Thermal neutron capture effects in radioactive and stable nuclide systems

Neutron capture effects in meteorites and lunar surface samples have been successfully used in the past to study exposure histories and shielding conditions. In recent years, however, it turned out that neutron capture effects produce a nuisance for some of the short‐lived radionuclide systems. The most prominent example is the ¹⁸²Hf‐¹⁸²W system in iron meteorites, for which neutron capture effects lower the ¹⁸²W/¹⁸⁴W ratio, thereby producing too old apparent ages. Here, we present a thorough study of neutron capture effects in iron meteorites, ordinary chondrites, and carbonaceous chondrites, whereas the focus is on iron meteorites. We study in detail the effects responsible for neutron production, neutron transport, and neutron slowing down and find that neutron capture in all studied meteorite types is not, as usually expected, exclusively via thermal neutrons. In contrast, most of the neutron capture in iron meteorites is in the epithermal energy range and there is a significant contribution from epithermal neutron capture even in stony meteorites. Using sophisticated particle spectra and evaluated cross section data files for neutron capture reactions we calculate the neutron capture effects for Sm, Gd, Cd, Pd, Pt, and Os isotopes, which all can serve as neutron‐dose proxies, either in stony or in iron meteorites. In addition, we model neutron capture effects in W and Ag isotopes. For W isotopes, the GCR‐induced shifts perfectly correlate with Os and Pt isotope shifts, which therefore can be used as neutron‐dose proxies and permit a reliable correction. We also found that GCR‐induced effects for the ¹⁰⁷Pd‐¹⁰⁷Ag system can be significant and need to be corrected, a result that is in contrast to earlier studies.     

Neutron capture cross-sections of stable xenon isotopes and their application in stellar nucleosynthesis

The neutron capture cross-sections of^{124, 132, 134}Xe have been measured by the activation technique at 25 keV neutron energy. These data were supplemented by calculated capture cross-sections for^{128, 129, 130, 131}Xe via the statistical model. The complete set of capture cross-sections obtained in this way served to determine the solar xenon abundance throughs-process systematics and to study a variety of isotopic anomalies.     

Galactic Chemical Evolution of the Lighter Neutron Capture Elements Sr, Y and Zr

Based on a three-component Galaxy chemical evolutionary model satisfying a large set of Galactic and extragalactic constraints, we compute the chemical evolution of the lighter neutron capture elements (e.g., Sr, Y and Zr) taking into account contributions from three processes. We compare our model results with available observational results and find that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis.     

Predictions of MESSENGER Neutron Spectrometer measurements for Mercury's north polar region

From radar images of Mercury's poles and MESSENGER Neutron Spectrometer (NS) measurements obtained during the spacecraft's flybys of Mercury, predictions of neutron count rates and their uncertainties are calculated for Mercury's north polar region as of the end of the MESSENGER primary orbital mission. If Mercury's poles contain large amounts of water ice, as has been suggested on the basis of the radar data, then during the one-year-long orbital mission the NS should detect signals indicative of excess polar hydrogen with a significance of at least 4σ, where σ is the standard deviation derived from Poisson counting statistics. If the polar deposits are not enriched with hydrogen, but are dominated by other elements, such as sulfur, then the MESSENGER neutron measurements should be able to confirm the absence of deposits having surface concentrations in excess of 50 wt% H₂O on permanently shadowed floors of craters near Mercury's north pole. Because of the large spatial footprint of the NS data, individual polar deposits will not be spatially resolved, but longitudinal asymmetries may be detected if residual systematic uncertainties are sufficiently low.     

