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At least 73 records · Page 4Linked to original sources
PubMed2006

Analytical study of diffusive relativistic shock acceleration.

Particle acceleration in relativistic shocks is studied analytically in the test-particle, small-angle scattering limit, for an arbitrary velocity-angle diffusion function D. The particle spectral index s is found to be sensitive to D, particularly downstream and at certain angles. The analysis, confirmed numerically, justifies and generalizes previous results for isotropic diffusion. It can be used to test collisionless shock models and to observationally constrain D. For example, strongly forward- or backward-enhanced diffusion downstream is ruled out by gamma-ray burst afterglow observations.

Journal Article↗
PubMed1994

Whitefly transmission and efficient ssDNA accumulation of bean golden mosaic geminivirus require functional coat protein.

Bean golden mosaic geminivirus (BGMV) has a bipartite genome composed of two circular ssDNA components (DNA-A and DNA-B) and is transmitted by the whitefly, Bemisia tabaci. DNA-A encodes the viral replication proteins and the coat protein. To determine the role of BGMV coat protein systemic infection and whitefly transmission, two deletions and a restriction fragment inversion were introduced into the BGMV coat protein gene. All three coat protein mutants produced systemic infections when coinoculated with DNA-B onto Phaseolus vulgaris using electric discharge particle acceleration "particle gun." However, they were not sap transmissible and coat protein was not detected in mutant-infected plants. In addition, none of the mutants were transmitted by whiteflies. With all three mutants, ssDNA accumulation of DNA-A and DNA-B was reduced 25- to 50-fold and 3- to 10-fold, respectively, as compared to that of wild-type DNA. No effect on dsDNA-A accumulation was detected and there was 2- to 5-fold increase in dsDNA-B accumulation. Recombinants between the mutated DNA-A and DNA-B forms were identified when the inoculated coat protein mutant was linearized in the common region.

Animals↗
PubMed2000

6Li from Solar Flares.

By introducing a hitherto ignored 6Li producing process, due to accelerated 3He reactions with 4He, we show that accelerated particle interactions in solar flares produce much more 6Li than 7Li. By normalizing our calculations to gamma-ray data, we demonstrate that the 6Li produced in solar flares, combined with photospheric 7Li, can account for the recently determined solar wind lithium isotopic ratio, obtained from measurements in lunar soil, provided that the bulk of the flare-produced lithium is evacuated by the solar wind. Further research in this area could provide unique information on a variety of problems, including solar atmospheric transport and mixing, solar convection and the lithium depletion issue, and solar wind and solar particle acceleration.

Journal Article↗
PubMed2005

A new population of very high energy gamma-ray sources in the Milky Way.

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources.

Journal Article↗
PubMed1991

Accelerated heavy particles and the lens. VI. RBE studies at low doses.

We report on the prevalence, hazard, and relative biological effectiveness (RBE) for various stages of lens opacification in rats induced by very low doses of fast argon ions of LET 88 keV/microns, compared to those for X rays. Doses of argon ions from 0.01 to 0.25 Gy were used and RBEs of these ions relative to X rays estimated using a nonparametric technique. At the end of the follow-up period, which encompasses a significant fraction of the animals' lifetime, 90% confidence intervals for the RBE of the argon ions relative to X rays were 4-8 at 0.25 Gy, 10-40 at 0.05 Gy, and 50-100 at 0.01 Gy. Our results are consistent with the point-estimate neutron RBEs in Japanese A-bomb survivors, though broad confidence bounds are present in the Japanese results. If a reasonable extrapolation to higher doses is used, our results are also consistent with data reported earlier at higher doses for argon-ion cataractogenesis in rats, mice, and rabbits. We conclude from these results that at very low doses the RBE for cataractogenesis from HZE particles in space is considerably more than 20, and use of a quality factor of at least 50 would be prudent.

Animals↗
PubMed1989

Accelerated heavy particles and the lens. III. Cataract enhancement by dose fractionation.

For a number of biological end points it has been shown that, in contrast to low linear energy transfer (LET) radiation, dose fractionation of high-LET radiation does not result in a reduction in overall effectiveness. Studies were conducted to determine the effect of fractionating the exposures to heavy ion doses on the development of cataracts. Rat eyes were exposed to single doses of 1, 5, and 25 cGy of 570 MeV/amu40Ar ions and to 2, 4, and 10 Gy of 250 kVp X rays. These were compared to unirradiated controls and eyes which were exposed to the same total dose delivered in four fractions over 12 h. While in all cases fractionation of the exposure to X rays produced significant reduction in cataractogenic potential, fractionating doses of 40Ar ions caused a dose- and stage-dependent enhancement in the development of cataracts.

