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NASA NTRS1992

Magnetic field decay in isolated neutron stars

Three mechanisms that promote the loss of magnetic flux from an isolated neutron star - Ohmic decay, ambipolar diffusion, and Hall drift - are investigated. Equations of motions are solved for charged particles in the presence of a magnetic field and a fixed background of neutrons, while allowing for the creation and destruction of particles by weak interactions. Although these equations apply to normal neutrons and protons, the present interpretations of their solutions are extended to cover cases of neutron superfluidity and proton superconductivity. The equations are manipulated to prove that, in the presence of a magnetic force, the charged particles cannot be simultaneously in magnetostatic equilibrium and chemical equilibrium with the neutrons. The application of the results to real neutron stars is discussed.

Goldreich, Peter↗

NASA NTRS1993

Thermal evolution of neutron stars with internal frictional heating

It has been suggested that the frictional interaction of neutron superfluids with normal matter in the inner crust of neutron stars dissipates rotational energy of superfluids and generates heat. Incorporating a general formula of internal heating into the detailed numerical codes of thermal evolution, we examine the effects of the internal heating on thermal evolution of neutron stars. We find that when a very stiff equation of state is used, it takes as long as about 2 x 10 exp 4 yr for the interior of a neutron star to reach the isothermal state, even if a strong heat source is placed in a thin layer of the inner crust. This time scale reduces to a few hundred years or less when medium to soft equations of state are used. A neutron star cools by neutrino emissions during the earlier stages referred to as the neutrino cooling era, while during the later photon cooling era it cools primarily by emission of photons from its surface. We show that heating rates expected in the current superfluid-crust interaction model can greatly increase the surface temperature in the photon cooling era, significantly changing the thermal evolution of relatively old neutron stars if a stiff equation of state is adopted.

Umeda, Hideyuki↗

NASA NTRS1991

Plasma physics of accreting neutron stars

Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

Ghosh, Pranab↗

NASA NTRS1993

The Los Alamos neutron spectrometer for the Lunar Scout-1 mission

We review the current status of the Los Alamos program to develop a neutron spectrometer for the Lunar Scout-1 mission, which is the first of two such missions to obtain global compositional, gravity, topography, and image maps of the lunar surface during nominal one-year missions. The neutron spectrometer will measure fast and slow (epithermal and thermal) neutrons in the ranges of 0.5 MeV to 25 MeV and 0.01 eV to more than 1 keV, respectively. The neutron spectrometer will consist of two independent instruments, a fast-neutron one, and a thermal and epithermal one. The measured neutron fluxes are very sensitive to hydrogen in the top meter of the lunar surface and provide additional information about lunar composition.

Auchampaugh, George↗

NASA NTRS1993

Redistribution of subsurface neutrons caused by ground ice on Mars

As part of an ongoing effort to understand data that will be returned by the Mars Observer Gamma Ray Spectrometer (MOGRS) experiment, we calculate neutron and gamma ray fluxes for simple models describing near-surface ground ice on Mars. Our goal is to determine the limits in information content of remotely sensed neutrons and gamma rays is delineating the abundance of ground ice and its distribution with depth. Such ice is expected based on simple theoretical calculations of the H2O environment of the martian surface and atmosphere. We find it produces a marked redistribution of neutrons with depth. Neutron-capture gamma ray production closely follows the flux of thermal neutrons and is likewise redistributed with depth. Calculated effects at Mars mapping orbit are sufficiently large that, if ice is present at the depths and in the quantities expected, the MOGRS should provide a robust detection of its presence. Although in the information content of gamma rays and neutrons alone is not sufficient to define the many parameters needed to specify its abundance and emplacement below the surface, their combined use might allow separation of the ice abundance from its vertical distribution within the regolith.

