Neutron multiplicity monitor observations during 1965.
Cosmic ray secondary nucleon production of multiple neutrons observed and energy spectra measured, using multiplicity monitor
ENERGY SPECTRUM↗
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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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Nonthermal accretion disk models around neutron stars
We consider the structure and emission spectra of nonthermal accretion disks around both strongly and weakly magnetized neutron stars. Such disks may be dissipating their gravitational binding energy and transferring their angular momentum via semicontinuous magnetic reconnections. We consider specifically the structure of the disk-stellar magnetospheric boundary where magnetic pressure balances the disk pressure. We consider energy dissipation via reconnection of the stellar field and small-scale disk turbulent fields of opposite polarity. Constraints on the disk emission spectrum are discussed.
Tavani, M.↗
Nuclear model calculations and their role in space radiation research
Proper assessments of spacecraft shielding requirements and concomitant estimates of risk to spacecraft crews from energetic space radiation requires accurate, quantitative methods of characterizing the compositional changes in these radiation fields as they pass through thick absorbers. These quantitative methods are also needed for characterizing accelerator beams used in space radiobiology studies. Because of the impracticality/impossibility of measuring these altered radiation fields inside critical internal body organs of biological test specimens and humans, computational methods rather than direct measurements must be used. Since composition changes in the fields arise from nuclear interaction processes (elastic, inelastic and breakup), knowledge of the appropriate cross sections and spectra must be available. Experiments alone cannot provide the necessary cross section and secondary particle (neutron and charged particle) spectral data because of the large number of nuclear species and wide range of energies involved in space radiation research. Hence, nuclear models are needed. In this paper current methods of predicting total and absorption cross sections and secondary particle (neutrons and ions) yields and spectra for space radiation protection analyses are reviewed. Model shortcomings are discussed and future needs presented. c2002 COSPAR. Published by Elsevier Science Ltd. All right reserved.
NASA Center JSC↗
An Improved Neutron Transport Algorithm for HZETRN
Long term human presence in space requires the inclusion of radiation constraints in mission planning and the design of shielding materials, structures, and vehicles. In this paper, the numerical error associated with energy discretization in HZETRN is addressed. An inadequate numerical integration scheme in the transport algorithm is shown to produce large errors in the low energy portion of the neutron and light ion fluence spectra. It is further shown that the errors result from the narrow energy domain of the neutron elastic cross section spectral distributions, and that an extremely fine energy grid is required to resolve the problem under the current formulation. Two numerical methods are developed to provide adequate resolution in the energy domain and more accurately resolve the neutron elastic interactions. Convergence testing is completed by running the code for various environments and shielding materials with various energy grids to ensure stability of the newly implemented method.
Slaba, Tony C.↗
The Systematic Interpretation of Cosmic Ray Data (The Transport Project)
The Transport project's primary goals were to: (1) Provide measurements of critical fragmentation cross sections; (2) Study the cross section systematics; (3) Improve the galactic cosmic ray propagation methodology; and (4) Use the new cross section measurements to improve the interpretation of cosmic ray data. To accomplish these goals a collaboration was formed consisting of researchers in the US at Louisiana State University (LSU), Lawrence Berkeley Laboratory (LBL), Goddard Space Flight Center (GSFC), the University of Minnesota (UM), New Mexico State University (NMSU), in France at the Centre d'Etudes de Saclay and in Italy at the Universita di Catania. The US institutions, lead by LSU, were responsible for measuring new cross sections using the LBL HISS facility, analysis of these measurements and their application to interpreting cosmic ray data. France developed a liquid hydrogen target that was used in the HISS experiment and participated in the data interpretation. Italy developed a Multifunctional Neutron Spectrometer (MUFFINS) for the HISS runs to measure the energy spectra, angular distributions and multiplicities of neutrons emitted during the high energy interactions. The Transport Project was originally proposed to NASA during Summer, 1988 and funding began January, 1989. Transport was renewed twice (1991, 1994) and finally concluded at LSU on September, 30, 1997. During the more than 8 years of effort we had two major experiment runs at LBL, obtained data on the interaction of twenty different beams with a liquid hydrogen target, completed the analysis of fifteen of these datasets obtaining 590 new cross section measurements, published nine journal articles as well as eighteen conference proceedings papers, and presented more than thirty conference talks.
Guzik, T. Gregory↗
The difference in the spectra of gamma-rays during radiation capture of thermal neutrons and of neutrons with the reactor spectrum
Spectral differences of gamma rays during radiation capture of thermal neutrons and of neutrons with reactor spectrum
GAMMA RADIATION↗
Ge-diode detector combined with crystal-diffraction spectrometer permits high-resolution gamma ray spectroscopy
Crystal-diffraction spectrometer, combined with a lithium-drifted Ge-diode detector, performs high-resolution gamma ray spectroscopy on the complicated neutron-capture gamma ray spectra. The system is most useful in the 1-3 MeV energy range and improves the signal to background ratio.
