180 research outputs found
Device and method of optically orienting biaxial crystals for sample preparation
An optical instrument we refer to as the “biaxial orientation device” has been developed for finding the optical plane, acute bisectrix, and obtuse bisectrix in biaxial crystals by means of optically aligning conoscopically formed melatopes and measuring the angular coordinates of the melatopes, where the angular values allow for determination of the optical plane containing the optical axes using a vector algebra approach. After determination of the optical plane, the instrument allows for the sample to be aligned in the acute bisectrix or obtuse bisectrix orientations and to be transferred to a simple mechanical component for subsequent grinding and polishing, while preserving the orientation of the polished faces relative to the optical plane, acute bisectrix, and obtuse bisectrix during the grinding and polishing process. Biaxial crystalline material samples prepared in the manner are suitable for accurate spectroscopic absorption measurements in the acute bisectrix and obtuse bisectrix directions as well as perpendicular to the optical plane
Reducing dose for digital cranial radiography : The increased source to the image-receptor distance approach
This investigation proposes that an increased source to the image-receptor distance (SID) technique can be used to optimize occipital frontal and lateral cranial radiographs acquired with direct digital radiography. Although cranial radiography is not performed on a routine basis, it should nonetheless be optimized to keep the dose to the patient as low as reasonably achievable, particularly because it can form part of the facial bone and sinus series. Dose measurements were acquired at various SIDs, and image quality was assessed using visual grading analysis. Statistically significant reductions in the effective dose between 19.2% and 23.9% were obtained when the SID was increased from the standard 100 to 150 cm (P ≤.05), and visual grading analysis scores indicate that image quality remained diagnostically acceptable for both projections. This investigation concludes that increasing the SID effectively optimizes occipital frontal and lateral skull radiographs. Radiology departments must be advised of the benefits of this technique with the goal of introducing an updated reference SID of 150 cm into clinical practice.Peer reviewe
A Framework for Modeling Polycyclic Aromatic Hydrocarbon Emission in Galaxy Evolution Simulations
We present a new methodology for simulating mid-infrared emission from
polycyclic aromatic hydrocarbons (PAHs) in galaxy evolution simulations. To do
this, we combine theoretical models of PAH emission features as they respond to
varying interstellar radiation fields, grain size distributions, and ionization
states with a new on-the-fly model for dust evolution in hydrodynamic galaxy
simulations. We apply these models to 3 idealized arepo galaxy evolution
simulations within the smuggle physics framework. We use these simulations to
develop numerical experiments investigating the buildup of PAH masses and
luminosities in galaxies in idealized analogs of the Milky Way, a dwarf galaxy,
and starburst disk. Our main results follow. Galaxies with high specific star
formation rates have increased feedback energy per unit mass, and are able to
efficiently shatter dust grains, driving up the fraction of ultra small grains.
At the same time, in our model large radiation fields per unit gas density
convert aliphatic grains into aromatics. The fraction of dust grains in the
form of PAHs (q_PAH) can be understood as a consequence of these processes, and
in our model PAHs form primarily from interstellar processing (shattering) of
larger grains rather than from the growth of smaller grains. We find that the
hardness of the radiation field plays a larger role than variations in the
grain size distribution in setting the total integrated PAH luminosities,
though cosmological simulations are necessary to fully investigate the complex
interplay of processes that drive PAH band luminosities in galaxies. Finally,
we highlight feature PAH strength variations, cautioning against the usage of
emission templates with constant feature strength ratios.Comment: Submitted to ApJ; comments welcom
The Origins of [CII] Emission in Local Star-forming Galaxies
The [CII] 158um fine-structure line is the brightest emission line observed
in local star-forming galaxies. As a major coolant of the gas-phase
interstellar medium, [CII] balances the heating, including that due to
far-ultraviolet photons, which heat the gas via the photoelectric effect.
However, the origin of [CII] emission remains unclear, because C+ can be found
in multiple phases of the interstellar medium. Here we measure the fractions of
[CII] emission originating in the ionized and neutral gas phases of a sample of
nearby galaxies. We use the [NII] 205um fine-structure line to trace the
ionized medium, thereby eliminating the strong density dependence that exists
in the ratio of [CII]/[NII] 122um. Using the FIR [CII] and [NII] emission
detected by the KINGFISH and Beyond the Peak Herschel programs, we show that
60-80% of [CII] emission originates from neutral gas. We find that the fraction
of [CII] originating in the neutral medium has a weak dependence on dust
temperature and the surface density of star formation, and a stronger
dependence on the gas-phase metallicity. In metal-rich environments, the
relatively cooler ionized gas makes substantially larger contributions to total
[CII] emission than at low abundance, contrary to prior expectations.
Approximate calibrations of this metallicity trend are provided.Comment: 8 pages, accepted for publication in Ap
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