Feldman, W. C.↗

NASA NTRS1997

Assay of the Martian Regolith with Neutrons

The purpose of the research is to combine experiments and Monte Carlo transport of neutrons through volume of soil in an attempt to model neutron leakage from planetary surfaces. Emphasis is given to the change of neutron spectra as a function of water content and location. During the first stage of effort, two experiments were conducted in which leakage of neutrons from a Pu-Be source through about 30 g/cm(exp 2) of soil were measured with several counters. A Monte Carlo code, MCNP, has been used to model many of the 100 individual runs of the experiment. Hydrogen is the element that has the most dramatic effect on the neutron spectrum and its effect on the neutron spectrum is almost the same whether it is in the form of water or polyethylene. In order to simulate various water configurations, sheets of polyethylene have been used between layers of soil as well as water in several concentrations up to 18%. Comparison of experimental results to theoretical predictions made with the MCNP code were disappointing for low concentrations of water. We have made extensive calculations to see if room return could be the cause of the discrepancies. Water concentrations of the 'dry' soil were measured by two different laboratories and differed only by 0.5%. We have made calculations to optimize the next experiment and are investigating other methods of determining the water content of 'dry' soil.

Drake, Darrell M.↗

NASA NTRS2004

Mars: The Terra Arabia Low Epithermal Neutron Flux Anomaly

The High Energy Neutron Spectrometer (HEND) and Neutron Specrometer, part of the Mars Odyssey Gamma Ray Spectrometer suite of instruments, measured anomalously low epithermal neutron flux in two low-latitude areas, Terra Arabia and SW of Olympus Mons (SWOM). The low epithermal neutron flux, an indirect measure of Hydrogen abundance, is indicative of relatively high water content (in this case up to 8 mass percent) in the upper 1-2 m of the surface layer. The epithermal flux within the 60 degree latitude zone does not correlate with bedrock geology or topography but partially correlates (for Arabia) with thermal inertia. According to theoretical estimates for the current geologic epoch, ground ice should not be stable at this depth at these low latitudes so it was concluded that the anomalies are due to the presence of chemically bound water (e.g. clays, hydroxides or hydrosalts). Fast neutron flux, which is indicative of the presence of water at 20-30 cm depth, does not correlate in this latitude zone with the epithermal neutron flux. As a further step of the analysis a geotraverse including study of 152 high resolution MOC images within the Arabia anomaly was done by to find out if the area inside the anomaly differs from the neighboring areas by the presence of fluvial channels and layered deposits (possible links to chemically bound water) or in thickness and apparent texture of the surface layer. No differences were found.

Basilevsky, A. T.↗

NASA NTRS2003

Neutron Particle Effects on a Quad-Redundant Flight Control Computer

This paper describes a single-event upset experiment performed at the Los Alamos National Laboratory. A closed-loop control system consisting of a Quad-Redundant Flight Control Computer (FCC) and a B737 simulator was operated while the FCC was exposed to a neutron beam. The purpose of this test was to analyze the effects of neutron bombardment on avionics control systems operating at altitudes where neutron strikes are probable. The neutron energy spectrum produced at the Los Alamos National Laboratory is similar in shape to the spectrum of atmospheric neutrons but much more intense. The higher intensity results in accelerated life tests that are representative of the actual neutron radiation that a FCC may receive over a period of years.

Eure, Kenneth↗

NASA NTRS2010

A Unique Outside Neutron and Gamma Ray Instrumentation Development Test Facility at NASA's Goddard Space Flight Center

An outside neutron and gamma ray instrumentation test facility has been constructed at NASA's Goddard Space Flight Center (GSFC) to evaluate conceptual designs of gamma ray and neutron systems that we intend to propose for future planetary lander and rover missions. We will describe this test facility and its current capabilities for operation of planetary in situ instrumentation, utilizing a l4 MeV pulsed neutron generator as the gamma ray excitation source with gamma ray and neutron detectors, in an open field with the ability to remotely monitor and operate experiments from a safe distance at an on-site building. The advantage of a permanent test facility with the ability to operate a neutron generator outside and the flexibility to modify testing configurations is essential for efficient testing of this type of technology. Until now, there have been no outdoor test facilities for realistically testing neutron and gamma ray instruments planned for solar system exploration

