arXiv subjects

Patrick N. Peplowski

Publications and source records attributed to Patrick N. Peplowski.

9 recordsLinked to original sources

arXiv2026

Precision cross section measurements of neutron-induced non-elastic gamma production reactions at 14 MeV

We present a technique for high-precision absolute measurements of gamma-ray production cross sections (n,xg) induced by 14 MeV neutrons. The technique is based on the Associated Particle Imaging (API) method, which tags individual neutrons emitted from a deuterium-tritium source with their associated alpha-particles, enabling coincidence-based suppression of lower-energy neutrons and room background signals, while also providing neutron flux measurements with uncertainties on the order of 1%. This compact, laboratory-scale technique has the potential to address gaps and discrepancies in existing cross section libraries at a fraction of the cost of large-scale dedicated facilities, with direct applications in active neutron interrogation, detector calibration, nuclear fusion science, and Monte Carlo simulations, among others. We demonstrate the technique through proof-of-concept experiments on thin and thick samples of natural Fe (1 mm, 8 mm) and natural C (2 mm, 10 mm). For Fe, gamma-ray production cross sections were measured for the 846.78 and 1238.33 keV transitions from the first and second excited states of 56Fe. For C, the (n,n'g) cross section was measured for the 4438.91 keV transition from the first excited state of 12C. Measurements were performed at 110 and 48 degrees relative to the neutron beam to characterize gamma-ray anisotropy. For the thin samples, the measured cross sections are 723 +/- 70 mb (56Fe, 846.78 keV, 110 deg), 303 +/- 42 mb (56Fe, 1238.33 keV, 110 deg), 755 +/- 106 mb (56Fe, 846.78 keV, 48 deg), 321 +/- 149 mb (56Fe, 1238.33 keV, 48 deg), 141 +/- 16 mb (12C, 4438.91 keV, 110 deg), and 279 +/- 105 mb (12C, 4438.91 keV, 48 deg). Uncertainties are dominated by counting statistics and detector efficiency calibration, both of which can be reduced in future experiments to achieve overall uncertainties of 5% or better.

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arXiv2023

Mixing model of Phobos' bulk elemental composition for the determination of its origin: Multivariate analysis of MMX/MEGANE data

The formation process of the two Martian moons, Phobos and Deimos, is still debated with two main competing hypotheses: the capture of an asteroid or a giant impact onto Mars. In order to reveal their origin, the Martian Moons eXploration (MMX) mission by Japan Aerospace Exploration Agency (JAXA) plans to measure Phobos' elemental composition by a gamma-ray and neutron spectrometer called MEGANE. This study provides a model of Phobos' bulk elemental composition, assuming the two formation hypotheses. Using the mixing model, we established a MEGANE data analysis flow to discriminate between the formation hypotheses by multivariate analysis. The mixing model expresses the composition of Phobos in 6 key lithophile elements that will be measured by MEGANE (Fe, Si, O, Ca, Mg, and Th) as a linear mixing of two mixing components: material from Mars and material from an asteroid as represented by primitive meteorite compositions. The inversion calculation includes consideration of MEGANE's measurement errors ($E_P$) and derives the mixing ratio for a given Phobos composition, based on which the formation hypotheses are judged. For at least 65\% of the modeled compositions, MEGANE measurements will determine the origin uniquely ($E_P$ = 30\%), and this increases from 74 to 87\% as $E_P$ decreases from 20 to 10\%. Although the discrimination performance depends on $E_P$, the current operation plan for MEGANE predicts an instrument performance for $E_P$ of 20--30\%, resulting in ~70\% discrimination between the original hypotheses. MEGANE observations can also enable the determination of the asteroid type of the captured body or the impactor. The addition of other measurements, such as MEGANE's measurements of the volatile element K, as well as observations by other MMX remote sensing instruments, will also contribute to the MMX mission's goal to constrain the origin of Phobos.

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arXiv2021

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 $τ_n=887 \pm 14_\text{stat}{\:^{+7}_{-3\:\text{syst}}}$ s, which is within 1$σ$ 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.

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arXiv2020

Space-based Measurements of Neutron Lifetime: Approaches to Resolving the Neutron Lifetime Anomaly

Free neutrons have a measured lifetime of 880 s, but disagreement between existing laboratory measurements of ~10 s have persisted over many years. This uncertainty has implications for multiple physics disciplines, including standard-model particle physics and Big-Bang nucleosynthesis. Space-based neutron lifetime measurements have been shown to be feasible using existing data taken at Venus and the Moon, although the uncertainties for these measurements of tens of seconds prevent addressing the current lifetime discrepancy. We investigate the implementation of a dedicated space-based experiment that could provide a competitive and independent lifetime measurement. We considered a variety of scenarios, including measurements made from orbit about the Earth, Moon, and Venus, as well as on the surface of the Moon. For a standard-sized neutron detector, a measurement with three-second statistical precision can be obtained from Venus orbit in less than a day; a one-second statistical precision can be obtained from Venus orbit in less than a week. Similarly precise measurements in Earth orbit and on the lunar surface can be acquired in less than 40 days (three-second precision) and ~300 days (one-second precision). Systematic uncertainties that affect a space-based neutron lifetime measurement are investigated, and the feasibility of developing such an experiment is discussed.