Animals↗
PubMed1993

Accelerated heavy particles and the lens. VII: The cataractogenic potential of 450 MeV/amu iron ions.

PURPOSE: To determine the cataractogenic potential dose of high velocity iron ions as a fixation of dose administered singly or fractionated. The dose is critical to risk assessment and to theories of radiation action and cataractogenesis. METHODS: Twenty-eight-day-old rats were examined by slit-lamp biomicroscopy on a weekly-bi-weekly basis for more than 2 yr after radiation exposure. For the acute exposure study doses of 1, 2, 5, 25, and 50 cGy were evaluated. The fractionated regimens involved total doses of 2, 25, and 50 cGy. The reference radiation consisted of 50, 100, 200, or 700 cGy of 250 kilovolt (peak) x-rays. RESULTS: In accordance with previous findings in the rat using 570 MeV/amu 40Ar ions, the relative biologic effectiveness increased rapidly with decreasing dose, reaching values as high as 100. Unlike 40Ar ions, fractionation of the 56Fe doses did not produce a consistent enhancement at any of the doses examined. CONCLUSIONS: The data support the previous findings of a high cataractogenic potential for high linear energy transfer (LET) radiation. The effectiveness for the production of cataracts increases with decreasing dose relative to x-rays and is independent of dose protraction. Although the present study did not reveal a consistent enhancement of effect when the ions were applied in fractions, the results are consistent with at least one theory of the inverse dose-rate effect observed for high-LET radiation.

Animals↗
PubMed1993

Accelerated heavy particles and the lens. VIII. Comparisons between the effects of acute low doses of iron ions (190 keV/microns) and argon ions (88 keV/microns).

The nature of the RBE-LET relationship for radiation-induced effects in vivo is not well known in the high-LET range above about 100 keV/microns. Here, we compare the cataractogenic effects of acute doses of 190 keV/microns iron ions on the eyes of rats with those of 88 keV/microns argon ions. The RBEs of the two radiations cannot be distinguished statistically, both being between 50 and 200 at our lowest dose of 0.01 Gy and decreasing to between 2 and 14 at our highest dose of 0.5 Gy; these values are consistent with results obtained in vivo, both for cataractogenesis and for oncogenesis. For this end point, therefore, the RBE-LET relationship probably varies very slowly between 88 and 190 keV/microns. On the basis of these studies with acute doses of 88 and 190 keV/microns particles, the detailed distribution in LET of the very high-LET galactic cosmic-ray dose to which astronauts in deep space are exposed may not be critical for the prediction of biological hazard. Such a conclusion might simplify the task of high-LET radiation risk estimation in space.

Animals↗
PubMed2007

Dipole hearing measurements in elasmobranch fishes.

The hearing thresholds of the horn shark Heterodontus francisci and the white-spotted bamboo shark Chiloscyllium plagiosum were measured using auditory evoked potentials (AEP) in response to a dipole sound stimulus. The audiograms were similar between the two species with lower frequencies yielding lower particle acceleration thresholds. The particle acceleration audiograms showed more sensitive hearing at low frequencies than previous elasmobranch audiograms, except for the lemon shark Negaprion brevirsotris. Auditory evoked potential signals were also recorded while the dipole stimulus was moved to different locations above the head and body. The strongest AEP signals were recorded from the area around the parietal fossa, supporting previous experiments that suggested this region is important for elasmobranch hearing. This is the first time that hearing experiments have been conducted using a dipole stimulus with elasmobranchs, which more closely mimics the natural sounds of swimming prey.

Animal Communication↗
PubMedDate not supplied

[Nonparametric method of determining the RBE coefficients of accelerated charged particles by the incidence of neoplasms in rats].

The nonparametric method was used to determine RBE coefficients of accelerated charged particles (helium ions of 4 GeV/nucleon and 645 MeV protons) by the incidence of tumors localized in different rat organs or by the absence of tumors. The nonparametric method permitted to find the dose dependence of the RBE coefficients and to make statistical analysis of the results obtained with due regard for come features of developing damages which were not revealed by conventional methods of determining RBE coefficients.

Animals↗