Bodnarik, J.↗

NASA NTRS2003

Neutron Measurements for Radiation Protection in Low Earth Orbit - History and Future

The neutron environment inside spacecraft has been of interest from a scientific and radiation protection perspective since early in the history of manned spaceflight. With 1:.1e exception of a few missions which carried plutonium-fueled radioisotope thermoelectric generators, all of the neutrons inside the spacecraft are secondary radiations resulting from interactions of high-energy charged particles with nuclei in the Earth's atmosphere, spacecraft structural materials, and the astronaut's own bodies. Although of great interest, definitive measurements of the spacecraft neutron field have been difficult due to the wide particle energy range and the limited available volume and power for traditional techniques involving Bonner spheres. A multitude of measurements, however, have been made of the neutron environment inside spacecraft. The majority of measurements were made using passive techniques including metal activation fo ils, fission foils, nuclear photoemulsions, plastic track detectors, and thermoluminescent detectors. Active measurements have utilized proton recoil spectrometers (stilbene), Bonner Spheres eRe proportional counter based), and LiI(Eu)phoswich scintillation detectors. For the International Space Station (ISS), only the plastic track! thermoluminescent detectors are used with any regularity. A monitoring program utilizing a set of active Bonner spheres was carried out in the ISS Lab module from March - December 200l. These measurements provide a very limited look at the crew neutron exposure, both in time coverage and neutron energy coverage. A review of the currently published data from past flights will be made and compared with the more recent results from the ISS. Future measurement efforts using currently available techniques and those in development will be also discussed.

Golightly, M. J.↗

NASA NTRS2011

Reconciling LCROSS and Orbital Neutron Water Abundance Estimates in Cabeus Crater

The Lunar Prospector Neutron Spectrometer (LPNS) first revealed Cabeus crater (84.9 deg S, 35.5degW) as having the highest inferred hydrogen on the Moon. Because of the broad LPNS footprint (approximately 40 km FWHM), the apparent peak water-equivalent hydrogen (WEH) concentration is only approximately 0.25 wt%, but could be much higher in smaller areas than the spectrometer footprint. Earlier image reconstruction work suggested that areas within permanent shadow have abundances approximately 1 wt% WEH. However, the LCROSS impact yielded total water estimates, ice plus vapor, of between 3 and 10 wt%. The large disagreement between LCROSS and apparent orbital values imply that either the ice is buried, by perhaps as much as 50 to 100 cm; or the ice distribution within Cabeus is spatially inhomogeneous, or both. Modeling reveals that the areal extent of a "shallow permafrost zone" is far greater than the area of permanent shadow. Ice can be virtually stable for billions of years within a few tens of centimeters of the surface in these areas. However, the LCROSS impact took place in an area of permanent shadow. If stably-trapped volatiles can be found in locales that receive occasional, oblique sunlight, landed missions may target these sites and eventual resource exploitation may be done more easily. Are orbital neutron data consistent with areally-extensive, volatile-rich cold traps? Orbital epithermal neutron data over the northern half of Cabeus (near the LCROSS impact site) are consistent with 0.2 wt% WEH or less in the "permafrost zone" near the crater. On the other hand, pixon reconstructions that confine the hydrogen enhancements to permanent shadow result in higher abundance estimates -- around 1 wt% if homogeneously mixed. But if the PSR abundance is increased to 10 wt%, consistent with the sum of all H-bearing compounds seen by LCROSS, a much larger-than-observed reduction in neutron count rate would be seen from orbit. It is likely that volatiles are inhomogeneously distributed, due to both impact processes and emplacement history. Two possibilities may bring consistency to the orbital and LCROSS measurements. Inhomogeneous lateral distribution: Consider the extreme case of a bimodal distribution within the crater -- dry and wet. In this case the epithermal leakage flux seen from orbit is a mixture of two different values, weighted according to fractional areas. Two possible outcomes, depending on whether the inferred leakage flux for the PSR or "permafrost" areas are considered. In the first case, approximately 40% of the PSR may be "wet", the remainder dry (and LCROSS was slightly lucky). However, if the whole area of permafrost is considered, then as little as 20% of the area will be as "wet" as the LCROSS results (and LCROSS was quite lucky). Inhomogeneous depth distribution: The leakage flux of thermal and epithermal neutrons depends on depth of burial of an icy layer beneath dry ferroan anorthosite soil (FAn). For the Cabeus PSR, the pixon reconstruction values for the epithermal flux allows a range of abundance and burial depth, while that of the thermal+epi detector constrains this range. (Uncertainties in iron abundance in the FAn can have significant impact on thermal neutron leakage flux estimates.) Between 20% and 40% of the Cabeus floor may be "wet", or alternatively a 5-10 wt% "wet" layer exists between 50 and 100 cm beneath a layer of dry regolith within the PSR. But volatile abundances of 5 wt% or more, distributed uniformly and homogeneously throughout the Cabeus PSR do not agree with orbital measurements

