Cosmic Ray Bombardment Research Articles

Formation of unsaturated hydrocarbons in interstellar ice analogues by cosmic rays

The formation of double and triple C-C bonds from the processing of pure c-C6H12 (cyclohexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly-charged, and energetic ions (219 MeV O^{7+} and 632 MeV Ni^{24+}) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ices analogs. The measurements were performed inside a high vacuum chamber at the heavy-ion accelerator GANIL (Grand Accel\'erat\'eur National d'Ions Lourds) in Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometry at different ion fluences. Dissociation cross section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources indicates that the initial composition of grains in theses environments should contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several species containing double or triple bounds were identified in the radiochemical products, such as hexene, cyclohexene, benzene, OCN-, CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggest an alternative scenario for the production of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.

A numerical model to develop long-term sediment budgets using isotopic sediment fingerprints

Developing accurate long-term, basin-scale sediment budgets using isotopic sediment fingerprints requires a sediment routing model that not only accounts for a range of sediment source terms (e.g. tributaries, surface erosion and erosion of bluffs and terraces) but also considers the variation in time of volume and tracer concentration for the sediment stored in the floodplain. This is accomplished here using a tracer routing model that accounts for production and decay of radioisotopes in the floodplain. The numerical model focuses on the average (i.e. across many hydrographs or years) budget of sediment and tracers at reach scale. To account for storage and remobilization of bulk sediment and/or tracer material, the model represents the floodplain as a system that can gain or lose mass depending on overbank deposition and net bank erosion rates. Isotopic tracers within the floodplain reservoir can be produced as a function of cosmic ray bombardment or atmospheric fallout, and can decay according to a first-order rate equation. Governing equations are derived using a simplified geometry that treats rivers at reach scale: channel sinuosity and migration rates are user-specified parameters, exchange of sediment and tracers between the river and floodplain is modeled at each cross section, and governing equations are derived in a 1D, width-averaged formulation. When the system reaches mobile equilibrium, the sediment deposited on the floodplain through overbank deposition is balanced by the sediment eroded from the floodplain through channel migration and by sediment contributed from external sources. The model is applied to a generic river system and is shown to converge over time to an equilibrium condition that is consistent with an independent analytical solution.

Ion irradiation of carbonaceous interstellar analogues

Context. A 3.4 μm absorption band (around 2900 cm-1), assigned to aliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in the diffuse interstellar medium, but disappears or is modified in dense clouds. This spectral difference between different phases of the interstellar medium reflects the processing of dust in different environments. Cosmic ray bombardment is one of the interstellar processes that make carbonaceous dust evolve. Aims. We investigate the effects of cosmic rays on the interstellar 3.4 μm absorption band carriers. Methods. Samples of carbonaceous interstellar analogues (a-C:H and soot) were irradiated at room temperature by swift ions with energy in the MeV range (from 0.2 to 160 MeV). The dehydrogenation and chemical bonding modifications that occurred during irradiation were studied with IR spectroscopy. Results. For all samples and all ions/energies used, we observed a decrease of the aliphatic C-H absorption bands intensity with the ion fluence. This evolution agrees with a model that describes the hydrogen loss as caused by the molecular recombination of two free H atoms created by the breaking of C-H bonds by the impinging ions. The corresponding destruction cross section and asymptotic hydrogen content are obtained for each experiment and their behaviour over a large range of ion stopping powers are inferred. Using elemental abundances and energy distributions of galactic cosmic rays, we investigated the implications of these results in different astrophysical environments. The results are compared to the processing by UV photons and H atoms in different regions of the interstellar medium. Conclusions. The destruction of aliphatic C-H bonds by cosmic rays occurs in characteristic times of a few 108 years, and it appears that even at longer time scales, cosmic rays alone cannot explain the observed disappearance of this spectral signature in dense regions. In diffuse interstellar medium, the formation by atomic hydrogen prevails over the destruction by UV photons (destruction by cosmic rays is negligible in these regions). Only the cosmic rays can penetrate into dense clouds and process the corresponding dust. However, they are not efficient enough to completely dehydrogenate the 3.4 μm carriers during the cloud lifetime. This interstellar component should be destroyed in interfaces between diffuse and dense interstellar regions where photons still penetrate but hydrogen is in molecular form.

