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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

An Adaptive Reference Scheme to Extend the Functional Range of Optical Backscatter Reflectometry in Extreme Environments

Optical frequency domain reflectometry (OFDR) measurements are performed by recording the interference pattern of light backscattered by density fluctuations or Bragg gratings along the length of an optical fiber. Changes in local temperature or strain in the fiber cause shifts in the backscattered light spectrum, which can be calibrated to the applied temperature or strain. Comparing the backscattered optical spectra from each reflection site with a reference (unstrained) spectrum allows for quantification of local spectral shifts. While mature OFDR-based technologies can provide high-precision spatially distributed measurements over kilometer lengths, the post-processing approaches used to recover spectral shifts from raw optical intensity data can limit the use of OFDR in harsh environments. This paper presents a novel approach to post-processing OFDR data which extends the usability of optical fibers exposed to harsh environments. This approach is the first to use the spectral shift quality, calculated for two OFDR measurements, to identify an appropriate reference scan from which the spectral shift can be determined. Here, three experimental data sets are used to test the algorithm: (1) an optical fiber heated to >950°C, (2) an aluminum-embedded optical fiber under strain from differential thermal expansion, and (3) an optical fiber exposed to a total neutron flux of 2×10 13 n/cm 2 /s over 40 hours in a nuclear test reactor. Results show that using a metric of quality to select an appropriate reference measurement extends the functional range of OFDR strain and temperature sensors. Furthermore, this algorithm can be applied to existing OFDR data.

47 OTHER INSTRUMENTATION↗
NASA NTRS1998

A Search for X-Ray Evidence of a Compact Companion to the Unusual Wolf-Rayet Star HD 50896 (EZ CMa)

We analyze results of a approx.25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays (greater than or equal to 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from less than or equal to 0.4 keV up to at least approx. 2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N(sub H) approx. 10(exp 22)/sq cm. The absorption-corrected X-ray luminosity L(sub x)(0.5 - 10 keV) = 10(exp 32.85) erg/s gives L(sub x)/ L(sub bol) approx. 10(exp -6), a value that is typical of WN stars. No X-ray variability was detected. Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near approx. 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion.

Skinner, Stephen L.↗
NASA NTRS1994

The nature of the compact X-ray source in the supernova remnant G27.4+0.0

High-resolution X-ray imaging data obtained with ROSAT is used to constrain the nature of the central compact source in the supernova remnant G27.4+0.0. Diffuse emission is seen from throughout the approximately 4 min diameter radio shell, while the central source remains unresolved at approximately 3 sec. We combine archival data from the Einstein HRI, IPC, and MPC with the ROSAT HRI data to define the X-ray spectra of the diffuse and point-like emission. The bulk of the shell radiation is consistent with that of a approximately 10(exp 7) K plasma, although a higher temperature component is also suggested by the data; coupled with the remnant's size and distance, we derive an age of between 500 and 2500 yr. The point source has a substantially harder spectrum, with a power-law photon index less than or approximately equal to 1. A search for periodic modulation from the point source yields upper limits ranging from 10%-35% for periods between 0.025 and 1000 s, depending on the assumed pulse shape. No aperiodic variability on timescales of from 10(exp 3) to 10(exp 8) s is required, although a factor of approximately 2 change between the Einstein and ROSAT eras is possible. We show that the point source cannot represent thermal emission from the surface of a young neutron star and is unlikely to be explained as nonthermal, Crab-like X-ray pulses or a small synchrotron nebula. The most likely models involve accretion-powered systems -- either a wind-fed neutron star with a massive companion or a low-mass X-ray binary. In all probability, this is the youngest X-ray binary in the Galaxy.