Elphic, Richard↗

NASA NTRS2011

Saturn Neutron Exosphere as Source for Inner and Innermost Radiation Belts

Energetic proton and electron measurements by the ongoing Cassini orbiter mission are expanding our knowledge of the highest energy components of the Saturn magnetosphere in the inner radiation belt region after the initial discoveries of these belts by the Pioneer 11 and Voyager 2 missions. Saturn has a neutron exosphere that extends throughout the magnetosphere from the cosmic ray albedo neutron source at the planetary main rings and atmosphere. The neutrons emitted from these sources at energies respectively above 4 and 8 eV escape the Saturn system, while those at lower energies are gravitationally bound. The neutrons undergo beta decay in average times of about 1000 seconds to provide distributed sources of protons and electrons throughout Saturn's magnetosphere with highest injection rates close to the Saturn and ring sources. The competing radiation belt source for energetic electrons is rapid inward diffusion and acceleration of electrons from the middle magnetosphere and beyond. Minimal losses during diffusive transport across the moon orbits, e.g. of Mimas and Enceladus, and local time asymmetries in electron intensity, suggest that drift resonance effects preferentially boost the diffusion rates of electrons from both sources. Energy dependences of longitudinal gradient-curvature drift speeds relative to the icy moons are likely responsible for hemispheric differences (e.g., Mimas, Tethys) in composition and thermal properties as at least partly produced by radiolytic processes. A continuing mystery is the similar radial profiles of lower energy (<10 MeV) protons in the inner belt region. Either the source of these lower energy protons is also neutron decay, but perhaps alternatively from atmospheric albedo, or else all protons from diverse distributed sources are similarly affected by losses at the moon' orbits, e.g. because the proton diffusion rates are extremely low. Enceladus cryovolcanism, and radiolytic processing elsewhere on the icy moon and ring surfaces, are additional sources of protons via ionization and charge exchange from breakup of water molecules. But one must then account somehow for local acceleration to the observed keV-MeV energies, since moon sweeping and E-ring absorption would remove protons diffusing inward from the middle magnetosphere. Although the main rings block further inward diffusion from the inner radiation belts, the exospheric neutron-decay source, combined with much slower diffusion of protons relative to electrons, may produce an innermost radiation belt in the gap between the upper atmosphere and the D-ring. This innermost belt will first be explored in-situ during the final proximal orbits of the Cassini mission.

Cooper, John↗

NASA NTRS2011

From X-Ray Telescopes to Neutron Focusing

In the case of neutrons the refractive index is slightly less than unity for most elements and their isotopes. Consequently, thermal and cold neutrons can be reflected from smooth surfaces at grazing-incidence angles. Hence, the optical technologies developed for x-ray astronomy can be applied for neutron focusing. The focusing capabilities of grazing incidence neutron imaging optics have been successfully demonstrated using nickel mirrors. The mirrors were fabricated using an electroformed nickel replication process at Marshall Space Flight Center. Results of the neutron optics experiments will be presented. Challenges of the neutron imaging optics as well as possible applications of the optics will be discussed.

