Measurement of the free neutron lifetime using the neutron spectrometer on NASA's Lunar Prospector mission

We use data from the Lunar Prospector Neutron Spectrometer to make the second space-based measurement of the free neutron lifetime finding ${\tau }_n=887\pm {14}_{\text{stat}}{}_{-3\text{syst}}^{+7}$ 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 nonuniform 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.

Figure 1

(a) LP altitude during the initial elliptical orbits. (b) The measured count rates in the Cd- and Sn-covered NS detectors. (c) The modeled count rates in the Cd- and Sn- covered detectors as described in Sec. 3. (d) The residuals of the thermal neutron counts (i.e., the difference between the Sn- and Cd-covered detectors) normalized by the uncertainty due to counting statistics. The grey regions show the data used to produce the final result based on the cuts described in Sec. 4. Time on the x axis is measured from January 1st, 1998.

Figure 2

Low-altitude 32-s (solid) and mission averaged (dotted) LPNS spectra from both the Cd- and Sn-covered ${}^3\mathrm{He}$ gas proportional counters. The grey shaded region shows those channels involved in determining the measured neutron counts. The dashed curves show the high-altitude background, measured during LP's approach to the Moon when the spacecraft altitude was $>1.5\times {10}^4$ km.

Figure 3

(a) The five regions with distinct compositions included in the neutron count-rate models: procellarum KREEP terrane (PKT), south pole Aitken terrane (SPAT), Feldspathic highland terrane (FHT), pure anorthosite terrane (PAN), and non-PKT maria (nPKT). (b) The model thermal neutron counts, i.e., the difference between the Sn-covered and Cd-covered detectors. (c) The measured thermal detector counts. The grey contours in (b) and (c) show the regions defined in (a).

Figure 4

(a) The result of the mcnpx-based efficiency calculation for a Sn-covered ${}^3\mathrm{He}$ tube to ${10}^{-9}$ MeV neutrons. (b) An example of angle-averaged detector response used in the count rate simulation, based on the instantaneous response shown in (a). The coordinates are conventional spherical polars where the spacecraft's rotation axis coincides with the y axis and the NS detector's long axis coincides with the z axis.

Figure 5

A cartoon of the LP spacecraft and NS looking along the spacecraft rotation axis. The locations of the Sn-covered and Cd-covered tubes are shown in red and blue, respectively. The axis of rotation, pointing out of the page, is indicated with the circled dot.

Figure 6

${\chi }^2$ statistic based on a comparison between the GRS counts and solid-angle subtended by the Moon, with a linear scaling applied to minimize the misfit, for different temporal offsets applied to the spacecraft ephemerides. The number of degrees of freedom $\nu$ is shown in the figure.

Figure 7

(a) ${\chi }^2$ comparison of the thermal count model and data with differing ${\tau }_n$ values using all of the observations shown in Fig. 1. The number of degrees of freedom $\nu$ is shown in the figure. (b) Histogram of the residuals of the best-fit model normalized by the uncertainty due to counting statistics. (c) and (d) are similar to (a) and (b), respectively, but make use only of the observations taken at longitudes greater than ${180}^{\circ }$ during the nine initial highly elliptical orbits.

Figure 8

${\chi }^2$ comparison of the modelled restricted thermal neutron count and data with differing ${\tau }_n$ values. The red curve shows a comparison with the reference model as in Fig. 7. The grey curves are based on models using perturbations from the reference composition. The black curve compares the data with a model including a latitude-dependent perturbation to the count rates intended to simulate the effect of temperature variation. The dotted lines indicate the locations of the minima of each of the curves.

Figure 9

Particle Data Group and other recent measurements of ${\tau }_n$ [31, 32, 33, 34], along with those of this paper and Wilson et al. [6]. The shaded regions represent the standard error on the uncertainty-weighted mean lifetime in each class of measurement.