439 research outputs found
Nuclear interactions of low-energy cosmic rays with the interstellar medium
Cosmic rays of kinetic energies below ~1 GeV per nucleon are thought to play
a key role in the chemistry and dynamics of the interstellar medium. They are
also thought to be responsible for nucleosynthesis of the light elements Li,
Be, and B. However, very little is known about the flux and composition of
low-energy cosmic rays since the solar modulation effect makes impossible a
direct detection of these particles near Earth. We first discuss the
information that the light elements have brought to cosmic-ray studies. We then
discuss the prospects for detection of nuclear gamma-ray line emission produced
by interaction of low-energy cosmic rays with interstellar nuclei.Comment: 6 pages, 3 figures. Invited review presented at the conference on
Cosmic Rays and the Interstellar Medium (CRISM-2011), Montpellier, France,
June 26-July 1. To be published in a special issue of MEMORIE della Societa
Astronomica Italian
Shape of the 4.438 MeV gamma-ray line of ^12C from proton and alpha-particle induced reactions on ^12C and ^16O
We calculated in detail the angular distribution of gamma-rays and the
resulting shape of the gamma-ray line produced by the nuclear deexcitation of
the 4.439 MeV state of ^12C following proton and alpha-particle interactions
with ^12C and ^16O in the energy range from threshold to 100 MeV per nucleon,
making use of available experimental data. In the proton energy range from 8.6
to 20 MeV, the extensive data set of a recent accelerator experiment on
gamma-ray line shapes and angular distributions was used to deduce
parameterizations for the gamma-ray emission of the 2^+, 4.439 MeV state of
^12C following inelastic proton scattering off ^12C and proton induced
spallation of ^16O. At higher proton energies and for alpha-particle induced
reactions, optical model calculations were the main source to obtain the needed
reaction parameters for the calculation of gamma-ray line shapes and angular
distributions. Line shapes are predicted for various interaction scenarios of
accelerated protons and alpha-particles in solar flares.Comment: REVTeX, 9 pages, 8 figures, 4 tables, to be published by Phys. Rev.
Deexcitation nuclear gamma-ray line emission from low-energy cosmic rays in the inner Galaxy
Recent observations of high ionization rates of molecular hydrogen in diffuse
interstellar clouds point to a distinct low-energy cosmic-ray component.
Supposing that this component is made of nuclei, two models for the origin of
such particles are explored and low-energy cosmic-ray spectra are calculated
which, added to the standard cosmic ray spectra, produce the observed
ionization rates. The clearest evidence of the presence of such low-energy
nuclei between a few MeV per nucleon and several hundred MeV per nucleon in the
interstellar medium would be a detection of nuclear \gamma-ray line emission in
the range E_ 0.1 - 10 MeV, which is strongly produced in their collisions with
the interstellar gas and dust. Using a recent \gamma-ray cross section
compilation for nuclear collisions, \gamma-ray line emission spectra are
calculated alongside with the high-energy \gamma-ray emission due to {\pi} 0
decay, the latter providing normalization of the absolute fluxes by comparison
with Fermi-LAT observations of the diffuse emission above E \gamma = 0.1 GeV.
Our predicted fluxes of strong nuclear \gamma-ray lines from the inner Galaxy
are well below the detection sensitivies of INTEGRAL, but a detection,
especially of the 4.4-MeV line, seems possible with new-generation \gamma-ray
telescopes based on available technology. We predict also strong \gamma-ray
continuum emission in the 1-8 MeV range, which in a large part of our model
space for low-energy cosmic rays exceeds considerably estimated instrument
sensitivities of future telescopes.Comment: 22 pages, 7 figures, accepted for publication in ApJ; figures 6 and 7
replace
Gamma-ray lines from cosmic-ray interactions with interstellar dust grains
As pointed out by Lingenfelter and Ramaty (1977), the shapes of some
gamma-ray lines produced by cosmic-ray interactions with the interstellar
medium potentially contain valuable information on the physical properties of
dust grains, including their compositions and size distributions. The most
promising of such lines are at 847, 1369, 1779 and 6129 keV, from 56-Fe*,
24-Mg*, 28-Si* and 16-O*, respectively. We performed detailed calculations of
their profiles using, in particular, available laboratory measurements combined
with optical model calculations to evaluate the energy distributions of the
recoiling excited nuclei. We show that the line shapes are mainly sensitive to
relatively large interstellar grains, with radii greater than 0.25 microns.
