2,788 research outputs found
Optimal operating conditions and characteristics of acetone/CaF_2 detector for inverse photoemission spectroscopy
Performance and characteristics of a band-pass photon detector using acetone
gas and CaF_2 window (acetone/CaF_2) have been studied and compared with an
ethanol/MgF_2 detector. The optimal operating conditions are found to be 4 mbar
acetone pressure and 745+/-20 V anode voltage. The count rate obtained by us is
about a factor of 3 higher than what has been reported earlier for the acetone
detector. Unlike other gas filled detectors, this detector works in the
proportional region with very small dead time (4 micro sec). A detector
band-pass of 0.48+/-0.01 eV FWHM is obtained.Comment: Review of Scientific Instruments 76, 066102 (2005
Influence of Ni doping on the electronic structure of Ni_2MnGa
The modifications in the electronic structure of Ni_{2+x}Mn_{1-x}Ga by Ni
doping have been studied using full potential linearized augmented plane wave
method and ultra-violet photoemission spectroscopy. Ni 3d related electron
states appear due to formation of Ni clusters. We show the possibility of
changing the minority-spin DOS with Ni doping, while the majority-spin DOS
remains almost unchanged. The total magnetic moment decreases with excess Ni.
The total energy calculations corroborate the experimentally reported changes
in the Curie temperature and the martensitic transition temperature with x.Comment: 4 pages, 4 figures, accepted in Phys. Rev.
Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements
We present new astrometric measurements from our ongoing monitoring campaign
of the HR 8799 directly imaged planetary system. These new data points were
obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and
2014. In addition, we present updated astrometry from previously published
observations in 2007 and 2008. All data were reduced using the SOSIE algorithm,
which accounts for systematic biases present in previously published
observations. This allows us to construct a self-consistent data set derived
entirely from NIRC2 data alone. From this dataset, we detect acceleration for
two of the planets (HR 8799b and e) at 3. We also assess possible
orbital parameters for each of the four planets independently. We find no
statistically significant difference in the allowed inclinations of the
planets. Fitting the astrometry while forcing coplanarity also returns
consistent to within 1 of the best fit values, suggesting that if
inclination offsets of 20 are present, they are not detectable
with current data. Our orbital fits also favor low eccentricities, consistent
with predictions from dynamical modeling. We also find period distributions
consistent to within 1 with a 1:2:4:8 resonance between all planets.
This analysis demonstrates the importance of minimizing astrometric systematics
when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A
Modulated structure in the martensite phase of Ni1.8Pt0.2MnGa: a neutron diffraction study
7M orthorhombic modulated structure in the martensite phase of Ni1.8Pt0.2MnGa
is reported by powder neutron diffraction study, which indicates that it is
likely to exhibit magnetic field induced strain. The change in the unit cell
volume is less than 0.5% between the austenite and martensite phases, as
expected for a volume conserving martensite transformation. The magnetic
structure analysis shows that the magnetic moment in the martensite phase is
higher compared to Ni2MnGa, which is in good agreement with magnetization
measurement
Fabrication of an inverse photoemission spectrometer to study unoccupied electronic states
We discuss the performance of an inverse photoemission spectrometer that has been recently fabricated in our laboratory. The photon detector is of band-pass type with acetone gas filling and CaF2 window (acetone/ CaF2). We determine the optimal operating conditions of the detector to be 4 mbar acetone pressure and 745 ± 20 V anode voltage. At these operating conditions, the count rate is improved by a factor of three to what has been reported earlier by Funnemann and Merz. We show that unlike other gas-filled detectors, acetone/CaF2 detector works in the proportional region. Its dead time is negligible and addition of multiplier gas like argon worsens its performance. The performance of this detector has been compared with an ethanol/MgF2 detector. High count rate, reasonable resolution, negligible dead time, no requirement of multiplier or quench gas, satisfactory stability, and ease of handling make the acetone/CaF2 detector an attractive candidate for use in inverse photoemission spectroscopy (IPES). A low-energy electron gun of Stoffel-Johnson type has been fabricated and the beam current profile has been determined as a function of electron kinetic energy. The IPES spectra of polycrystalline Ag and Ta are shown and compared with data in the literature. By fitting the IPES Fermi edge of Ag, we find the overall resolution of the spectrometer to be 0.55 eV and the band-pass of acetone/CaF2 detector to be 0.48 eV FWHM
Phase-Dependent Properties of Extrasolar Planet Atmospheres
Recently the Spitzer Space Telescope observed the transiting extrasolar
planets, TrES-1 and HD209458b. These observations have provided the first
estimates of the day side thermal flux from two extrasolar planets orbiting
Sun-like stars. In this paper, synthetic spectra from atmospheric models are
compared to these observations. The day-night temperature difference is
explored and phase-dependent flux densities are predicted for both planets. For
HD209458b and TrES-1, models with significant day-to-night energy
redistribution are required to reproduce the observations. However, the
observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Landfill Leaching: an Experimental Investigation Using Column Apparatus
Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv
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