Solar hard X-ray halo from decaying neutrons

Aims. To quantify the solar X-ray halo resulting from inner bremsstrahlung in beta decay of neutrons generated by cosmic ray bombardment of the solar surface. Methods. We show analytically how the angular form of this X-ray halo directly reflects the energy distribution of neutrons escaping the Sun. Previous Monte Carlo calculations of solar albedo neutron production are used to normalise an assumed parametric form for the escaping neutron distribution and thus to estimate the halo’s intensity. The main remaining assumptions, that neutrons escape radially and that gravitational deceleration may be neglected, affect a negligible fraction of all solar neutrons. Results. Observations of this halo would have clear diagnostic interest for cosmic ray propagation in the inner heliosphere. Unfortunately it is unlikely to be observable in competition with the cosmic X-ray background.

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH + 5CH 4, C 7H + 7, C 4H + 7, and H 4C 7N +. The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titan's atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH 4 at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.

Investigating the Mechanism for the Formation of Nitrous Oxide [N2O(X1Σ+)] in Extraterrestrial Ices

The formation of nitrous oxide, N2O(X 1Σ+), in interstellar space and in ices on Pluto and Triton has been experimentally investigated. A molecular nitrogen (N2) and carbon dioxide (CO2) ice mixture was irradiated at 10 K with 5 keV electrons to simulate the electronic interaction effects of Galactic cosmic-ray bombardment of extraterrestrial ice samples over a time of 5 × 106 yr. By monitoring the experiment with a Fourier transform infrared spectrometer on line and in situ, the temporal evolution of the 2235 cm-1 absorption band of nitrous oxide was found to follow pseudo-first-order kinetics. This indicates that the mechanism of formation is most likely a reaction between ground-state molecular nitrogen, N2(X 1Σ), and an oxygen atom, either in the ground state (3P) or in the first electronically excited state (1D), within the matrix cage through an addition of the oxygen atom to a nonbonding electron pair on the nitrogen molecule. The observation of nitrous oxide together with the kinetics and dynamics studies investigated in this paper underline the role of nonequilibrium processes in low-temperature ice matrices, aid in the understanding of chemical reaction pathways that exist in extraterrestrial ices, and assist a prospective identification of nitrous oxide on the surfaces of Pluto and Triton.

The morphology of interstellar water ice

We have studied, by infrared absorption spectroscopy, the effects of ion irradiation (with 200 keV protons) on the morphology/porosity of thin amorphous water ice (H2O) samples at 15 K. We have found that the intensity of the OH dangling bond feature decreases after ioni irradiation and that the amount of carbon monoxide (CO) absorbed in water ice decreases as the fluence of impinging ions increases. These results indicate that the porosity of amorphous water ice decreases after ion irradiation. Here we present the experimental results and suggest that because of cosmic ray bombardment water ice in interstellar grain mantles is compact instructure.

An age-colour relationship for main-belt S-complex asteroids.

Asteroid collisions in the main belt eject fragments that may eventually land on Earth as meteorites. It has therefore been a long-standing puzzle in planetary science that laboratory spectra of the most populous class of meteorite (ordinary chondrites, OC) do not match the remotely observed surface spectra of their presumed (S-complex) asteroidal parent bodies. One of the proposed solutions to this perplexing observation is that 'space weathering' modifies the exposed planetary surfaces over time through a variety of processes (such as solar and cosmic ray bombardment, micro-meteorite bombardment, and so on). Space weathering has been observed on lunar samples, in Earth-based laboratory experiments, and there is good evidence from spacecraft data that the process is active on asteroid surfaces. Here, we present a measurement of the rate of space weathering on S-complex main-belt asteroids using a relationship between the ages of asteroid families and their colours. Extrapolating this age-colour relationship to very young ages yields a good match to the colour of freshly cut OC meteorite samples, lending strong support to a genetic relationship between them and the S-complex asteroids.

