Magnetostratigraphic record of the Late Miocene onset of the East Asian monsoon, and Pliocene uplift of northern Tibet

Widespread eolian red clay underlying the Plio-Pleistocene loess-palaeosol succession in northern China has been dated magnetostratigraphically back to 8.35 Ma, indicating that the East Asian monsoon started at about the same time as the Indian monsoon. An initial sedimentation rate of 11 m/Myr increased gradually to 17.5 m/Myr by 6 Ma, and then decreased to 6 m/Myr between 5 Ma and 3.5 Ma. A marked increase in sedimentation rate and grain size beginning between 3.5 Ma and 3.1 Ma indicates that the East Asian winter monsoon strengthened at this time, and intensified further after 2.6 Ma. The temporal coincidence of the stronger winter monsoon and the Pliocene uplift of northwestern Tibet just before the onset of the Northern Hemisphere glaciation indicate that the three events could be causally linked. (C) 2001 Published by Elsevier Science B.V

A compact tunable x-ray source based on parametric x-ray generation by moderate energy linacs

Parametric x-radiation can be described as the diffraction of virtual photons associated with the electric field of a relativistic charged particle passing through a crystal. In analogy with Bragg reflection of x-rays, these diffracted photons appear as real photons, with an energy which satisfies Bragg's law for the reflecting crystal planes. We describe the results of experiments performed on the Naval Postgraduate School linac which were designed to explore the basic properties of PXR in order to assess its potential application as a compact tunable x-ray source. Experiments using a mosaic graphite radiator show that this radiator produced multiple order, narrow bandwidth reflections from 5 - 45 keV. The measured production efficiency is found to exceed that predicted for spectral orders n > 1. We demonstrated the tunability of PXR by rotating the crystal in order to change the Bragg angle relative to the incident 90 MeV electron beam.This work was partially supported by the Defense Nuclear Agency, the Naval Postgraduate School and USDOE SBIR Contract (No. DE-FG03-9lER81099

Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau

     Over the past two decades, magnetoclimatological studies of loess-paleosol sequences in the Chinese Loess Plateau (CLP) have made outstanding achievements, which greatly promote the understanding of East Asian paleomonsoon evolution, inland aridification of Asia, and past global climate changes. Loess magnetic properties of the CLP have been well studied. In contrast, loess magnetic properties from outside the CLP in China have not been fully understood. We have little knowledge about the magnetic properties of loess in the Ili Basin, an intermontane depression of the Tianshan (or Tien Shan) Mountains. Here, we present the results of rock magnetic measurements of the Ili loess including mass magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM), high/low temperature dependence of susceptibility (TDS) and hysteresis, as well as X-ray diffraction (XRD) for mineral analysis. Based on the comparison with loess-paleosol sequences in the CLP (hereafter referred to as the Chinese loess), we discuss the possible magnetic susceptibility enhancement mechanism of the Ili loess. The results show that 1) the total magnetic mineral concentration of the Ili loess is far lower than that of the Chinese loess, though they have similar magnetic mineral compositions. The ferrimagnetic minerals in the Ili loess are magnetite and maghemite, and the antiferromagnetic mineral is hematite; XRD analysis also identifies the presence of ilmenite. The ratio of maghemite is lower in the Ili loess than in the Chinese loess, but the ratios of magnetite and hematite are higher in the Ili loess than in the Chinese loess. 2) The granularity of magnetic minerals in the Ili loess, dominated by pseudo-single domain (PSD) and multi-domain (MD) grains, is generally much coarser than that of the Chinese loess. Ultrafine pedogenically-produced magnetic grains have a very limited contribution to the susceptibility enhancement. Rather, PSD and MD particles of magnetite and maghemite are the main contributors to the enhancement of susceptibility in the Ili loess. 3) The susceptibility enhancement mechanism for the Ili loess is complicated and superimposes both a wind velocity/vigor model (Alaskan or Siberian model) and the in situ ultrafine grain pedogenic model; the former might play an important role in the Ili loess. 4) Magnetic susceptibility enhancements of the Ili loess are related not only to the eolian input of the source area, but also to the local climate, landform, and geological background. Therefore, great care should be taken when reconstructing paleoclimate using magnetic susceptibility data from the Ili loess

Observation of soft-x-ray spatial coherence from resonance transition radiation

We have observed the spatial distribution of coherent or resonant transition radiation (RTR) in the soft-x-ray region of the spectrum (1-2 keV). Resonance transition radiators were constructed and tested at two accelerators using electron-beam energies ranging from 50 to 228 MeV. These radiators emitted soft x-rays in a circularly symmetrical annulus with a half-angle divergence of 2.5-9.0 mrad. The angle of peak emission was found to increase with electron-beam energy, in contrast to the incoherent case, for which the angle of emission varied inversely with electron-beam energy. By careful selection of foil thickness and spacing, one may design radiators whose angle of emission varies over a range of charged-particle energies. A particular RTR mode (r=m=1) was found to give a sharp annular ring that becomes more accentuated as the number of foils is increased. The RTR effect has application in particle detection, beam diagnostics, x-ray source brightness enhancement, and x-ray free-electron-laser emission.This work was supported by the Department of Energy under the Small Business Innovative Research (SBIR) program, Grant Number DE-AC03-86ER80428, Canadian Natural Science and Engineering Research Council (NSERC), and the Naval Postgraduate SchoolApproved for public release; distribution is unlimited