813 research outputs found
Beam Orientation Optimization for Intensity Modulated Radiation Therapy using Adaptive l1 Minimization
Beam orientation optimization (BOO) is a key component in the process of IMRT
treatment planning. It determines to what degree one can achieve a good
treatment plan quality in the subsequent plan optimization process. In this
paper, we have developed a BOO algorithm via adaptive l_1 minimization.
Specifically, we introduce a sparsity energy function term into our model which
contains weighting factors for each beam angle adaptively adjusted during the
optimization process. Such an energy term favors small number of beam angles.
By optimizing a total energy function containing a dosimetric term and the
sparsity term, we are able to identify the unimportant beam angles and
gradually remove them without largely sacrificing the dosimetric objective. In
one typical prostate case, the convergence property of our algorithm, as well
as the how the beam angles are selected during the optimization process, is
demonstrated. Fluence map optimization (FMO) is then performed based on the
optimized beam angles. The resulted plan quality is presented and found to be
better than that obtained from unoptimized (equiangular) beam orientations. We
have further systematically validated our algorithm in the contexts of 5-9
coplanar beams for 5 prostate cases and 1 head and neck case. For each case,
the final FMO objective function value is used to compare the optimized beam
orientations and the equiangular ones. It is found that, our BOO algorithm can
lead to beam configurations which attain lower FMO objective function values
than corresponding equiangular cases, indicating the effectiveness of our BOO
algorithm.Comment: 19 pages, 2 tables, and 5 figure
Resonant Interaction Between a Weak Gravitational Wave and a Microwave Beam in the Double Polarized States Through a Static Magnetic Field
We investigate the resonant interaction to the weak gravitational waves in a
coupling electromagnetic system, which consists of a Gaussian beam with the
double polarized transverse electric modes, a static magnetic field and the
fractal membranes. We find that under the syncroresonance condition a
high-frequency GW (HFGW) of h=10^-30,v_g=3GHz may produce the perturbative
photon flux (PPF) of 2.15*10/s in a surface of 0.01m^2. The PPF can be pumped
out from the background photon fluxes and one might obtain the amplified signal
photon flux of 2.15*10^4s^-1 by cascade fractal membranes. It appears to be
worthwhile to study this effect for the detection of the high-frequency relic
GWs in quintessential inflationary models and the HFGWs expected by possible
laboratory schemes.Comment: 10page
Continuous twin screw rheo-extrusion of an AZ91D magnesium alloy
© The Minerals, Metals & Materials Society and ASM International 2012The twin screw rheo-extrusion (TSRE) is designed to take advantage of the nondendritc microstructure and thixotropic characterization of semisolid-metal slurries and produce simple metal profiles directly from melts. The extrusion equipment consists of a rotor-stator high shear slurry maker, a twin screw extruder, and a die assembly. The process is continuous and has a potential for significantly saving energy, manufacturing cost, and enhancing efficiency. The present investigation was carried out to study the process performance for processing rods of an AZ91D magnesium alloy and the microstructure evolution during processing. The semisolid slurry prepared by the process was characterized by uniformly distributed nondendritic granular primary phase particles. AZ91D rods with uniform and fine microstructures and moderate mechanical properties were produced. For the given slurry making parameters, decreasing extrusion temperature was found to improve microstructures and properties. The mechanisms of particle granulation and refinement and the effect of processing parameters on process performance and thermal management are discussed. © 2012 The Minerals, Metals & Materials Society and ASM International.EPSRC (UK) and Rautomead Lt
Numerical simulation of time delay interferometry for eLISA/NGO
eLISA/NGO is a new gravitational wave detection proposal with arm length of
10^6 km and one interferometer down-scaled from LISA. Just like LISA and
ASTROD-GW, in order to attain the requisite sensitivity for eLISA/NGO, laser
frequency noise must be suppressed to below the secondary noises such as the
optical path noise, acceleration noise etc. In previous papers, we have
performed the numerical simulation of the time delay interferometry (TDI) for
LISA and ASTROD-GW with one arm dysfunctional by using the CGC 2.7 ephemeris.