Helfand, David J.↗
DOE OSTI2022

On the effect of scalar flux weighing of linearly anisotropic scattering matrices in few-group transport calculations

The majority of the codes available for homogenized group constant generation for deterministic transport calculations apply the approximation of scalar flux weighting during energy group condensation of higher-order anisotropic scattering matrices. In this paper, we discuss the effect of scalar flux weighting of the linearly anisotropic scattering matrices in the frame of S P{sub 3} and S{sub 12} calculations performed for a two-dimensional VVER-440 reactor benchmark. To compare group constants generated for two neutron energy groups, an infinite pin cell was homogenized with Serpent 2 and ERANOS ECCO. Serpent 2 applies scalar flux, while ERANOS ECCO performs current weighting of the linearly anisotropic scattering matrices during energy group condensation. For analyzing the effect of the various weighting options, three simple reactor models were built assuming different core sizes using standard rectangular assemblies with 15*15 fuel pins. Diffusion, SP{sub 3} and S{sub 12} calculations were performed for the 3 models using group constants generated with Serpent 2 and ERANOS ECCO. The effect of scalar flux weighting of linearly anisotropic scattering matrices is shown by comparing the decrease in reactivity due to the decreased reactor size, as well as the assembly power distribution to reference results obtained with Serpent 2 Monte Carlo calculations. Neglecting higher than linearly anisotropic scattering and indirect application of diffusion coefficients in higher-order transport calculations is advised if angular flux-moment spectra weighted higher-order scattering matrices cannot be generated. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2025

Structural characterization of highly alloyed (Al,Gd)N thin films

Highly alloyed (Al,Gd)N is of potential interest in a variety of applications, including neutron detection and in devices such as non-volatile memory. Gd has been shown to have very low equilibrium solubility in AlN at room temperature; however, non-equilibrium deposition techniques such as sputtering are able to deposit thin films, which incorporate large amounts of Gd. Here, we characterize a highly-alloyed (Al,Gd)N combinatorial thin film grown by RF sputtering on a GaN substrate, looking for any evidence of chemical or phase segregation or structural disorder in the films. Compositions with between 13% and 32% Gd (on a cation basis) were studied. No evidence was found for chemical or phase segregation in any studied composition. Higher degrees of Gd incorporation led to greater structural disorder in the film and a tendency toward amorphization; however, electron diffraction shows that the film does not become fully amorphous at any of the studied compositions, instead retaining textured local order even at 32% Gd. Electron energy loss spectra suggest that the material retains a locally wurtzite-like tetrahedral bonding environment at all studied compositions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Status Update on the Development of Transducers and Bonding Techniques for Enabling Acoustic Measurements of Damage in Microreactor Components

This report provides an overview of potential sensors and sensor-bonding techniques to enable the online acoustic interrogation of microreactor components and enhance structural health monitoring capabilities. The report focuses primarily on optical fiber–based acoustic sensors and describes initial experimental progress toward the deployment of these sensors for microreactor applications. The general approach is to monitor the resonant frequencies of microreactor components and search for evidence of structural defects that could indicate imminent failure. If properly identified, then the components could be repaired during the next reactor outage to prevent costly unplanned shutdowns. The ability to monitor the structural health of components could also reduce the need for time-consuming visual inspections and reduce staffing to improve microreactor economic viability. Increased sensor density is also one of the first steps to moving toward eventual semiautonomous operation. The expected microreactor conditions in which acoustic sensors must survive are characterized, including temperatures, neutron fluences, thermomechanical strains, and vibrational frequencies. Optical fiber–based acoustic sensors are identified as an attractive candidate for acoustic monitoring because of their high accuracy, immunity to electromagnetic interference, and resiliency in high-temperature, high-radiation environments. Optical fiber–based intrinsic sensors, such as type-II fiber-Bragg gratings and Fabry-Pérot Cavities (FPCs), are particularly attractive for a microreactor environment because of their high temperature stability, and FPCs also enable higher frequency interrogation with a lower sensitivity to radiation-induced drift. This report describes multiple interrogation systems, but the best interrogation system for a given situation will depend on the specific microreactor application, including the desired acoustic vibrational amplitudes, modes, and resonant frequencies. Initial experiments included fabricating three FPCs, tack-welding these FPCs to stainless-steel pipes or rods and performing room-temperature acoustic sensing tests to capture the vibrational frequency content. Peaks were identified in the measured frequency spectra and compared with the theoretical fundamental frequencies obtained from Euler-Bernoulli beam theory. Two of the three FPCs measured vibrational frequencies that generally matched those obtained from theory. Future work will include similar testing on pipes or other microreactor components with intentional flaws to evaluate the ability to determine changes in resonant frequencies. Finally, these tests will be repeated at high temperatures, potentially with an applied thermomechanical stress, to include environmental conditions similar to those for a microreactor application.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2022