Namenson, A. I.↗
Cascade model of gamma-ray bursts: Power-law and annihilation-line components
If, in a neutron star magnetosphere, an electron is accelerated to an energy of 10 to the 11th or 12th power eV by an electric field parallel to the magnetic field, motion of the electron along the curved field line leads to a cascade of gamma rays and electron-positron pairs. This process is believed to occur in radio pulsars and gamma ray burst sources. Results are presented from numerical simulations of the radiation and photon annihilation pair production processes, using a computer code previously developed for the study of radio pulsars. A range of values of initial energy of a primary electron was considered along with initial injection position, and magnetic dipole moment of the neutron star. The resulting spectra was found to exhibit complex forms that are typically power law over a substantial range of photon energy, and typically include a dip in the spectrum near the electron gyro-frequency at the injection point. The results of a number of models are compared with data for the 5 Mar., 1979 gamma ray burst. A good fit was found to the gamma ray part of the spectrum, including the equivalent width of the annihilation line.
Harding, A. K.↗
Cascade model of gamma-ray bursts
If, in a neutron star magnetosphere, an electron is accelerated to an energy of 10 to the 11th or 12th power eV by an electric field parallel to the magnetic field, motion of the electron along the curved field line leads to a cascade of gamma rays and electron-positron pairs. This process is believed to occur in radio pulsars and gamma ray burst sources. Results are presented from numerical simulations of the radiation and photon annihilation pair production processes, using a computer code previously developed for the study of radio pulsars. A range of values of initial energy of a primary electron was considered along with initial injection position, and magnetic dipole moment of the neutron star. The resulting spectra was found to exhibit complex forms that are typically power law over a substantial range of photon energy, and typically include a dip in the spectrum near the electron gyro-frequency at the injection point. The results of a number of models are compared with data for the 5 Mar., 1979 gamma ray burst. A good fit was found to the gamma ray part of the spectrum, including the equivalent width of the annihilation line.
Sturrock, P. A.↗
Comparison of Radiation Transport Codes, HZETRN, HETC and FLUKA, Using the 1956 Webber SPE Spectrum
Protection of astronauts and instrumentation from galactic cosmic rays (GCR) and solar particle events (SPE) in the harsh environment of space is of prime importance in the design of personal shielding, spacec raft, and mission planning. Early entry of radiation constraints into the design process enables optimal shielding strategies, but demands efficient and accurate tools that can be used by design engineers in every phase of an evolving space project. The radiation transport code , HZETRN, is an efficient tool for analyzing the shielding effectiveness of materials exposed to space radiation. In this paper, HZETRN is compared to the Monte Carlo codes HETC-HEDS and FLUKA, for a shield/target configuration comprised of a 20 g/sq cm Aluminum slab in front of a 30 g/cm^2 slab of water exposed to the February 1956 SPE, as mode led by the Webber spectrum. Neutron and proton fluence spectra, as well as dose and dose equivalent values, are compared at various depths in the water target. This study shows that there are many regions where HZETRN agrees with both HETC-HEDS and FLUKA for this shield/target configuration and the SPE environment. However, there are also regions where there are appreciable differences between the three computer c odes.
Heinbockel, John H.↗
NICER Detection of Strong Photospheric Expansion during a Thermonuclear X-Ray Burst from 4U 1820–30
The Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS) observed strong photospheric expansion of the neutron star in 4U1820–30 during a Type I X-ray burst. A thermonuclear helium flash in the star’s envelope powered a burst that reached the Eddington limit. Radiation pressure pushed the photosphere out to ∼200 km, while the blackbody temperature dropped to 0.45 keV. Previous observations of similar bursts were performed with instruments that are sensitive only above 3 keV, and the burst signal was weak at low temperatures. NICERʼs 0.2–12 keV passband enables the first complete detailed observation of strong expansion bursts. The strong expansion lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent radius, before the photosphere finally settled back down at 3 s after the burst onset. In addition to thermal emission from the neutron star, the NICER spectra reveal a second component that is well fit by optically thick Comptonization. During the strong expansion, this component is six times brighter than prior to the burst, and it accounts for 71% of the flux. In the moderate expansion phase, the Comptonization flux drops, while the thermal component brightens, and the total flux remains constant at the Eddington limit. We speculate that the thermal emission is reprocessed in the accretion environment to form the Comptonization component, and that changes in the covering fraction of the star explain the evolution of the relative contributions to the total flux.