Neutron capture effects on samarium,europium, and gadolinium in Apollo 15 deep drill‐core samples

Abstract— The isotopic compositions of Sm and Gd in seven lunar samples from the Apollo 15 deep drill core were determined to discuss the effects of neutron capture near the lunar surface. Large isotopic deviations of ¹⁵⁰Sm/¹⁴⁹Sm, ¹⁵⁶Gd/¹⁵⁵Gd, and ¹⁵⁸Gd/¹⁵⁷Gd derived from neutron capture effects were observed in all samples. Although neutron capture products in lunar samples were investigated extensively in the 1970s, our precise isotopic measurements resulted in several new findings. The neutron fluence in the Apollo 15 drill core is a function of depth with a symmetric peak at 190 g/cm² depth from the surface, confirming the results of earlier investigations. Neutron fluence values calculated from the isotopic shifts by comparison to artificially irradiated standard reagents were (5.16–7.49) × 10¹⁶ n/cm². These values are 1.3 to 1.4x larger than those previously reported. Variations of ε_Sm/ε_Gd with depth are interpreted as being due to variations in the neutron energy spectrum. Here ε_Sm and ε_Gd are defined as in previous studies of lunar neutron stratigraphy. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers. In addition to large isotopic shifts for ¹⁴⁹Sm, ¹⁵⁰Sm, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd, and ¹⁵⁸Gd, isotopic enrichments of ¹⁵²Gd and ¹⁵⁴Gd derived from neutron capture for ¹⁵¹Eu and ¹⁵³Eu, respectively, were also observed in all samples.     

An Upper Bound on Neutron Star Masses

We derive an upper bound on neutron star masses by using model equations of state in the nuclear matter density region and the causality limited equation of state in the ultradense region. Supposing that the model equations of state describe neutron star matter at nuclear matter density correctly we find as bound 3.75 M_⊙. For large fiducial densities one gets a maximum mass which is above a previous estimate.     

Neutron absorption constraints on the composition of 4 Vesta

Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock‐forming elements. From a circular, polar low‐altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole‐rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg‐rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine‐rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.     

Neutron star models based on an improved equation of state

Using an equation of state for cold degenerate matter which takes nuclear forces and nuclear clustering into account, neutron star models are constructed. Stable models were obtained in the mass range above 0.065M and density range 10^14.08 to 10^15.4 gm/cm³. All of these models were found to be bound. The outer crystalline layer of the star was found to have a thickness of 200 m or more depending on the mass of the model.     

The Effect of Interstellar Medium Parameters on the Accretion by Neutron Stars

The effect of the magnetic field trapped into the interstellar gas on the accretion by neutron stars is considered. The account of the magnetic field decreases the accretion by 2 orders. As a result of accretion the luminosity of the neutron stars is found to be ∼ 10²⁸ – 10²⁹ erg/s and so their observation as X-ray sources is hardly probable.     

Influence of Neutron Star Parameters on Evolutions of Different Types of Pulsar

Influences of the mass, moment of inertia, rotation, absence of stability in the atmosphere and some other parameters of neutron stars on the evolution of pulsars are examined. It is shown that the locations and evolutions of soft gamma repeaters, anomalous X-ray pulsars and other types of pulsar on the period versus period derivative diagram can be explained adopting values of B < 10¹⁴ G for these objects if they have smaller mass (e.g. about 0.5 Solar mass) compared to the conventionally adopted values of mass. This approach gives the possibility to explain many properties of different types of pulsar.     

磁化中子星发射线的激射不稳定性理论

在中子星磁轴吸积柱的上部,少数高能电子通过磁镜点反射,可使部份电子的速度分布形成非热分布,由此激发激射(Maser)不稳定性。波被放大,发射出频率近似为电子迴旋频率及其倍频的相干辐射。用此模型计算了HerX-1的迴旋线发射。发现不稳定性增长率与吸积柱中电子数密度成正比,因而比非相干散射产生的连续辐射随电子数密度增长更快;而且发射线的强度和能量均与脉冲相位关联。这个理论可解释近期的HerX-1观测结果。     

