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NASA NTRS1983

On neutron star structure and the millisecond pulsar

The millisecond pulsar is the first observed example of a neutron star spinning rapidly enough to approach the Jacobi bifurcation point and thus affords the possibility of constraining neutron star physics. The pulsar must be rotating below the critical frequency at which its equilibrium configuration would become nonaxisymmetric, since the lifetime of this configuration against decay by gravitational radiation is very short. This critical frequency may be used to set a lower limit of 2 x 10 to the 14th g/cu cm on the density of the star. If the mass is 0.5-1.5 solar mass, several of the stiffer neutron star equations of state may be ruled out, and the radius should be less than 16 km. The condition for axisymmetry also imposes an upper limit on the rotation rate to which neutron stars may be spun up by accretion disks in binary systems.

Harding, A. K.↗
NASA NTRS1983

Structure of neutron star envelopes

The envelopes of nonmagnetic neutron stars are studied using the best available opacities and equation of state. The general relativistic equations of the structure and evolution of nonmagnetic neutron stars are discussed, and it is shown that they can be reduced to a single equation for calculating the thermal structure of neutron star envelopes. The physical input needed to solve the thermal structure equation is investigated and the numerical results of envelope model calculations are presented. It is shown that the thermal structure of neutron star envelopes is determined by the single parameter T(s) to the 4th/g(s), where T(s) is the effective surface temperature and g(s) the surface gravity of the star. This result is used to derive a number of other scaling relations, and the effects of general relativity on the envelope thermal structure are examined in detail. The results of a sensitivity analysis of the regional opacity needed to obtain a reliable relationship between the temperatures of the inner and outer boundaries of the envelope is presented.

Gudmundsson, E. H.↗
NASA NTRS1984

Electromagnetic damping of neutron star oscillations

A simple model of magnetic field perturbations driven by neutron star oscillations is used to estimate the electromagnetic power radiated by g-modes and torsional oscillations. The calculation assumes that the neutron star has a frozen-in magnetic field which is perturbed by the oscillatory motions of the surface. The disturbances propagate into the vacuum as outgoing electromagnetic waves. The relative effectiveness of Joule heating of the neutron star crust by pulsation-induced electric currents is estimated. It is concluded that electromagnetic damping is the dominant energy dissipation mechanism for quadrupole g-mode oscillations of neutron stars. For dipole spheroidal modes, both electromagnetic radiation and Joule heating are important, and there is no gravitational radiation emitted by these modes.

Mcdermott, P. N.↗
NASA NTRS1985

Search for solar neutrons using NM-64 equipment

Two years (1980 to 1982) neutron monitor data from the Chacaltaya (geographic coordinates: N16.32 deg W68. 15 deg; cutoff rigidity: 13.1 GV; altitude: 5,300 m a.s.l.) station has been scanned; the sampling time of the 12NM-64 neutron monitor is 5 min. The nucleonic component increases have been correlated with 66 hard X-, gamma rays satellite data from solar origin, as reported by several groups. Typical neutron monitor time profiles of the events are presented. Chree-analysis was performed discriminating the events according to its solar coordinates. Ground data from solar limb locii are more enhanced at the time of the onset than other geometrically visible flares. Chree histograms of neutron monitor output profiles are also presented from geometrically invisible events from the Chacaltaya station.

Martinic, N. J.↗
NASA NTRS1985

Neutron-induced 2.2 MeV background in gamma ray telescopes

Neutron-induced gamma ray production is an important source of background in Compton scatter gamma ray telescopes where organic scintillator material is used. Most important is deuteron formation when atmospheric albedo and locally produced neutrons are thermalized and subsequently absorbed in the hydrogenous material. The resulting 2.2 MeV gamma ray line radiation essentially represents a continuous isotropic source within the scintillator itself. Interestingly, using a scintillator material with a high hydrogen-to-carbon ratio to minimize the scintillator material with a high hydrogen-to-carbon ratio to minimize the neutron-induced 4.4 MeV carbon line favors the np reaction. The full problem of neutron-induced background in Compton scatter telescopes has been previously discussed. Results are presented of observations with the University of California balloon-borne Compton scatter telescope where the 2.2 MeV induced line emission is prominently seen.