Keek, L.↗
Cyclotron Lines in Highly Magnetized Neutron Stars
Cyclotron lines, also called cyclotron resonant scattering features are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 1012 Gauss. Also, possible proton cyclotron lines from single neutron stars with even stronger magnetic fields are briefly discussed. With regard to electron cyclotron lines, we present an updated list of XRBPs that show evidence of such absorption lines. The first such line was discovered in a 1976 balloon observation of the accreting binary pulsar Hercules X-1, it is considered to be the first direct measurement of the magnetic field of a neutron star. As of today (end 2018), we list 35 XRBPs showing evidence of one ore more electron cyclotron absorption line(s). A few have been measured only once and must be confirmed (several more objects are listed as candidates). In addition to the Tables of objects, we summarize the evidence of variability of the cyclotron line as a function of various parameters (especially pulse phase, luminosity and time), and add a discussion of the different observed phenomena and associated attempts of theoretical modeling. We also discuss our understanding of the underlying physics of accretion onto highly magnetized neutron stars. For proton cyclotron lines, we present tables with seven neutron stars and discuss their nature and the physics in these objects.
Staubert, R.↗
One-photon pair annihilation in magnetized relativistic plasmas
In supersonic magnetic fields, electron-positron pairs may annihilate into single photons producing spectral features above 1 MeV. The paper calculates the exact one-photon annihilation rate in the general case where pairs may annihilate from excited Landau states, extending the previous studies which were restricted to pairs in the ground state. Asymptotic expressions for annihilation spectra and rates in the limit of large pair quantum numbers are also derived. It is found that the rate of annihilation from excited states can exceed the rate from the ground state by orders of magnitude in fields less than about 2 x 10 to the 12th G. This allows one-photon annihilation to be competitive with the two-photon process at typical neutron star field strengths. Annihilation spectra from a Maxwellian pair plasma at transrelativistic temperatures show fine structure near threshold on a scale (h/2pi)omega sub B as the result of contributions from individual pair states, which blend into a smooth continuum at higher energies.
Harding, A. K.↗
Neutron yields from 435 MeV/nucleon Nb stopping in Nb and 272 MeV/nucleon Nb stopping in Nb and Al
Neutron fluences were measured from 435 MeV/nucleon Nb ions stopping in a Nb target and 272 MeV/nucleon Nb ions stopping in targets of Nb and Al for neutrons above 20 MeV and at laboratory angles between 3 degrees and 80 degrees. The resultant spectra were integrated over angles to produce neutron energy distributions and over energy to produce neutron angular distributions. The total neutron yields for each system were obtained by integrating over the angular distributions. The angular distributions from all three systems are peaked forward, and the energy distributions from all three systems show an appreciable yield of neutrons with velocities greater than the beam velocity. Comparison of the total neutron yields from the two Nb + Nb systems suggests that the average neutron multiplicity decreases with decreasing projectile energy. Comparison of the total yields from the two 272 MeV/nucleon systems suggests that the total yields show the same dependence on projectile and target mass number as do total inclusive neutron cross sections. The data are compared with Boltzmann-Uehling-Uhlenbeck model calculations.
NASA Discipline Radiation Health↗
Gamma-ray spectra analysed by computer programme using the peak area method.
Computer program for instrumental neutron activation analysis of chemical abundances in meteorites, examining gamma-ray spectra
ELEMENT ABUNDANCE↗
The ORNL-SNAP shielding program
The effort in the ORNL-SNAP shielding program is directed toward the development and verification of computer codes using numerical solutions to the transport equation for the design of optimized radiation shields for SNAP power systems. A brief discussion is given for the major areas of the SNAP shielding program, which are cross-section development, transport code development, and integral experiments. Detailed results are presented for the integral experiments utilizing the TSF-SNAP reactor. Calculated results are compared with experiments for neutron and gamma-ray spectra from the bare reactor and as transmitted through slab shields.
Mynatt, F. R.↗
An integrated radiation physics computer code system.
An integrated computer code system for the semi-automatic and rapid analysis of experimental and analytic problems in gamma photon and fast neutron radiation physics is presented. Such problems as the design of optimum radiation shields and radioisotope power source configurations may be studied. The system codes allow for the unfolding of complex neutron and gamma photon experimental spectra. Monte Carlo and analytic techniques are used for the theoretical prediction of radiation transport. The system includes a multichannel pulse-height analyzer scintillation and semiconductor spectrometer coupled to an on-line digital computer with appropriate peripheral equipment. The system is geometry generalized as well as self-contained with respect to material nuclear cross sections and the determination of the spectrometer response functions. Input data may be either analytic or experimental.
Steyn, J. J.↗
Skylab neutron environment experiment (Science Demonstration SD-34 (TV108)). Description and preliminary results
Neutron and proton induced radioactivity at various locations within Skylab were measured. Samples of five metals were formed into activation packets and deployed at the following locations on the Skylab 4 mission: orbital workshop film vault, water storage tank, and two opposing orbital workshop internal locations. Radioactive nuclides were produced in the packets by nuclear interactions of high-energy protons and secondary neutrons within Skylab. Low-level gamma ray spectroscopy measurements were made on the returned packets to determine the incident neutron and proton fluxes and spectra and their variations with mass distribution.
Fishman, G. J.↗