Update of the Nuclear Criticality Slide Rule Calculations: Plutonium systems – Delayed Fission Gamma

IRSN (France), LLNL (USA) and ORNL (USA) began a long-term collaboration effort in 2015 to update the nuclear criticality Slide Rule for the emergency response to a nuclear criticality accident. The Slide Rule permits the estimation of neutron and gamma dose rates and integrated doses based upon estimated fission yields, as a function of distance from the fission source, and time after criticality accidents for different critical systems. This paper presents results from the fourth phase of the current update of the Slide Rule project, in which delayed fission-product gamma (DFG) dose rates of unreflected plutonium critical systems were compared by several modern 3D radiation transport codes (MCNP, COG, SCALE), using updated flux-to-dose conversion factors. Dose rates are calculated for fissile material at five moderation ratios (H/Pu), at 1 m above the ground as a function of distance (between 30 cm and 1,200 m) from the external surface of the source to the center of the detector, and for periods between 1 s and 1,000 min after the critical instantaneous event. Further efforts have been devoted to the determination of the delayed gamma source, by comparing the time-dependent energy spectra obtained from several methods. Overall, DFG dose rates calculated by each participant led to consistent results. Extra effort is under way to identify the cause of the remaining differences, by comparing precisely the gamma source, and particularly nuclides inventories.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1985

Ultraviolet spectroscopic observations of HD 77581(Vela X-1 = 4U 0900-40)

Ultraviolet spectra of HD 77581 obtained with the IUE satellite in December 1982 and January 1983 are analyzed. The effective temperature is found to be 25,000 + or - 1000 K by comparing its photospheric absorption lines with those in four well-studied B0-B1 supergiants. The distance of 1.9 + or - 0.2 kpc is derived from its luminosity log L/solar-L = +5.53, which is obtained from the effective temperature and from its radius. Circumstellar matter is detected not only in the C IV and Si IV lines but also in Al III, Fe III, C II, and Mg II lines. Profiles of the C IV and Si IV lines show a similar dependence on the orbital phase as in 1978. Two distinct sharp components are found in the shortward-shifted absorptions of the Al III and Fe III lines. Once component is present only in the second half of the orbital period, and its origin may be a 'trailing wake' behind the neutron star. The other component is seen all over the period, and it can be interpreted as originating in a cool expanding region of gas in the orbital plane.

Sadakane, K.↗
NASA NTRS2012

Pulsar Pair Cascades in Magnetic Fields with Offset Polar Caps

Neutron star magnetic fields may have polar caps (PC) that are offset from the dipole axis, through field-line sweepback near the light cylinder or non-symmetric currents within the star. The effects of such offsets on electron-positron pair cascades are investigated, using simple models of dipole magnetic fields with small distortions that shift the PCs by different amounts or directions. Using a Monte Carlo pair cascade simulation, we explore the changes in the pair spectrum, multiplicity and energy flux across the PC, as well as the trends in pair flux and pair energy flux with spin-down luminosity, L(sub sd). We also give an estimate of the distribution of heating flux from returning positrons on the PC for different offsets. We find that even modest offsets can produce significant increases in pair multiplicity, especially for pulsars that are near or beyond the pair death lines for centered PCs, primarily because of higher accelerating fields. Pair spectra cover several decades in energy, with the spectral range of millisecond pulsars (MSPs) two orders of magnitude higher than for normal pulsars, and PC offsets allow significant extension of all spectra to lower pair energies. We find that the total PC pair luminosity L(sub pair) is proportional to L(sub sd), with L(sub pair) approximates 10(exp -3) L(sub sd) for normal pulsars and L(sub pair) approximates 10(exp -2) L(sub sd) for MSPs. Remarkably, the total PC heating luminosity for even large offsets increases by less than a factor of two, even though the PC area increases by much larger factors, because most of the heating occurs near the magnetic axis.