Zanrosso, E. M.↗
NASA NTRS1985

Monte Carlo calibration of the SMM gamma ray spectrometer for high energy gamma rays and neutrons

The Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission spacecraft was primarily designed and calibrated for nuclear gamma ray line measurements, but also has a high energy mode which allows the detection of gamma rays at energies above 10 MeV and solar neutrons above 20 MeV. The GRS response has been extrapolated until now for high energy gamma rays from an early design study employing Monte Carlo calculations. The response to 50 to 600 MeV solar neutrons was estimated from a simple model which did not consider secondary charged particles escaping into the veto shields. In view of numerous detections by the GRS of solar flares emitting high energy gamma rays, including at least two emitting directly detectable neutrons, the calibration of the high energy mode in the flight model has been recalculated by the use of more sophisticated Monte Carlo computer codes. New results presented show that the GRS response to gamma rays above 20 MeV and to neutrons above 100 MeV is significantly lower than the earlier estimates.

Cooper, J. F.↗
NASA NTRS1983

The solar cosmic ray neutron event on June 3, 1982

The observation of solar neutrons from the earth has been reported. The observations were carried out June 3, 1982, following the intense gamma-ray line flare at 1143 UT. The emission spectra of the neutrons was calculated over an energy range from 40 MeV to 1.2 GeV, on the basis of measurements from a gamma-ray/neutron scintillation spectrometer, and the IGY Jungfraujoch neutron monitor at an altitude of 3500 m. Some possible terrestrial effects of the solar neutrons are discussed. It is shown that the calculated spectra conform to the requirement of ion acceleration to GeV energies a few seconds after the start of the impulsive phase of a solar ion flare event.

Debrunner, H.↗
NASA NTRS1983

Solar flare neutron fluxes derived from interplanetary charged particle measurements

The first observation of interplanetary protons produced by the decay of solar neutrons made by Evenson et al. (1983) after a solar flare which occurred on June 3, 1982, is expanded, extending the measurement of the spectrum of the decay protons to higher energy. The spectrum of the decay protons for the June 21, 1980, neutron event observed by Chupp et al. (1982) is also determined. The measurements suggest that neutron emission from solar flares is isotropic and that different flares emit neutrons with similar spectra. The importance of such measurements in the study of interplanetary propagation of charged particles is discussed.

Evenson, P.↗
NASA NTRS1984

Probing the interiors of neutron stars

This paper reviews the information on neutron stars that has been obtained from observations of pulsars and pulsing X-ray stars, with emphasis on results from timing studies. Included are summaries of the most recent theoretical work on neutron star structure and the current evidence concerning neutron star masses and radii. Also included are discussions of large frequency jumps and small-scale timing irregularities, the description of the latter phenomenon in terms of random noise processes, and the use of both as probes of the internal structure and dynamical properties of neutron stars. Finally, some of the most promising directions for future work are briefly noted.

Lamb, F. K.↗
NASA NTRS1984

Solar-flare neutrons and gamma-rays

The numbers and spectra of the accelerated protons and nuclei that produce the neutrons and gamma-rays observed in solar flares are derived, and the results are compared with interplanetary observations of flare protons. The two most widely studied flare acceleration mechanisms, stochastic and diffusive shock acceleration, are discussed, and the arguments favoring the thick-target interaction model for neutron and gamma-ray production at the sun are briefly reviewed. The pertinent results of the theory of neutron and gamma-ray production are presented. The number and spectrum of the accelerated particles are derived from observations of nuclear deexcitation lines and the 2.223 MeV line from several flares. The June 21, 1980 and June 3, 1982 flares, from which a wealth of neutron, gamma-ray and energetic-particle data has recently become available, are discussed.

Murphy, R. J.↗
NASA NTRS1983

Determination of neutron flux distribution by using ANISN, a one-dimensional discrete S sub n ordinates transport code with anisotropic scattering

The purpose of this project was to use a one-dimensional discrete coordinates transport code called ANISN in order to determine the energy-angle-spatial distribution of neutrons in a 6-feet cube rock box which houses a D-T neutron generator at its center. The project was two-fold. The first phase of the project involved adaptation of the ANISN code written for an IBM 360/75/91 computer to the UNIVAC system at JSC. The second phase of the project was to use the code with proper geometry, source function and rock material composition in order to determine the neutron flux distribution around the rock box when a 14.1 MeV neutron generator placed at its center is activated.