Hypervelocity capture of particles in aerogel: Dependence on aerogel properties

Capture of high-speed (hypervelocity) particles in aerogel at ambient temperatures of 175-763 K is reported. This extends previous work which has mostly focussed on conducting experiments at ambient laboratory temperatures, even though aerogels are intended for use in cosmic dust capture cells in space environments which may experience a range of temperatures (e.g., the NASA Stardust mission which collected dust at 1.81 AU and putative Mars atmospheric sampling missions). No significant change in track length (normalised to impactor size) was found over the range 175-600 K, although at 763 K a significant reduction (30%) was found. By contrast, entrance hole diameter remained constant only up to 400 K, above this sudden changes of up to 50% were observed. Experiments were also carried out at normal laboratory temperature using a wide range of aerogel densities and particle sizes. It was found that track length normalised to particle size varies inversely with aerogel density. This is a power law dependence and not linear as previously reported, with longer tracks at lower densities. Glass projectiles (up to 100 μm size) were found to undergo a variety of degrees of damage during capture. In addition to the well known acquisition of a coating (partial or complete) of molten aerogel the mechanical damage includes pitting and meridian fractures. Larger (500 μm diameter) stainless steel spheres also showed damage during capture. In this case melting and ablation occurs, suggesting surficial temperatures during impact in excess of 1400 °C. The response of the aerogel itself to passage of particles through it is reported. The presence of fan-like fractures around the tracks is attributed to cone cracking similar to that in glasses of normal density, with the difference that here it is a repetitive process as the particles pass through the aerogel.     

Photogrammetric measurements of Olympus Mons on Mars

Mariner 9 took many pictures of the giant Olympus Mons during its year in orbit around Mars. Control points have been identified on the top of Olympus Mons, on the volcanic shield, and on the surrounding plains, and their locations have been measured on the television pictures. These measurements were used to compute the aerographic coordinates and the planetary radii of the points. The radii at some of the points were derived from radar elevation measurements and from radio occultation measurements. The mountain rises about 21 km above its base.     

Lidar atmospheric measurements on Mars and Earth

The LIDAR instrument operating from the surface of Mars on the Phoenix Mission measured vertical profiles of atmospheric dust and water ice clouds at temperatures around −65 °C. An equivalent lidar system was utilized for measurements in the atmosphere of Earth where dust and cloud conditions are similar to Mars. Coordinated aircraft in situ sampling provided a verification of lidar measurement and analysis methods and also insight for interpretation of lidar derived optical parameters in terms of the dust and cloud microphysical properties. It was found that the vertical distribution of airborne dust above the Australian desert is quite similar to what is observed in the planetary boundary layer above Mars. Comparison with the in situ sampling is used to demonstrate how the lidar derived optical extinction coefficient is related to the dust particle size distribution. The lidar measurement placed a constraint on the model size distribution that has been used for Mars. Airborne lidar measurements were also conducted to study cirrus clouds that form in the Earth’s atmosphere at a similar temperature and humidity as the clouds observed with the lidar on Mars. Comparison with the in situ sampling provides a method to derive the cloud ice water content (IWC) from the Mars lidar measurements.     

Sheath effects on DEMETER ion drift measurements

The “Instrument d’Analyse du Plasma” on DEMETER includes an ion drift meter used to measure the direction of the incoming ram plasma ( [Berthelier et al., 2006a] and [Berthelier et al., 2006b]). Given the velocity of the satellite, and expected flow velocities of plasma along DEMETER's orbit, it is estimated that at mid latitudes, the direction of incident plasma as measured by IAP should be within approximately 2° of the ram direction. Yet, significantly larger angular deviations are measured routinely. An important assumption made in the interpretation of onboard instruments, such as IAP, is that neither the spacecraft nor the instrument significantly perturb the plasma that is being measured. In view of the large observed angular deviations, this paper examines the possible effect of the electrostatic sheath surrounding IAP. This is done with the 3D PIC simulation code PTetra. The model uses a full 3D particle in cell code with unstructured tetrahedral mesh capable of accurately representing the satellite geometry. The mesh is also adaptive so as to provide a fine spatial resolution in the vicinity of the particle sensor where it is needed, and a coarse resolution in regions where plasma parameters vary over a longer scale length. Calculation results show that while particle deflection associated with the electrostatic sheath near IAP can account for appreciable angular deflections for representative ionospheric plasmas, they are typically smaller than the ones observed. Additionally, the model is unable to reproduce the latitudinal dependence of the observed large deflection angles. It is concluded that sheath effects may cause appreciable distortions on the IAP type of ion flow meter instruments, and on other particle sensors in general. The larger observed deviations and their latitudinal dependence, however, must be attributable to other physical processes not accounted for in the model.     