Harding, Alice K.↗
DOE OSTI2024

Spectroscopic Measurements and Models of Energy Deposition in the Substrate of Quantum Circuits by Natural Ionizing Radiation

Naturally occurring background radiation is a potential source of correlated decoherence events in superconducting qubits that will challenge error-correction schemes. In order to characterize the radiation environment in an unshielded laboratory representative of superconducting qubits’ environments, we performed broadband, spectroscopic measurements of background radiation events inside a millikelvin refrigerator. The spectrometer was designed to mimic the size and composition of a quantum circuit. Specifically, we measured the background radiation spectra in silicon substrates of two thicknesses, 500 and 1500 µm, and one area, 25 mm 2 . The observed spectra span energies from a few kilo-electron-volts up to nearly 10 MeV, are nearly featureless, and decrease in intensity by a factor of 40 000 between 100 keV and 3 MeV for the 500-µm substrate. We integrate the spectra to obtain the average event rates and deposited power levels. These quantities correspond to a rate of 0.023 events per second and a power of 4.9 keV s -1 , when counting events that deposit at least 40 keV for the 500-µm-thick substrate. We find that the cryogenic measurements are in good agreement with predictions based on simple measurements of the terrestrial gamma-ray flux outside the refrigerator, published models of cosmic-ray fluxes, a crude model of the cryostat, and radiation-transport simulations. This model requires no free parameters to predict the background radiation spectra in the silicon substrates. The agreement between measurements and predictions demonstrates that the model we present can be used to assess the relative contributions of terrestrial and cosmic-ray sources to background radiation interactions in silicon substrates of varying thickness. These spectroscopic measurements are performed with a novel combination of superconducting microresonators located on micromachined silicon islands that define the interaction volume with background radiation. The resonators transduce deposited energy to a readily detectable electrical signal. Microresonator readout closely resembles dispersive superconducting qubit readout, so similar devices—with or without micromachined islands—are suitable for integration with superconducting quantum circuits as detectors for background radiation events. For our specific laboratory conditions, we find that gamma-ray emissions from radioisotopes are responsible for the majority of events that deposit E < 1 ⁢Me⁢V. We present results demonstrating that the background radiation spectrum contains relevant contributions from cosmic-ray particles other than muons, particularly a tail of multi-mega-electron-volt events due to protons and neutrons. These observations suggest several paths to reducing the impact of background radiation on quantum circuits, supported by an empirically validated model for generating reliable predictions of radiation interactions with silicon substrates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1996

Measurements of the linear energy transfer spectra on the Mir orbital station and comparison with radiation transport models

A tissue equivalent proportional counter designed to measure the linear energy transfer spectra (LET) in the range 0.2-1250 keV/micrometer was flown in the Kvant module on the Mir orbital station during September 1994. The spacecraft was in a 51.65 degrees inclination, elliptical (390 x 402 km) orbit. This is nearly the lower limit of its flight altitude. The total absorbed dose rate measured was 411.3 +/- 4.41 microGy/day with an average quality factor of 2.44. The galactic cosmic radiation (GCR) dose rate was 133.6 microGy/day with a quality factor of 3.35. The trapped radiation belt dose rate was 277.7 microGy/day with an average quality factor of 1.94. The peak rate through the South Atlantic Anomaly was approximately 12 microGy/min and nearly constant from one pass to another. A detailed comparison of the measured LET spectra has been made with radiation transport models. The GCR results are in good agreement with model calculations; however, this is not the case for radiation belt particles and again points to the need for improving the AP8 omni-directional trapped proton models.