Ghorai, S. K.↗
NASA NTRS1987

Neutron tori and the origin of r-process elements

It is shown here that if an accretion disk gets hotter than a few MeV, nuclei in the infalling matter are dissociated into their constituent neutrons and protons. Neutrons released by dissociation of matter falling at high accretion rates into a black hole or neutron star accumulate in a dense 'neutron torus'. The occurrence of the classical rapid or r-process nucleosynthesis in such a torus is investigated.

Hogan, C. J.↗
NASA NTRS1989

Gravitational effects on planetary neutron flux spectra

The effects of gravity on the planetary neutron flux spectra for planet Mars, and the lifetime of the neutron, were investigated using a modified one-dimensional diffusion accelerated neutral-particle transport code, coupled with a multigroup cross-section library tailored specifically for Mars. The results showed the presence of a qualitatively new feature in planetary neutron leakage spectra in the form of a component of returning neutrons with kinetic energies less than the gravitational binding energy (0.132 eV for Mars). The net effect is an enhancement in flux at the lowest energies that is largest at and above the outermost layer of planetary matter.

Feldman, W. C.↗
NASA NTRS1989

Relativistic neutrons in active galactic nuclei

The acceleration of protons to relativistic energies in active galactic nuclei leads to the creation of relativistic neutrons which escape from the central engine. The neutrons decay at distances of up to 1-100 pc, depositing their energies and momenta in situ. Energy deposition by decaying neutrons may inhibit spherical accretion and drive a wind, which could be responsible for the velocity fields in emission-line regions and the outflow of broad absorption line systems. Enhanced pressure in the neutron decay region may also help to confine emission line clouds. A fraction of the relativistic proton energy is radiated in gamma-rays with energies which may be as large as about 100,000 GeV.

Sikora, Marek↗
NASA NTRS1989

Neutron and high speed photogrammetric arcjet diagnosis

Two methods for real time internal diagnostics of arcjet engines are described. One method uses cold, thermal, or epithermal neutrons. Cold neutrons are used to detect the presence and location of hydrogenous propellants. Thermal neutrons are used to delineate the edge contours of anode and cathode surfaces and to measure stress/strain. Epithermal neutrons are used to measure temperatures on arcjet surfaces, bulk material temperatures, and point temperatures in bulk materials. It is found that this method, with an exposure time of 10 min, produces at temperature accuracy for W or Re of + or - 2.5 C. The other method uses visible-light high-speed photogrammetry to obtain images of the transient behavior of the arc during start-up and to relate this behavior to electrial supply characteristics such as voltage, current, and ripple.

Stewart, P. A. E.↗
NASA NTRS1990

Neutron and gamma irradiation effects on power semiconductor switches

The performance characteristics of high power semiconductor switches subjected to high levels of neutron fluence and gamma dose must be known by the designer of the power conditioning, control and transmission subsystem of space nuclear power systems. Location and the allowable shielding mass budget will determine the level of radiation tolerance required by the switches to meet performance and reliability requirements. Neutron and gamma ray interactions with semiconductor materials and how these interactions affect the electrical and switching characteristics of solid state power switches is discussed. The experimental measurement system and radiation facilities are described. Experimental data showing the effects of neutron and gamma irradiation on the performance characteristics are given for power-type NPN Bipolar Junction Transistors (BJTs), and Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs). BJTs show a rapid decrease in gain, blocking voltage, and storage time for neutron irradiation, and MOSFETs show a rapid decrease in the gate threshold voltage for gamma irradiation.

Schwarze, G. E.↗
NASA NTRS1989

Cosmic-ray neutron albedo dose in low-earth orbits

An earth albedo neutron environmental model is proposed which provides a way to estimate neutron exposure in low-earth orbit. It is shown that, in the predominantly low inclination orbits (i=28.5 deg) used in the U.S. space program, the neutron exposures are relatively low (0.7 cSv/y). The neutron exposures are more significant for polar orbital missions and even high inclination missions, such as Skylab (i=57 deg).

Wilson, J. W.↗