carlo neutron transport modeling reduce systematics in space-based τn measurements?
• RQ4 Does the space-based measurement of τn agree with the accepted value and the current discrepancy between bottle and beam experiments?

Key findings

• The measured neutron lifetime is τn = 887 ± 14stat +7−3 syst s, consistent with the accepted value within 1σ.
• The inclusion of non-uniform elemental composition in the neutron transport model significantly reduced systematic uncertainty compared to previous space-based measurements.
• The measurement demonstrates that planetary bodies with well-characterized surface compositions, such as the Moon, are viable platforms for high-precision τn measurements.
• The result supports the feasibility of future space-based experiments to achieve systematics comparable to laboratory-based bottle and beam experiments.
• The study confirms that the neutron flux escaping from a planetary body is sensitive to τn and can be used as a natural clock for neutron decay.
• The method reduces the dominant source of systematic error in prior space-based measurements by modeling depth-dependent composition and neutron moderation.