Gubarev, M. V.↗

NASA NTRS2011

PING Gamma Ray and Neutron Measurements of a Meter-Sized Carbonaceous Asteroid Analog

Determining the elemental composition of carbonaceous (spectral type C) asteroids is still one of the basic problems when studying these objects. The only main source of elemental composition information for asteroids is from their optical, NIR and IR properties, which include their spectral reflectance characteristics, albedo, polarization, and the comparison of optical spectroscopy with meteorite groups corresponding to asteroids of every spectral type. Unfortunately, these sources reflect observations from widely contrasting spatial scales that presently yield a void in the continuum of microscopic and macroscopic evidence, a lack of in situ measurement confirmation, and require deeper sensing techniques to discern the nature of these asteroids. The Probing In situ with Neutrons and Gamma rays (PING) instrument is ideally suited to address this problem because it can be used to determine the bulk elemental composition, H and C content, the average atomic weight and density of the surface and subsurface layers of C-type asteroids, and can provide measurements used to determine the difference between and distinguish between different types of asteroids. We are currently developing the PING instrument that combines gamma ray and neutron detectors with a 14 Me V pulsed neutron generator to determine the in-situ bulk elemental abundances and geochemistry of C-type asteroids with a spatial resolution of 1 m down to depths of tens of cm to 1 m. One aspect of the current work includes experimentally testing and optimizing PING on a known meter-sized Columbia River basalt C-type asteroid analog sample that has a similar composition and the same neutron response as that of a C-type asteroid. An important part of this effort focuses on utilizing timing measurements to isolate gamma rays produced by neutron inelastic scattering, neutron capture and delayed activation processes. Separating the gamma ray spectra by nuclear processes results in higher precision and sensitivity elemental composition measurements. Using gated data acquisition techniques allows for the unambiguous identification of gamma ray lines from different isotopes and nuclear processes, especially in situations when limited detector resolution results in overlapping gamma ray lines that cannot be individually resolved. In this paper, we will present the PING basalt layering experimental data, taken at the test facility at NASA Goddard Space Flight Center using the time tagged event-by-event data analysis technique, compared to our MCNPX computer simulation results for the C-type asteroid and basalt layering simulant models. Comparison of these data will show the advantages, validity, and measurement sensitivity of PING's nuclear interrogation methods to obtain more precise and sensitive in situ bulk elemental composition and density measurements of the subsurface of asteroids.

Bodnarik, J.↗

NASA NTRS2011

Optical Sensors for Monitoring Gamma and Neutron Radiation

For safety and efficiency, nuclear reactors must be carefully monitored to provide feedback that enables the fission rate to be held at a constant target level via adjustments in the position of neutron-absorbing rods and moderating coolant flow rates. For automated reactor control, the monitoring system should provide calibrated analog or digital output. The sensors must survive and produce reliable output with minimal drift for at least one to two years, for replacement only during refueling. Small sensor size is preferred to enable more sensors to be placed in the core for more detailed characterization of the local fission rate and fuel consumption, since local deviations from the norm tend to amplify themselves. Currently, reactors are monitored by local power range meters (LPRMs) based on the neutron flux or gamma thermometers based on the gamma flux. LPRMs tend to be bulky, while gamma thermometers are subject to unwanted drift. Both electronic reactor sensors are plagued by electrical noise induced by ionizing radiation near the reactor core. A fiber optic sensor system was developed that is capable of tracking thermal neutron fluence and gamma flux in order to monitor nuclear reactor fission rates. The system provides near-real-time feedback from small- profile probes that are not sensitive to electromagnetic noise. The key novel feature is the practical design of fiber optic radiation sensors. The use of an actinoid element to monitor neutron flux in fiber optic EFPI (extrinsic Fabry-Perot interferometric) sensors is a new use of material. The materials and structure used in the sensor construction can be adjusted to result in a sensor that is sensitive to just thermal, gamma, or neutron stimulus, or any combination of the three. The tested design showed low sensitivity to thermal and gamma stimuli and high sensitivity to neutrons, with a fast response time.