The Effects of Ion Irradiation on the Evolution of the Carrier of the 3.4 Micron Interstellar Absorption Band

Carbon grains in the interstellar medium evolve through exposure to UV photons, heat, gas, and cosmic rays. Understanding their formation, evolution, and destruction is an essential component of evaluating the composition of the dust available for newly forming planetary systems. The 3.4 lm absorption band, attributed to the aliphatic C”H stretch vibration, is a useful probe of the degree to which energetic processing affects hydrogenated carbon grains. Here we report on the effects of ion bombardment of two different kinds of nano-size hydrogenated carbon grains with different hydrogen content. Grain samples, both with and without a mantle of H2O ice, were irradiated with 30 keV He + to simulate cosmic-ray processing in both diffuse and dense interstellar medium conditions. The ion fluences ranged between 1:5 � 10 13 and 7:9 � 10 15 ions cm � 2 . Infrared and Raman spectroscopy were used to study the effects of ion irradiation on grains. In both the dense and diffuse interstellar medium simulations, ion bombardment led to a reduction of the 3.4 lm band intensity. To discuss the effects of cosmic-ray irradiation of interstellar hydrogenated carbon materials we adopt the approximation of 1 MeV monoenergetic protons. An estimate of the C”H bond destruction cross section by 1 MeV protons was made based on experiments using 30 keV He + ions and model calculations. In combination with results from our previous studies, which focused on UV irradiation and thermal H atom bombardment, the present results indicate that the C”H bond destruction by fastcolliding charged particles is negligible with respect to that of UV photons in the diffuse ISM. However, in dense cloud regions, cosmic-ray bombardment is the most significant C”H bond destruction mechanism when the optical depth corresponds to values of the visual extinction larger than � 5 mag. The results presented here strengthen the new interpretation of the evolution of the interstellar aliphatic component (i.e., the C”H bonds in the CH2 and CH3 groups) as evidenced by the presence of the 3.4 lm absorption band in the diffuse medium and the absence of such a signature in the dense cloud environment. The evolutionary transformation of carbon grains, induced by H atoms, UV photons, and cosmic rays, indicates that C”H bonds are readily formed, in situ, in the diffuse interstellar medium and are destroyed in the dense cloud environment.

Measurements of the concentrations of 26Al in lunar rocks 15555 and 60025

The authors have measured the concentrations of 26Al in lunar rock samples 15555,859 and 60025,28 with a multidimensional gamma-ray spectrometer, obtaining concentrations of 167±14 and 255±10 dpm/kg for the two rocks, respectively. Computer modelings were carried out on both samples of the 26Al production rates from cosmic-ray bombardment using consistent values of cosmic-ray parameters and reasonable values of surface erosion to see if an inconsistency in values needed for a fit between measurement and modeling could be found which would suggest the presence of surface-implanted 26Al in one or both samples. The final result was that both of the samples could be fitted by the computer models using omnidirectional SEP flux and rigidity shape values of 125 protons/cm 2/s>10 MeV and 125 MV/unit charge, respectively, and with the reasonable surface erosion rates of 0–0.24 mm/Myr for rock 15555,859, and 1.6 mm/Myr for 60025,28.

Les nucleides cosmogeniques produits in-situ; de nouveaux outils en geomorphologie quantitative

Abstract Measurement of 10 Be and 26 Al concentrations produced by cosmic ray bombardment within the quartz mineral fraction of surficial deposits and exposed bedrocks (in situ-production) is rapidly becoming an important quantitative tool in geomorphology. Whereas conventional methods provide age control on stratigraphic profiles, surface exposure dating using in situ-produced 10 Be and 26 Al is particularly well-suited not only for continuous dating but also for quantifying spatial variations and rates of geomorphic processes.

