29,134 research outputs found
Tracer techniques for urine volume determination and urine collection and sampling back-up system
The feasibility, functionality, and overall accuracy of the use of lithium were investigated as a chemical tracer in urine for providing a means of indirect determination of total urine volume by the atomic absorption spectrophotometry method. Experiments were conducted to investigate the parameters of instrumentation, tracer concentration, mixing times, and methods for incorporating the tracer material in the urine collection bag, and to refine and optimize the urine tracer technique to comply with the Skylab scheme and operational parameters of + or - 2% of volume error and + or - 1% accuracy of amount of tracer added to each container. In addition, a back-up method for urine collection and sampling system was developed and evaluated. This back-up method incorporates the tracer technique for volume determination in event of failure of the primary urine collection and preservation system. One chemical preservative was selected and evaluated as a contingency chemical preservative for the storage of urine in event of failure of the urine cooling system
Iron K-alpha Emission from X-ray Reflection: Predictions for Gamma-Ray Burst Models
Recent observations of several gamma-ray burst (GRB) afterglows have shown
evidence for a large amount of X-ray line emitting material, possibly arising
from ionized iron. A significant detection of an X-ray spectral feature, such
as that found in the Chandra observation of GRB 991216, may provide important
constraints on the immediate environment of the burst and hence on progenitor
models. The large Fe K-alpha equivalent widths inferred from the X-ray
observations favor models in which the line is produced when the primary X-ray
emission from the source strikes Thomson-thick material and Compton scatters
into our line of sight. We present such reflection spectra here, computed in a
fully self-consistent manner, and discuss the range of ionization parameters
that may be relevant to different models of GRBs. We argue that the presence of
a strong hydrogen-like K-alpha line is unlikely, because Fe-XXVI photons would
be trapped resonantly and removed from the line core by Compton scattering. In
contrast, a strong narrow emission line from He-like Fe-XXV is prominent in the
model spectra. We briefly discuss how these constraints may affect the line
energy determination in GRB 991216.Comment: 8 pages, 3 figures, Ap.J. Letters accepte
The phase diagram of ice: a quasi-harmonic study based on a flexible water model
The phase diagram of ice is studied by a quasi-harmonic approximation. The
free energy of all experimentally known ice phases has been calculated with the
flexible q-TIP4P/F model of water. The only exception is the high pressure ice
X, in which the presence of symmetric O-H-O bonds prevents its modeling with
this empirical interatomic potential. The simplicity of our approach allows us
to study ice phases at state points of the T-P plane that have been omitted in
previous simulations using free energy methods based on thermodynamic
integration. The effect in the phase diagram of averaging the proton disorder
that appears in several ice phases has been studied. It is found particularly
relevant for ice III, at least for cell sizes typically used in phase
coexistence simulations. New insight into the capability of the employed water
model to describe the coexistence of ice phases is presented. We find that the
H-ordered ices IX and XIV, as well as the H-disordered ice XII, are
particularly stable for this water model. This fact disagrees with experimental
data. The unexpected large stability of ice IX is a property related to the
TIP4P-character of the water model. Only after omission of these three stable
ice phases, the calculated phase diagram becomes in reasonable qualitative
agreement to the experimental one in the T-P region corresponding to ice Ih,
II, III, V, and VI. The calculation of the phase diagram in the quantum and
classical limits shows that the most important quantum effect is the
stabilization of ice II due to its lower zero-point energy when compared to
that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table
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