4,695 research outputs found
Activities of \gamma-ray emitting isotopes in rainwater from Greater Sudbury, Canada following the Fukushima incident
We report the activity measured in rainwater samples collected in the Greater
Sudbury area of eastern Canada on 3, 16, 20, and 26 April 2011. The samples
were gamma-ray counted in a germanium detector and the isotopes 131I and 137Cs,
produced by the fission of 235U, and 134Cs, produced by neutron capture on
133Cs, were observed at elevated levels compared to a reference sample of
ice-water. These elevated activities are ascribed to the accident at the
Fukushima Dai-ichi nuclear reactor complex in Japan that followed the 11 March
earthquake and tsunami. The activity levels observed at no time presented
health concerns.Comment: 4 pages, 8 figure
High Gaussicity feedhorns for sub-/ millimeter wave applications
In feedhorn design, the power coupling to the fundamental free-space LG00 mode, or Gaussicity, is a good proxy for high performance, particularly the sidelobe and cross-polar levels and the near-field behavior. Gaussicity can be maximized by ensuring that the first few horn modes reach the aperture with the appropriate phase and amplitude relationship. We present two feedhorn designs for which the Gaussicity was maximized in order to achieve high performance. The first is a 94 GHz corrugated horn with a tanh-linear profile, manufactured by electroforming, which achieves a Gaussicity of 99.92% at band center and sidelobes at the -60 dB level. The second is a 340 GHz smooth-walled spline horn which achieves a Gaussicity of >99.2% over a 10% bandwidth, sidelobes below -30 dB and excellent near-field behavior. This design has been successfully fabricated in E-plane split block suitable for low volume manufacture, for example for imaging arrays.Postprin
A low upper-limit on the lithium isotope ratio in HD140283
We have obtained a high-S/N (900-1100), high-resolving-power (R=95000)
spectrum of the metal-poor subgiant HD 140283 in an effort to measure its
6Li/7Li isotope ratio. From a 1-D atmospheric analysis, we find a value
consistent with zero, 6Li/7Li = 0.001, with an upper limit of 6Li/7Li < 0.026.
This measurement supersedes an earlier detection (0.040 +/- 0.015(1sigma)) by
one of the authors. HD 140283 provides no support for the suggestion that
Population II stars may preserve their 6Li on the portion of the subgiant
branch where 7Li is preserved. However, this star does not defeat the
suggestion either; being at the cool end of subgiant branch of the Spite
plateau, it may be sufficiently cool that 6Li depletion has already set in, or
the star may be sufficiently metal poor that little Galactic production of 6Li
had occurred. Continued investigation of other subgiants is necessary to test
the idea. We also consider the implications of the HD 140283 upper limit in
conjunction with other measurements for models of 6Li production by cosmic rays
from supernovae and structure formation shocks.Comment: 8 pages, 4 figures; accepted for publication in Astronomy and
Astrophysic
Optimization and measurement of a smoothly profiled horn for a W-band gyro-TWA
In this paper, a smoothly profiled horn was designed and manufactured to replace a corrugated output horn for a W-band gyro-TWA with improved ultra-high vacuum compatibility. It was optimized for high vacuum integrity, low reflection, high fundamental Gaussian mode content, low side lobe levels and high directivity over the frequency bandwidth of 90 - 100 GHz. Over this operating frequency band the reflectivity was better than -37 dB and the coupling to a fundamental Gaussian mode was above 97%. The far field pattern showed a directivity of approximately 27 dB in the measurement with side lobes lower than -30 dB
Wideband corrugated feedhorns, for radar, communications, radiometry and quasi-optics
This work was supported in part by Engineering and Physical Sciences Research Council, U.K., under Grant EP/R013705/1, and in part by IAA Funding.A wide variety of desirable antenna beam patterns can be synthesized by optimal excitation and phasing of the HE11 and HE12 modes in scalar corrugated feedhorns. However, the bandwidth of such two-mode horns is often limited by modal dispersion. In this paper we introduce a class of low dispersion, two-mode feedhorns that can operate, in some cases, over operating bandwidths of 40-50%. We provide example designs that include horns with high coupling efficiency to: 1) a pure HE11 mode for single-mode excitation of corrugated pipe transmission lines; 2) a LG00 and LG02 combination for radiometry, with narrow beams; 3) a pure Laguerre Gaussian LG00 mode for quasi-optical instrumentation with constant phase centers; 4) a constant gain antenna for uniform illumination with frequency; 5) Airy patterns or “top hat” patterns for radar or communications applications, designed to maximize aperture efficiencies when used with larger reflect or lens antennas. More generally, we show methods to generate and phase multiple HE1n modes, to synthesize symmetric output beams at any desired frequency or gain.Publisher PDFPeer reviewe
A novel class of CoA-transferase involved in short-chain fatty acid metabolism in butyrate-producing human colonic bacteria
Peer reviewedPublisher PD
On the influence of the companion star in Eta Carinae: 2D radiative transfer modeling of the ultraviolet and optical spectra
We present 2D radiative transfer modeling of the Eta Carinae binary system
accounting for the presence of a wind-wind collision (WWC) cavity carved in the
optically-thick wind of the primary star. By comparing synthetic line profiles
with HST/STIS spectra obtained near apastron, we show that the WWC cavity has a
strong influence on multi-wavelength diagnostics. This influence is regulated
by the modification of the optical depth in the continuum and spectral lines.
We find that H-alpha, H-beta, and Fe II lines are the most affected by the WWC
cavity, since they form over a large volume of the primary wind. These spectral
lines depend on latitude and azimuth since, according to the orientation of the
cavity, different velocity regions of a spectral line are affected. For 2D
models with orientation corresponding to orbital inclination angle 110deg < i <
140deg and longitude of periastron 210deg < omega < 330deg, the blueshifted and
zero-velocity regions of the line profiles are the most affected. These orbital
orientations are required to simultaneously fit the UV and optical spectrum of
Eta Car, for a half-opening angle of the cavity in the range 50-70deg. We find
that the excess P-Cygni absorption seen in H-alpha, H-beta and optical Fe II
lines in spherical models becomes much weaker or absent in the 2D models, in
agreement with the observations. The observed UV spectrum of Eta Car, dominated
by Fe II absorption lines, is superbly reproduced by our 2D cavity models.
Small discrepancies still remain, as H-gamma and H-delta absorptions are
overestimated by our models. We suggest that photoionization of the wind of the
primary by the hot companion star is responsible for the weak absorption seen
in these lines. Our CMFGEN models indicate that the primary star has a
mass-loss rate of 8.5x10e-4 Msun/yr and wind terminal velocity of 420 km/s
around the 2000 apastron.Comment: 20 pages, 14 figures, accepted for publication in MNRA
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