Extraterrestrial Helium Trapped in Fullerenes in the Sudbury Impact Structure

Fullerenes (C60 and C70) in the Sudbury impact structure contain trapped helium with a 3He/4He ratio of 5.5 x 10(-4) to 5.9 x 10(-4). The 3He/4He ratio exceeds the accepted solar wind value by 20 to 30 percent and is higher by an order of magnitude than the maximum reported mantle value. Terrestrial nuclear reactions or cosmic-ray bombardment are not sufficient to generate such a high ratio. The 3He/4He ratios in the Sudbury fullerenes are similar to those found in meteorites and in some interplanetary dust particles. The implication is that the helium within the C60 molecules at Sudbury is of extraterrestrial origin.

Experimental and numerical simulation of Martian neutron distributions

Measurements of the gamma-ray and neutron distributions produced by cosmic-ray bombardment of planetary surfaces can provide important information about water content and near-surface elemental compositions. Accurate interpretation of such measurements depends heavily on comparisons with radiation-transport calculations, which involve not only the soil composition but also assumptions about (1) particle types and energy distributions of cosmic rays, (2) interaction mechanisms for high-energy protons and neutrons, (3) neutron transport at low energies, and (4) production of prompt gamma rays and radionuclides in proton- and neutron-induced reactions. The present experiment simulates the planetary situation by directing a beam of 800 MeV protons onto a large container of Martian-like sand. Measurements were made of (1) the spatial distributions for protons and neutrons inside the container via activation foils and (2) the energy spectrum for neutron leakage out of the container via time of flight. Several configurations of the target were used to represent different Martian surfaces. The measured neutron-energy spectra are in good agreement with calculations made using a computer code that couples medium-energy neutron production to low-energy neutron transport, and the measured spatial distributions match those predicted using calculated fluxes and known cross sections for radionuclide production. The analyses include discussions of soil water content, finite-size effects, and the implications for data from possible planetary missions.

Chlorine-36 production and distribution in Australia

Chlorine-36 is an unstable isotope of chlorine produced by cosmic-ray bombardment of the atmosphere and the Earth's surface as well as by absorption of cosmic-ray or natural radioactivity-derived neutrons in stable chlorine. Nuclear weapons detonations, nuclear reactors and the use of neutron probes in environmental studies add to the inventory of 36Cl. Radiochlorine is strongly associated with water in the atmosphere, environment, soils and groundwater. Chlorine-36 undergoes radioactive decay with a half-life of 301 ka. These properties make radiochlorine a valuable tracer in the study of aqueous and saline systems and a useful tool for dating closed systems (e.g. confined aquifers). Recent measurements of 36Cl/Cl, using the 14UD tandem accelerator at ANU, show that a variety of processes is important in groundwaters of the Murray-Darling Basin and in the Great Artesian Basin. The results also provide useful information on the levels of 36Cl in soils, salt lakes and other parts of the Australian environment. The results confirm that estimates of 36Cl precipitation rates are reasonable for southeastern Australia but that more information is needed for other regions. Further modelling of chloride movement is also needed, particularly for arid Australia.

Recent results on celestial gamma radiation from SMM

During close to ten years of highly successful operation, the Gamma Ray Spectrometer ( GRS) on the Solar Maximum Mission ( SMM) made significant celestial measurements, in addition to the discovery and monitoring of 56Co γ-ray lines from SN 1987A. These observations include limits set on a) the amount of 56Ni produced in Type Ia SN 1986G, b) radioactive products of Galactic nucleosynthetic events, including 7Be, 22Na, 44Ti, and 60Co, and c) Galactic emissions at 4.43 and 6.13 MeV from excitation of interstellar C and O by cosmic-ray bombardment. The spectrometer measured the diffuse Galactic spectrum from 0.3 to 8.5 MeV, revealing a power-law continuum, narrow lines at 0.511 and 1.809 MeV, and the positronium continuum below 0.5 MeV. Limits have also been placed on the temporal variability of a Galactic center source of 511 keV radiation. A total of 177 cosmic γ-ray bursts have been observed, 17 detected at energies above 10 MeV.