Mir Project↗
NASA NTRS1994

Energetic electrons in the magnetophere of Saturn

The energy spectra and angular distributions of electrons observed by Pioneer 11 as a function of radial distance in the inner magnetosphere of Saturn are reanalyzed and phase space densities are then calculated. The radial dependence of phase space density requires a distributed loss process. The loss is greatest in the region of the E ring (5.5 less than L less than 8.5) and is attributed to collisions with the ring particles in agreement with earlier work by Van Allen et al. (1980b). Quantitative analysis yields the following properties of the E ring: the particle radii are in the range of 4 x 10(exp -5) to 3.2 x 10(exp -4) cm and the thickness of the ring is approximately 3 R(sub s). Between the inner edge of the E ring (5.5 R(sub s)) and the outer edge of the A ring (2.3 R(sub s)) there are more energetic electrons than can be supplied by radial diffusion from an external source. Detailed calculations show that a cosmic ray albedo neutron decay (CRAND) source in the A and B rings is a plausible source for this excess. The radial diffusion coefficient required to explain the E ring absorption and CRAND source for electrons is 1 x 10(exp -12) greater than D(sub 0) greater than 3 x 10(exp -12) R(sub s)(exp 2)/s, assuming that D(sub LL) = D(sub 0)L(exp 3). As part of the reanalysis program, a method for the deconvolution of pitch angle distributions observed by simple detectors on a rotating spacecraft is developed. This process removes the instrumental response and rotational smear due to finite sampling periods and yields true angular distributions.

Randall, B. A.↗
NASA NTRS1994

Energetic electrons in the magnetosphere of Saturn

The energy spectra and angular distributions of electrons observed by Pioneer 11 as a function of radial distance in the inner magnetosphere of Saturn are reanalyzed and phase space densities are then calculated. The radial dependence of phase space density requires a distributed loss process. The loss is greatest in the region of the E ring (5.5 less than L less than 8.5) and is attributed to collisions with the ring particles in agreement with earlier work by Van Allen et al. (1980). Quantitative analysis yields the following properties of the E ring: the particle radii are in the range of 4 x 10(exp -5) to 3.2 x 10(exp -4) cm and the thickness of the ring is approximately 3 R(sub s). Between the inner edge of the E ring (5.5 R(sub s)) and the outer edge of the A ring (2.3 R(sub s)) there are more energetic electrons than can be supplied by radial diffusion from an external source. Detailed calculations show that a cosmic ray albedo neutron decay (CRAND) source in the A and B rings is a plausible source for this excess. The radial diffusion coefficient required to explain the E ring absorption and CRAND source for electrons is 1 x 10(exp -12) greater than D(sub 0) greater than 3 x 10(exp -12) R(exp 2, sub s)/s, assuming that D(sub LL) = D(sub 0)L(exp 3). As part of the reanalysis program, a method for the deconvolution of pitch angle distributions observed by simple detectors on a rotating spacecraft is developed. This process removes the instrumental response and rotational smear due to finite sampling periods and yields true angular distributions.

Randall, B. A.↗
NASA NTRS2023

Heavy Ion Acceleration in 3he-Rich Solar Energetic Particle Events: New Insights From Solar Orbiter

We present Solar Orbiter energetic particle observations of two 3 He-rich events with features more clearly observed than in prior studies. The event of 2022 November 9 observed from 0.59 au contained hundreds of ultra-heavy ions (UH, mass >78 amu) whereas previous observations at 1 au have shown only an occasional count or two. The event of 2023 April 8 observed from 0.29 au fortuitously had very low ambient activity, making it possible to observe spectra from the 3 He acceleration mechanism without contamination, revealing extremely low H and 4 He intensities arriving simultaneously with other ions observed in typical 3 He-rich events. Taken together with previous studies we believe these data show that 3 He-rich events have a single acceleration mechanism that is responsible for the unique abundance features of 3 He, heavy ions, and UH ions. Considering the acceleration model of Roth and Temerin (1997) that heats the ions over a broad range of gyrofrequencies away from those damped by H and 4 He, we calculate reasonable fits to the observed abundances O-Fe. A key result is that high values of, e.g., Fe/O typical of such events is not due to preferential Fe heating, but on the contrary is due mainly to the depletion of O which at elevated temperatures has a charge-to-mass ratio in the region of the waves damped by 4He. The model also naturally incorporates features of high ionization states and neutron-rich isotope enhancements that have been long-standing puzzles in observations of this type of flare.