Boyd, Clark D.↗

NASA NTRS2012

Estimation of Orbital Neutron Detector Spatial Resolution by Systematic Shifting of Differential Topographic Masks

We present a method and preliminary results related to determining the spatial resolution of orbital neutron detectors using epithermal maps and differential topographic masks. Our technique is similar to coded aperture imaging methods for optimizing photonic signals in telescopes [I]. In that approach photon masks with known spatial patterns in a telescope aperature are used to systematically restrict incoming photons which minimizes interference and enhances photon signal to noise. Three orbital neutron detector systems with different stated spatial resolutions are evaluated. The differing spatial resolutions arise due different orbital altitudes and the use of neutron collimation techniques. 1) The uncollimated Lunar Prospector Neutron Spectrometer (LPNS) system has spatial resolution of 45km FWHM from approx. 30km altitude mission phase [2]. The Lunar Rennaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) with two detectors at 50km altitude evaluated here: 2) the collimated 10km FWHM spatial resolution detector CSETN and 3) LEND's collimated Sensor for Epithermal Neutrons (SETN). Thus providing two orbital altitudes to study factors of: uncollimated vs collimated and two average altitudes for their effect on fields-of-view.

McClanahan, T. P.↗

NASA NTRS1999

Neutron yields from 155 MeV/nucleon carbon and helium stopping in aluminum

Neutron fluences have been measured from 155 MeV/nucleon 4He and 12C ions stopping in an Al target at laboratory angles between 10 and 160 deg. The resultant spectra were integrated over angle and energy above 10 MeV to produce total neutron yields. Comparison of the two systems shows that approximately two times as many neutrons are produced from 155 MeV/nucleon 4He stopping in Al and 155 MeV/nucleon 12C stopping in Al. Using an energy-dependent geometric cross-section formula to calculate the expected number of primary nuclear interactions shows that the 12C + Al system has, within uncertainties, the same number of neutrons per interaction (0.99 +/- 0.03) as does the 4He + Al system (1.02 +/- 0.04), despite the fact that 12C has three times as many neutrons as does 4He. Energy and angular distributions for both systems are also reported. No major differences can be seen between the two systems in those distributions, except for the overall magnitude. Where possible, the 4He + Al spectra are compared with previously measured spectra from 160 and 177.5 MeV/nucleon 4He interactions in a variety of stopping targets. The reported spectra are consistent with previously measured spectra. The data were acquired to provide data applicable to problems dealing with the determination of the radiation risk to humans engaged in long-term missions in space; however, the data are also of interest for issues related to the determination of the radiation environment in high-altitude flight, with shielding at high-energy heavy-ion accelerators and with doses delivered outside tumor sites treated with high-energy hadronic beams.

NASA Discipline Radiation Health↗

NASA NTRS2001

Estimation of neutron and other radiation exposure components in low earth orbit

The interaction of high-energy space radiation with spacecraft materials generates a host of secondary particles, some, such as neutrons, are more biologically damaging and penetrating than the original primary particles. Before committing astronauts to long term exposure in such high radiation environments, a quantitative understanding of the exposure and estimates of the associated risks are required. Energetic neutrons are traditionally difficult to measure due to their neutral charge. Measurement methods have been limited by mass and weight requirements in space to nuclear emulsion, activation foils, a limited number of Bonner spheres, and TEPCs. Such measurements have had limited success in quantifying the neutron component relative to the charged components. We will show that a combination of computational models and experimental measurements can be used as a quantitative tool to evaluate the radiation environment within the Shuttle, including neutrons. Comparisons with space measurements are made with special emphasis on neutron sensitive and insensitive devices. c2001 Elsevier Science Ltd. All rights reserved.

STS-57 Shuttle Project↗