Distributed reacceleration of cosmic rays Wandel, A.; Eichler, D.; Letaw, J. R. ...
Astrophysical journal/The Astrophysical journal,
05/1987, Letnik:
316, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We develop a model in which cosmic rays, in addition to their initial acceleration by a strong shock, are continuously reaccelerated (e.g., by weak shocks) while propagating through the galaxy. The ...equations describing this acceleration scheme are solved analytically (approximating ionization losses by a cutoff) and numerically. Solutions for the spectra of primary and secondary cosmic rays are given in a closed analytic form, and they allow a rapid search in parameter space for viable propagation models with distributed reacceleration included. The observed boron-to-carbon ratio can be reproduced by the reacceleration theory over a range of escape parameters, some of them quite different from the standard "leaky box" model. It is also shown that even a very modest amount of reacceleration by strong shocks causes the boron-to carbon ratio to level off at sufficiently high energies, and this effect may be observed in the CRNE data. Several other curiosities in the data may be explained naturally if a modest amount of distributed reacceleration is invoked, including (a) the apparent truncation at low energy in the otherwise exponential pathlength distribution associated with the leaky box model, (b) the sub-iron isotopic anomalies and other effects noted by Silberberg et al., and (c) the discrepancy between the reported 10Be lifetime and the lifetime of cosmic rays in the dense strata of the galactic disk.
Connectivity of vertebrate genomes: Paired-related homeobox (Prrx) genes in spotted gar, basal teleosts, and tetrapods Braasch , Ingo (University of Oregon, Eugene(Etats-Unis). Institute of Neuroscience); Guiguen , Yann (INRA , Rennes (France). UR 1037 Laboratoire de Physiologie et Génomique des Poissons ); Loker , Ryan (University of Oregon, Eugene(Etats-Unis). Institute of Neuroscience) ...
http://www.journals.elsevier.com/comparative-biochemistry-and-physiology-part-c-toxicology-and-pharmacology/recent-articles,
2014
Publication
Teleost fish are important models for human biology, health, and disease. Because genome duplication in a teleost ancestor (TGD) impacts the evolution of teleost genome structure and gene ...repertoires, we must discriminate gene functions that are shared and ancestral from those that are lineage-specific in teleosts or tetrapods to accurately apply inferences from teleost disease models to human health. Generalizations must account both for the TGD and for divergent evolution between teleosts and tetrapods after the likely two rounds of genome duplication shared by all vertebrates. Progress in sequencing techniques provides new opportunities to generate genomic and transcriptomic information from a broad range of phylogenetically informative taxa that facilitate detailed understanding of gene family and gene function evolution. We illustrate here the use of new sequence resources from spotted gar (Lepisosteus oculatus), a rayfin fish that diverged from teleosts before the TGD, as well as RNA-Seq data from gar and multiple teleost lineages to reconstruct the evolution of the Paired-related homeobox (Prrx) transcription factor gene family, which is involved in the development of mesoderm and neural crest-derived mesenchyme. We show that for Prrx genes, the spotted gar genome and gene expression patterns mimic mammals better than teleosts do. Analyses force the seemingly paradoxical conclusion that regulatory mechanisms for the limb expression domains of Prrx genes existed before the evolution of paired appendages. Detailed evolutionary analyses like those reported here are required to identify fish species most similar to the human genome to optimally connect fish models to human gene functions in health and disease.
Distributed acceleration of cosmic rays Silberberg, R.; Tsao, C. H.; Letaw, J. R. ...
Physical review letters,
09/1983, Letnik:
51, Številka:
13
Journal Article
Recenzirano
It is proposed that the acceleration of cosmic rays is distributed over their propagation through interstellar space. Thus after most nuclear fragmentation reactions in the interstellar medium, ...cosmic rays gain about a factor of 5 in energy. This hypothesis resolves several discrepancies in composition at different energies. The present results suggest that after the principal acceleration and during their galactic confinement, cosmic rays are further accelerated by the weak shocks of widely distributed old supernova remnants.
The single-event-upset rates due to neutron-induced nuclear recoils were calculated for Si and GaAs components using the HETC and MCNP codes and the ENDF database for (n, p) and (n, alpha ) ...reactions. For the same critical charge and sensitive volume, the upset rate in Si exceeds that of GaAs by a factor of about 1.7, mainly because more energy is transferred in neutron interactions with lighter Si nuclei. The upset rates due to neutrons are presented as functions of critical charge and atmospheric altitude. Upsets induced by cosmic-ray nuclei, secondary protons, and neutrons are compared.< >
Heavy cosmic ray nuclei are mostly attenuated with a shielding of 50 g/cm2 atmospheric gas. However, the shielding acts as a generator of neutrons, evaporated or knocked out of nuclei. These neutrons ...generate highly ionizing nuclear recoils that produce single-event upsets in microelectronic components. To attenuate the secondary neutron flux over 300 g/cm2 of atmospheric material is required. The numerous slow protons from nuclear interactions in shielding will also genetrate upsets in sensitive components, which have a low critical charge. At altitudes below 65,000 feet, most single-event upsets are due to these secondary particles. The upset rates due to neutrons and slow secondary protons from cosmic ray, solar flare particle, and trapped radiation particle interactions are presented as a function of the critical charge.
Galactic cosmic radiation (GCR) is an important component of the radiation dose to astronauts when Earth's magnetic shielding is low, for example, on spaceflights outside the magnetosphere and in ...high inclination orbits. In high inclination orbits, low energy (high LET) particles are transmitted through the field only at extreme latitudes, but can dominate the orbit-averaged dose. We have compiled existing measurements of heavy ion (HZE) doses where GCR is expected to be important: Apollo 16 and 17, Skylab, Apollo-Soyuz Test Project. The LET spectra, due to direct ionization from GCR and its nuclear fragments, for each of these missions has been estimated based on the NRL CREME environmental model and particle transport codes developed by us. The resulting comparisons validate the CREME model predictions of high-LET galactic cosmic ray fluxes to within a factor of two or three. Some systematic differences between model and data are identified.
Heavy nuclei in cosmic rays generate single event upsets in microelectronic components. We have calculated the LET-distributions and the fluxes of the various cosmic ray nuclei for elements 1 ≤ Z ≤ ...28. These radiation transport calculations extend to altitudes of 55,000 feet, i.e. to aircraft flight altitudes. The calculations agree well with the limited experimental data available. The results of calculation of the upset rate vs. the critical charge are presented.
The flux and LET-spectra of heavy cosmic ray nuclei and their secondary progeny have been calculated at aircraft flight altitudes. The associated frequency of single event upsets is presented and ...compared with neutron-induced events.