Solar flares↗
NASA NTRS2014

Mixed-field GCR Simulations for Radiobiological Research Using Ground Based Accelerators

Space radiation is comprised of a large number of particle types and energies, which have differential ionization power from high energy protons to high charge and energy (HZE) particles and secondary neutrons produced by galactic cosmic rays (GCR). Ground based accelerators such as the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL) are used to simulate space radiation for radiobiology research and dosimetry, electronics parts, and shielding testing using mono-energetic beams for single ion species. As a tool to support research on new risk assessment models, we have developed a stochastic model of heavy ion beams and space radiation effects, the GCR Event-based Risk Model computer code (GERMcode). For radiobiological research on mixed-field space radiation, a new GCR simulator at NSRL is proposed. The NSRL-GCR simulator, which implements the rapid switching mode and the higher energy beam extraction to 1.5 GeV/u, can integrate multiple ions into a single simulation to create GCR Z-spectrum in major energy bins. After considering the GCR environment and energy limitations of NSRL, a GCR reference field is proposed after extensive simulation studies using the GERMcode. The GCR reference field is shown to reproduce the Z and LET spectra of GCR behind shielding within 20% accuracy compared to simulated full GCR environments behind shielding. A major challenge for space radiobiology research is to consider chronic GCR exposure of up to 3-years in relation to simulations with cell and animal models of human risks. We discuss possible approaches to map important biological time scales in experimental models using ground-based simulation with extended exposure of up to a few weeks and fractionation approaches at a GCR simulator.

Kim, Myung-Hee Y.↗
NASA NTRS2001

Integration of the QMSFRG Database into the HZETRN Code

Accurate nuclear interaction data bases are needed for describing the transport of space radiation in matter including space craft structures, atmospheres, and tissues. Transport models support the identification and development of new material concepts for human and electronic part protection. Quantum effects are manifested in nuclear reactions in several ways including interference effects between terms in the multiple scattering series, the many-body nuclear wave functions (for e.g. the roles of shell structure and Fermi momentum) and nuclear clustering. The quantum multiple scattering fragmentation model (QMSFRG) is a comprehensive model for generating nuclear interaction databases for galactic cosmic ray (GCR) transport. Other nuclear databases including the NUCFRG model and Monte-Carlo simulation codes such as FLUKA, LAHET, HETC, and GEANT ignore quantum effects. These codes fail to describe many important features of nuclear reactions and are thus inaccurate for the evaluation of materials for radiation protection. Previously we have shown that quantum effects are manifested through constructive interference in forward production spectra, the effects of Fermi momentum on production spectra, cluster nuclei knockout, and the nuclear response function. Quantum effects are especially important for heavy ions with mass numbers less than 20 that dominate radiation transport in human tissues and for the materials that are expected to be superior in space radiation protection. We describe the integration of the QMSFRG model into the HZETRN transport code. Integration milestones include proper treatment of odd-even charge-mass effects in nuclear fragmentation and the momentum distribution of nucleon production from GCR primary heavy ions. We have also modified the two-body amplitudes in the model to include nuclear medium effects. In order to include a comprehensive description of the GCR isotopic composition in materials, we have described the isotopic composition of the GCR by extending the 59-isotope version of HZETRN to an 120-isotope version. The isotopic composition of most primary GCR elements (including H, He, C, N, O, Ne, Mg, Si, Ar, Ca, Cr, and Fe) are included in the extended model. We discuss results for the high-energy neutron composition inside materials, and the charge and mass distribution for benchmark GCR problems.