Some remarks on the capture of Triton and the origin of Pluto

Harrington and Van Flandern (1979, Icarus39, 131–136) suggests that the irregular features of the Neptunian satellite system and Pluto's escape were caused by an encounter with a massive external body. They rule out the alternative mechanism based on the capture of Triton (which seems more plausible because it does not appeal to any unobserved object) on the basis of an incorrect deduction from McCord's (1966, Astron. J.71, 585–590) analysis on the tidal decay of Triton's orbit. As a matter of fact, many recent results show that satellite captures are possible, and in the case of Triton several arguments support this interpretation.     

Gravitational lunar capture based on bicircular model in restricted four body problem

Gravitational capture is a useful phenomenon in the design of the low energy transfer (LET) orbit for a space mission. In this paper, gravitational lunar capture based on the Sun–Earth–Moon bicircular model (BCM) in the restricted four body problem is studied. By the mechanical analysis in the space near the Moon, we first propose a new parameter \(k\) , the corrected ratio of the radial force, to investigate the influence of the radial force on the capture eccentricity in the BCM. Then, a parametric analysis is performed to detect the influences on the corrected ratio \(k\) . Considering the restriction of time-of-flight and corrected ratio, we investigate, respectively, the minimum capture eccentricity and the corrected minimum capture eccentricity. Via numerical analysis, we discover two special regions on the sphere of capture, in which the capture point possesses the global minimum capture eccentricity and corrected capture eccentricity. They denote the optimal capture regions in terms of minimizing the fuel consumption of the maneuver. According to the results obtained, some suggestions on the design of the LET orbit are given.     

Electron capture strength on odd-A nucleus 59Co in explosive astrophysical environment

The Gamow-Teller (GT) transitions within massive stars play sumptuous role in the dynamics of core collapse supernovae. GT strength distributions and electron capture rates have been calculated for odd-A nucleus ⁵⁹Co within the proton-neutron quasiparticles random phase approximation (pn-QRPA) formalism. The pn-QRPA results are compared with other model calculations and (n,p) reaction experiment carried out at TRIUMF charge-exchange facility. The pn-QRPA calculated a total B(GT ₊) strength of 3.3 for ⁵⁹Co to be compared with the shell model value of 2.5 and the 1.9±0.1 in the (n,p) charge-exchange reaction. Aufderheide et al. (1993) extracted total strength equaling 2.4±0.3. The placement of GT centroid at 5.6 MeV by the pn-QRPA model is in reasonable agreement with the shell model centroid at 5.1 MeV whereas the measured GT centroid was placed at 4.4±0.3 MeV in the (n,p) experiment. Fuller, Fowler and Newman (FFN) (1980, 1982a, 1982b), placed the GT centroid at too low excitation energy of 2.0 MeV in the daughter nucleus ⁵⁹Fe, and this misplacement led to the enhancement of FFN rates. The suppressed pn-QRPA and shell model electron capture rates are in good agreement with each other. The rates are suggestive of higher value of Y _e (electron-to-baryon ratio) and may contribute to a more massive homologously collapsing core resulting in a more energetic shock. It might be interesting for the simulators to check the effect of these suppressed rates on the fine-tuning of the time rate of Y _e , the concomitant heavy element nucleosynthesis, and, on the energetics of the subsequent shock wave.