Line fluxes from the inner Galaxy are then predicted.Comment: 7 pages, 4 figures, to be published in New Astronomy Reviews
(proceedings of the Workshop "Astronomy with Radioactivities IV and Filling
the Sensitivity Gap in MeV Astronomy", Seeon, Germany, May 26-30, 2003
How the interface type manipulates the thermomechanical response of nanostructured metals : A case study on nickel
The presence of interfaces with nanoscale spacing significantly enhances the strength of materials, but also the rate controlling processes of plastic flow are subject to change. Due to the confined grain volumes, intragranular dislocation-dislocation interactions, the predominant processes at the micrometer scale, are replaced by emission of dislocations from and their subsequent accommodation at the interfaces. Both processes not only depend on the interfacial spacing, but also on the atomistic structure of the interface. Hence, a thorough understanding how these processes are affected by the interface structure is required to predict and improve the behavior of nanomaterials. The present study attempts to rationalize this effect by investigating the thermomechanical behavior of samples consisting of three different interfaces. Pure nickel samples with predominant fractions of low- and high-angle as well as twin boundaries with a similar average spacing around 150 nm are investigated using high temperature nanoindentation strain rate jump tests. Depending on the interface structure, hardness, strain rate sensitivity and apparent activation volumes evolve distinctively different with testing temperature. While in case of high-angle boundaries for all quantities a pronounced thermal dependence is found, the other two interface types behave almost athermal in the same temperature range. These differences can be rationalized based on the different interfacial diffusivity, affecting the predominant process of interfacial stress relaxation
Physical implications of INTEGRAL/SPI gamma-ray line measurements of the 2003 October 28 solar flare
The very powerful X-class solar flare of 2003 October 28 was detected with
the INTEGRAL spectrometer as an intense gamma-ray flash of about 15 minutes.
Despite the non-standard incidence of the solar gamma-rays, time-resolved
spectra including several nuclear gamma-ray lines were obtained. Such a
measurement with a high-energy-resolution instrument can provide valuable
information of the isotopic abundances of the ambient solar material, as well
as the composition, directionality and energy spectra of the accelerated
nuclei. First results on the measured gamma-ray line ratios and time history of
the neutron-capture line are presented.Comment: 4 pages, 4 figures (uses vietnam.sty). To appear in the proceedings
of the 5th Rencontres du Vietnam, "New Views on the Universe", Hanoi, Aug
5-11, 200
Properties of the energetic particle distributions during the October 28, 2003 solar flare from INTEGRAL/SPI observations
Analysis of spectra obtained with the gamma-ray spectrometer SPI onboard
INTEGRAL of the GOES X17-class flare on October 28, 2003 is presented. In the
energy range 600 keV - 8 MeV three prominent narrow lines at 2.223, 4.4 and 6.1
MeV, resulting from nuclear interactions of accelerated ions within the solar
atmosphere could be observed. Time profiles of the three lines and the
underlying continuum indicate distinct phases with several emission peaks and
varying continuum-to-line ratio for several minutes before a smoother decay
phase sets in. Due to the high-resolution Ge detectors of SPI and the
exceptional intensity of the flare, detailed studies of the 4.4 and 6.1 MeV
line shapes was possible for the first time. Comparison with calculated line
shapes using a thick target interaction model and several energetic particle
angular distributions indicates that the nuclear interactions were induced by
downward-directed particle beams with alpha-to-proton ratios of the order of
0.1. There are also indications that the 4.4 MeV to 6.1 MeV line fluence ratio
changed between the beginning and the decay phase of the flare, possibly due to
a temporal evolution of the energetic particle alpha-to-proton ratio.Comment: 24 pages, 10 figures, accepted for publication by A&
Delayed X- and Gamma-Ray Line Emission from Solar Flare Radioactivity
We have studied the radioactive line emission expected from solar active
regions after large flares, following the production of long-lived
radioisotopes by nuclear interactions of flare-accelerated ions. This delayed
X- and gamma-ray line emission can provide unique information on the
accelerated particle composition and energy spectrum, as well as on mixing
processes in the solar atmosphere. Total cross sections for the formation of
the main radioisotopes by proton, 3-He and alpha-particle reactions are
evaluated from available data combined with nuclear reaction theory.
Thick-target radioisotope yields are provided in tabular form, which can be
used to predict fluxes of all of the major delayed lines at any time after a
gamma-ray flare. The brightest delayed line for days after the flare is found
to be the 511 keV positron-electron annihilation line resulting from the decay
of several beta+ radioisotopes. After ~2 days however, the flux of the 511 keV
line can become lower than that of the 846.8 keV line from the decay of 56-Co.
Our study has revealed other delayed gamma-ray lines that appear to be
promising for detection, e.g. at 1434 keV from the radioactivity of both the
isomer and the ground state of 52-Mn, 1332 and 1792 keV from 60-Cu, and 931.1
keV from 55-Co. The strongest delayed X-ray line is found to be the Co Kalpha
at 6.92 keV, which is produced from both the decay of the isomer of 58-Co by
the conversion of a K-shell electron and the decay of 57-Ni by orbital electron
capture. Prospects for observation of these lines with RHESSI or future space
instruments are discussed.Comment: Accepted for publication in ApJS. 29 pages, 12 figure
Effect of boron doping on grain boundary cohesion in technically pure molybdenum investigated via meso-scale three-point-bending tests
Molybdenum has numerous advantageous functional and high-temperature properties. However, plastic deformation as well as structural applications are limited due to a propensity for brittle, intercrystalline failure, especially at low temperatures. It is well known that oxygen segregations have a detrimental effect, whereas it is assessed that carbon and/or boron have a beneficial effect on grain boundary cohesion. An advanced approach for the improvement of these interfaces is segregation engineering, e.g. the addition of cohesion enhancing elements segregating to the grain boundaries. To investigate early stages of crack formation, three-point bending tests on recrystallized commercially pure and boron micro-doped molybdenum were conducted between −28 \ub0C and room temperature. The tensile-loaded top surface of the specimens was examined post-mortem close to the final fracture area via scanning electron microscopy. The occurring separations of grains are investigated for distinct features and the chemical composition of the interface is complementary measured by atom probe tomography
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