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arXiv2020

Space-Based Measurement of the Neutron Lifetime using Data from the Neutron Spectrometer on NASA's MESSENGER Mission

We establish the feasibility of measuring the neutron lifetime via an alternative, space-based class of methods, which use neutrons generated by galactic cosmic ray spallation of planets' surfaces and atmospheres. Free neutrons decay via the weak interaction with a mean lifetime of around 880 s. This lifetime constrains the unitarity of the CKM matrix and is a key parameter for studies of Big-Bang nucleosynthesis. However, current laboratory measurements, using two independent approaches, differ by over 4$σ$. Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASA's MESSENGER spacecraft, which was not designed to make this measurement, we estimate the neutron lifetime to be $780\pm60_\textrm{stat}\pm70_\textrm{syst}$ s, thereby demonstrating the viability of this new approach.

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arXiv2019

Chemically-distinct regions within Venus' atmosphere revealed by MESSENGER-measured N2 concentrations

A defining characteristic of the planet Venus is its thick, CO2-dominated atmosphere. Despite over fifty years of robotic exploration, including thirteen successful atmosphere probes and landers, our knowledge of N2, the second-most-abundant compound in the atmosphere, is highly uncertain (von Zahn et al., 1983). We report the first measurement of the nitrogen content of Venus' atmosphere at altitudes between 60 and 100 km. Our result, 5.0 +/- 0.4 v% N2, is significantly higher than the value of 3.5 v% N2 reported for the lower atmosphere (<50 km altitude). We conclude that Venus' atmosphere contains two chemically-distinct regions, contrasting sharply with the expectation that it should be uniform across these altitude due to turbulent mixing (e.g. Oyama et al., 1980). That the lower-mass component is more concentrated at high altitudes suggests that the chemical profile of the atmosphere above 50-km altitude reflects mass segregation of CO2 and N2. A similar boundary between well-mixed and mass-segregated materials exists for Earth, however it is located at a substantially higher altitude of ~100 km. That Venus' upper and lower atmosphere are not in chemical equilibrium complicates efforts to use remote sensing measurements to infer the properties of the lower atmosphere and surface, a lesson that also applied to the growing field of exoplanet astronomy. The observation of periodic increases in SO2 concentrations in Venus' upper atmosphere, which has been cited as evidence for active volcanic eruptions at the surface (Esposito et al., 1984), may instead be attributable to atmosphere processes that periodically inject SO2 from the lower atmosphere into the upper atmosphere.

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arXiv2018

Image Reconstruction Techniques in Neutron and Gamma-Ray Spectroscopy: Improving Lunar Prospector Data

We present improved resolution maps of the Lunar Prospector Neutron Spectrometer thermal, epithermal and fast neutron data and Gamma-Ray Spectrometer Th-line fluxes via global application of pixon image reconstruction techniques. With the use of mock data sets, we show that the pixon image reconstruction method compares favorably with other methods that have been used in planetary neutron and gamma-ray spectroscopy. The improved thermal neutron maps are able to clearly distinguish variations in composition across the lunar surface, including within the lunar basins of Hertzsprung and Schrodinger. The improvement in resolution reveals a correlation between albedo and thermal neutron flux within the craters. The consequent increase in dynamic range confirms that Hertzsprung basin contains one of the most anorthositic parts of the lunar crust, including nearly pure anorthite over a region tens of km in diameter. At Orientale, the improvement in spatial resolution of the epithermal neutron data show that there is a mismatch between measures of regolith maturity that sample the surface and those that probe the near-subsurface, which suggests a complex layering scenario.

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arXiv2017

The Chemical Composition of Mercury

The chemical composition of a planetary body reflects its starting conditions modified by numerous processes during its formation and geological evolution. Measurements by X-ray, gamma-ray, and neutron spectrometers on the MESSENGER spacecraft revealed Mercury's surface to have surprisingly high abundances of the moderately volatile elements sodium, sulfur, potassium, chlorine, and thorium, and a low abundance of iron. This composition rules out some formation models for which high temperatures are expected to have strongly depleted volatiles and indicates that Mercury formed under conditions much more reducing than the other rocky planets of our Solar System. Through geochemical modeling and petrologic experiments, the planet's mantle and core compositions can be estimated from the surface composition and geophysical constraints. The bulk silicate composition of Mercury is likely similar to that of enstatite or metal-rich chondrite meteorites, and the planet's unusually large core is most likely Si rich, implying that in bulk Mercury is enriched in Fe and Si (and possibly S) relative to the other inner planets. The compositional data for Mercury acquired by MESSENGER will be crucial for quantitatively testing future models of the formation of Mercury and the Solar System as a whole, as well as for constraining the geological evolution of the innermost planet.

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arXiv2013

Olivine or Impact Melt: Nature of the "Orange" Material on Vesta from Dawn

NASA's Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types, a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), b) lobate patches with well-defined edges, and c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed "Leslie feature" first identified by Gaffey (1997) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.

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