Cucinotta, F. A.↗
DOE OSTI2022

Evolution of Bonding and Magnetism via Changes in Valence Electron Count in CuFe 2– x Co x Ge 2

A series of solid solutions, CuFe 2–x Co x Ge 2 (x = 0, 0.2, 0.4, 0.8, and 1.0), have been synthesized by arc-melting and characterized by powder X-ray and neutron diffraction, magnetic measurements, Mössbauer spectroscopy, and electronic band structure calculations. All compounds crystallize in the CuFe 2 Ge 2 structure type, which can be considered as a three-dimensional framework built of fused MGe6 octahedra and MGe 5 trigonal bipyramids (M = Fe and Co), with channels filled by rows of Cu atoms. As the Co content (x) increases, the unit cell volume decreases in an anisotropic fashion: the b and c lattice parameters decrease while the a parameter increases. The changes in all the parameters are nearly linear, thus following Vegard’s law. CuFe 2 Ge 2 exhibits two successive antiferromagnetic (AFM) orderings, corresponding to the formation of a commensurate AFM structure, followed by an incommensurate AFM structure observed at lower temperatures. Additionally, as the Co content increases, the AFM ordering temperature (TN) gradually decreases, and only one AFM transition is observed for x ≥ 0.2. The magnetic behavior of unsubstituted CuFe 2 Ge 2 was found to be sensitive to the preparation method. The temperature-dependent zero-field 57 Fe Mössbauer spectra reveal two hyperfine split components that evolve in agreement with the two consecutive AFM orderings observed in magnetic measurements. In contrast, the field-dependent spectra obtained for fields ≥ 2 T reveal a parallel arrangement of the moments associated with the two crystallographically unique metal sites. Electronic band structure calculations and chemical bonding analysis reveal a mix of strong M–M antibonding and non-bonding states at the Fermi level, in support of the overall AFM ordering observed in zero field. The substitution of Co for Fe reduces the population of the M–M antibonding states and the overall density of states at the Fermi level, thus suppressing the T N value.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

OSSE observations of the Crab pulsar

We present results of the Compton Gamma-Ray Observatory (CGRO) Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Crab pulsar, made during MJD 48373-48406 (1991 April 27 - 1991 May 30) and MJD 48798-48804 (1992 June 25 - 1992 July 1). Pulsar light curves and spectra over the approximately 0.05 to 10 MeV range are presented. The arrival time of the gamma-ray peak and the radio peak agree to within 30 microsec which is better than the approximately 300 microsec accuracy of the measurements. The overall pulse phase averaged spectrum in the 0.1 - 10 MeV range is well-fit by a power law of the form 0.05 x (E/0.13 MeV)exp(-(1.99 +/- 0.03)) photons/sq cm/s. The outer-gap model (with gap parameter equal to 0.46) provided to us by Ho agrees with the data to better than 20%. The spectra of the bridge and second peak are slightly harder than the first peak as measured by the hardness ratio (approximately 110 - 220 keV)/(approximately 50 - 105 keV): P1 = 0.54 +/- 0.01, P2 = 0.63 +/- 0.01, bridge = 0.68 +/- 0.03. The phase of the two peaks in the light curve is constant over the 50 - 550 keV range to within the accuracy of the measurements (better than 0.02 in phase). No evidence was found for variability of the light curve on timescales from 2 minutes (less than a factor of 1.8) to 1 year (less than a factor of 1.06), where these are 3 sigma upper limits. However, when we examine the historical database, we find, in agreement with Nolan et al. (1993), that there is evidence for a 13 year variation in the ratio of the intensity of peak 2 to peak 1. We show that if this is interpreted as being due to precession (which changes the relative view of the intrinsic gamma-ray pulse as seen on earth), the variation is consistent with models of neutron star structure. The optical data may be in conflict with the interpretation however. We found no statistically significant lines in the 50 - 550 keV range in the spectrum. The average 3 sigma upper limits in 10(exp -3) photons/sq cm/s for lines at 0.073, 0.078, 0.4, 0.44, 0.511, and 0.545 MeV are 0.3, 0.5, 0.6, 0.5, and 0.1.

Ulmer, M. P.↗