On the Mechanism of H2 Formation in the Interstellar Medium

The problem of the formation of molecular hydrogen in interstellar clouds is revisited. the role played by cosmic ray bombardment under certain circumstances is considered mainly in the light of the low formation rate of H2 on grains due to the reduced mobility of adsorbed H atoms on their amorphous surfaces at interstellar temperatures.

Ion desorption by high energy irradiation of ices and Astrophysical implications

Abstract We have studied by time of flight mass spectrometry the ion desorption from icy films stimulated by the irradiation of MeV/amu Kr ions. We have determined the mass of the desorbed species, and detected clusters of the form H(H2O)n+, O(H2O)n− and OH(H2O)n−, with n higher than 20. We have measured the variation of the desorption yields with the energy of the primary beam, for both the positively and negatively charged ions. The production of ions with high masses induced by the cosmic ray bombardment of icy grains may play an important role in cosmic chemistry.

High energy irradiations simulating cosmic‐ray‐induced planetary gamma ray production: I. Fe target

A series of 6 GeV proton irradiations at the Argonne National Laboratory's Zero Gradient Synchrocyclotron included the bombardment of two thick Fe targets for the purpose of simulating the cosmic ray bombardment of planetary objects in space. Cosmic ray bombardment leads to characteristic gamma ray production, from which, by measuring their energy and intensity, surface composition may be derived. In this experiment, gamma ray energy spectra were obtained with a germanium solid state detector during the bombardment. The spectra show much detail, with 46 of the gamma ray lines ascribed to the Fe targets. The background includes recoil‐broadened lines due to energetic neutron interactions in the Ge detector. A comparison with values predicted by modeling shows good agreement for Fe lines from neutron inelastic scattering and spallation reactions, and less satisfactory agreement for neutron capture reactions, the latter found to be due primarily to the difference in composition between the Fe target and the mean lunar abundance used in the modeling. Analysis of the irradiation results combined with continuum data obtained in lunar orbit indicates that 100 hours of measurement with a state‐of‐the‐art instrument should generate a spectrum containing approximately 20 lines due to Fe alone, with a 2σ sensitivity for detection of about 0.2%.

26Al‐production rates and 53Mn/26Al production rate ratios in nonantarctic chondrites and their application to bombardment histories

The long‐lived spallogenic radionuclides 26Al and 53Mn were systematically studied in a large number of nonantarctic meteorites by nondestructive γ‐γ‐coincidence technique and neutron activation, respectively. From the 26Al‐activities normalized to the main target element, silicon, an average production rate of 298±55 (dpm/kg Siequ) was derived. Based on 15 chondrites with exposure ages ≥12×106a and depth profiles of Dhurmsala and Keyes, an average production rate ratio [53Mn/26Al] prod × 1.48±0.15 [dpm/kg Fe / dpm/kg Siequ] was calculated, which seems to be depth‐independent for meteorites with preatmospheric radii R<35 cm. 53Mn/26Al‐radiation ages for 29 stones with short exposure ages were determined. A comparison of our results with the respective 53Mn‐ and 21Ne‐exposure ages generally shows a good agreement. The cosmic ray bombardment age scale covered by this method is 1×106<Trad<10×106a.

Beryllium-10 in Australasian Tektites: Evidence for a Sedimentary Precursor

Each of seven Australasian tektites contains about 1 x l0(8) atoms of beryllium-10 (half-life, 1.53 x 10(6) years) per gram. Cosmic-ray bombardment of the australites cannot have produced the measured amounts of beryllium-10 either at the earth's surface or in space. The beryllium-10 contents of these australites are consistent with a sedimentary precursor that adsorbed from precipitation beryllium-10 produced in the atmosphere. The sediments must have spent several thousand years at the earth's surface within a few million years of the tektite-producing event.