955 research outputs found
Reactivity of cellulose during hydrothermal carbonization of lignocellulosic biomass.
Hydrothermal carbonization (HTC) of pure cellulose (CE) and birchwood (BW) samples was carried out at temperatures between 160 and 280 °C, 0.5 h residence time and biomass-to-water ratio 1:5, to investigate the reactivity of cellulose in lignocellulosic biomass. Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) showed that the CE samples remained unaltered at temperatures up to 220 °C, but were significantly decomposed at 230 °C producing a thermal recalcitrant aromatic and high energy-dense material. FTIR showed that dehydration and aromatization reactions occurred at temperature equal or higher than 230 °C for the CE samples while a similar increase in aromatization for the BW hydrochars was evident only at temperatures equal or higher than 260 °C. Acid hydrolysis, TGA and FTIR suggested that a higher thermal resistance of naturally occurring cellulose in BW (when compared to CE sample) could be related to a ‘protecting shield’ offered by interlinked lignin in the plant matrix
Regge calculus from a new angle
In Regge calculus space time is usually approximated by a triangulation with
flat simplices. We present a formulation using simplices with constant
sectional curvature adjusted to the presence of a cosmological constant. As we
will show such a formulation allows to replace the length variables by 3d or 4d
dihedral angles as basic variables. Moreover we will introduce a first order
formulation, which in contrast to using flat simplices, does not require any
constraints. These considerations could be useful for the construction of
quantum gravity models with a cosmological constant.Comment: 8 page
A high-field adiabatic fast passage ultracold neutron spin flipper for the UCNA experiment
The UCNA collaboration is making a precision measurement of the β asymmetry (A) in free neutron decay using polarized ultracold neutrons (UCN). A critical component of this experiment is an adiabatic fast passage neutron spin flipper capable of efficient operation in ambient magnetic fields on the order of 1 T. The requirement that it operate in a high field necessitated the construction of a free neutron spin flipper based, for the first time, on a birdcage resonator. The design, construction, and initial testing of this spin flipper prior to its use in the first measurement of A with UCN during the 2007 run cycle of the Los Alamos Neutron Science Center's 800 MeV proton accelerator is detailed. These studies determined the flipping efficiency of the device, averaged over the UCN spectrum present at the location of the spin flipper, to be ϵ(overbar) = 0.9985(4)
Finiteness and Dual Variables for Lorentzian Spin Foam Models
We describe here some new results concerning the Lorentzian Barrett-Crane
model, a well-known spin foam formulation of quantum gravity. Generalizing an
existing finiteness result, we provide a concise proof of finiteness of the
partition function associated to all non-degenerate triangulations of
4-manifolds and for a class of degenerate triangulations not previously shown.
This is accomplished by a suitable re-factoring and re-ordering of integration,
through which a large set of variables can be eliminated. The resulting
formulation can be interpreted as a ``dual variables'' model that uses
hyperboloid variables associated to spin foam edges in place of representation
variables associated to faces. We outline how this method may also be useful
for numerical computations, which have so far proven to be very challenging for
Lorentzian spin foam models.Comment: 15 pages, 1 figur
Spectrum of Charged Black Holes - The Big Fix Mechanism Revisited
Following an earlier suggestion of the authors(gr-qc/9607030), we use some
basic properties of Euclidean black hole thermodynamics and the quantum
mechanics of systems with periodic phase space coordinate to derive the
discrete two-parameter area spectrum of generic charged spherically symmetric
black holes in any dimension. For the Reissner-Nordstrom black hole we get
, where the integer p=0,1,2,.. gives the charge
spectrum, with . The quantity , n=0,1,... gives
a measure of the excess of the mass/energy over the critical minimum (i.e.
extremal) value allowed for a given fixed charge Q. The classical critical
bound cannot be saturated due to vacuum fluctuations of the horizon, so that
generically extremal black holes do not appear in the physical spectrum.
Consistency also requires the black hole charge to be an integer multiple of
any fundamental elementary particle charge: , m=0,1,2,.... As a
by-product this yields a relation between the fine structure constant and
integer parameters of the black hole -- a kind of the Coleman big fix mechanism
induced by black holes. In four dimensions, this relationship is
and requires the fine structure constant to be a rational
number. Finally, we prove that the horizon area is an adiabatic invariant, as
has been conjectured previously.Comment: 21 pages, Latex. 1 Section, 1 Figure added. To appear in Class. and
Quant. Gravit
Spectrum of rotating black holes and its implications for Hawking radiation
The reduced phase space formalism for quantising black holes has recently
been extended to find the area and angular momentum spectra of four dimensional
Kerr black holes. We extend this further to rotating black holes in all
spacetime dimensions and show that although as in four dimensions the spectrum
is discrete, it is not equispaced in general. As a result, Hawking radiation
spectra from these black holes are continuous, as opposed to the discrete
spectrum predicted for four dimensional black holes.Comment: 11 pages, Revtex4. Minor changes to match version to appear in Class.
Quant. Gra
Effective action and semiclassical limit of spin foam models
We define an effective action for spin foam models of quantum gravity by
adapting the background field method from quantum field theory. We show that
the Regge action is the leading term in the semi-classical expansion of the
spin foam effective action if the vertex amplitude has the large-spin
asymptotics which is proportional to an exponential function of the vertex
Regge action. In the case of the known three-dimensional and four-dimensional
spin foam models this amounts to modifying the vertex amplitude such that the
exponential asymptotics is obtained. In particular, we show that the ELPR/FK
model vertex amplitude can be modified such that the new model is finite and
has the Einstein-Hilbert action as its classical limit. We also calculate the
first-order and some of the second-order quantum corrections in the
semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure
A next-generation inverse-geometry spallation-driven ultracold neutron source
The physics model of a next-generation spallation-driven high-current
ultracold neutron (UCN) source capable of delivering an extracted UCN rate of
around an-order-of-magnitude higher than the strongest proposed sources, and
around three-orders-of-magnitude higher than existing sources, is presented.
This UCN-current-optimized source would dramatically improve cutting-edge UCN
measurements that are currently statistically limited. A novel "Inverse
Geometry" design is used with 40 L of superfluid He (He-II), which acts as
a converter of cold neutrons (CNs) to UCNs, cooled with state-of-the-art
sub-cooled cryogenic technology to 1.6 K. Our design is optimized for a
100 W maximum heat load constraint on the He-II and its vessel. In our
geometry, the spallation target is wrapped symmetrically around the UCN
converter to permit raster scanning the proton beam over a relatively large
volume of tungsten spallation target to reduce the demand on the cooling
requirements, which makes it reasonable to assume that water edge-cooling only
is sufficient. Our design is refined in several steps to reach
s under our other restriction of 1 MW maximum
available proton beam power. We then study effects of the He-II scattering
kernel as well as reductions in due to pressurization to reach
s. Finally, we provide a design for the UCN
extraction system that takes into account the required He-II heat transport
properties and implementation of a He-II containment foil that allows UCN
transmission. We estimate a total useful UCN current from our source of
s from a 18 cm diameter guide 5 m from the source.
Under a conservative "no return" approximation, this rate can produce an
extracted density of cm in 1000~L external experimental
volumes with a Ni (335 neV) cut-off potential.Comment: Submitted to Journal of Applied Physic
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