The results are well below their respective limits which the laser frequency
noise is required to be suppressed. In this paper, we follow the same procedure
to simulate the time delay interferometry numerically. To do this, we work out
a set of 1000-day optimized mission orbits of the eLISA/NGO spacecraft starting
at January 1st, 2021 using the CGC 2.7 ephemeris framework. We then use the
numerical method to calculate the residual optical path differences in the
second-generation TDI solutions as in our previous papers. The maximum path
length difference, for all configurations calculated, is below 13 mm (43 ps).
It is well below the limit which the laser frequency noise is required to be
suppressed for eLISA/NGO. We compare and discuss the resulting differences due
to the different arm lengths for various mission proposals -- eLISA/NGO, an
NGO-LISA-type mission with a nominal arm length of 2 x 10^6 km, LISA and
ASTROD-GW.Comment: 17 pages, 13 figures, 3 tables, minor changes in description to match
the accepted version of Classical and Quantum Gravity. arXiv admin note: text
overlap with arXiv:1102.496
Orbital effects of a monochromatic plane gravitational wave with ultra-low frequency incident on a gravitationally bound two-body system
We analytically compute the long-term orbital variations of a test particle
orbiting a central body acted upon by an incident monochromatic plane
gravitational wave. We assume that the characteristic size of the perturbed
two-body system is much smaller than the wavelength of the wave. Moreover, we
also suppose that the wave's frequency is much smaller than the particle's
orbital one. We make neither a priori assumptions about the direction of the
wavevector nor on the orbital geometry of the planet. We find that, while the
semi-major axis is left unaffected, the eccentricity, the inclination, the
longitude of the ascending node, the longitude of pericenter and the mean
anomaly undergo non-vanishing long-term changes. They are not secular trends
because of the slow modulation introduced by the tidal matrix coefficients and
by the orbital elements themselves. They could be useful to indepenedently
constrain the ultra-low frequency waves which may have been indirectly detected
in the BICEP2 experiment. Our calculation holds, in general, for any
gravitationally bound two-body system whose characteristic frequency is much
larger than the frequency of the external wave. It is also valid for a generic
perturbation of tidal type with constant coefficients over timescales of the
order of the orbital period of the perturbed particle.Comment: LaTex2e, 24 pages, no figures, no tables. Changes suggested by the
referees include
Four pulsar discoveries in NGC 6624 by TRAPUM using MeerKAT
We report 4 new pulsars discovered in the core-collapsed globular cluster
(GC) NGC 6624 by the TRAPUM Large Survey Project with the MeerKAT telescope.
All of the new pulsars found are isolated. PSR J18233021I and PSR
J18233021K are millisecond pulsars with period of respectively 4.319 ms and
2.768 ms. PSR J18233021J is mildly recycled with a period of 20.899 ms, and
PSR J18233022 is a long period pulsar with a period of 2.497 s. The pulsars
J18233021I, J18233021J, and J18233021K have position and dispersion
measure (DM) compatible with being members of the GC and are therefore
associated with NGC 6624. Pulsar J18233022 is the only pulsar bright enough
to be re-detected in archival observations of the cluster. This allowed the
determination of a timing solution that spans over two decades. It is not
possible at the moment to claim the association of pulsar J18233022 with the
GC given the long period and large offset in position ( arcminutes) and
DM (with a fractional difference of 11 percent compared the average of the
pulsars in NGC 6624). The discoveries made use of the beamforming capability of
the TRAPUM backend to generate multiple beams in the same field of view which
allows sensitive searches to be performed over a few half-light radii from the
cluster center and can simultaneously localise the discoveries. The discoveries
reflect the properties expected for pulsars in core-collapsed GCs.Comment: Accepted for publication on Monthly Notices of the Royal Astronomical
Society. 11 pages, 6 figure
The TRAPUM L-band survey for pulsars in Fermi-LAT gamma-ray sources
More than 100 millisecond pulsars (MSPs) have been discovered in radio
observations of gamma-ray sources detected by the Fermi Large Area Telescope
(LAT), but hundreds of pulsar-like sources remain unidentified. Here we present
the first results from the targeted survey of Fermi-LAT sources being performed
by the Transients and Pulsars with MeerKAT (TRAPUM) Large Survey Project. We
observed 79 sources identified as possible gamma-ray pulsar candidates by a
Random Forest classification of unassociated sources from the 4FGL catalogue.
Each source was observed for 10 minutes on two separate epochs using MeerKAT's
L-band receiver (856-1712 MHz), with typical pulsed flux density sensitivities
of 100Jy. Nine new MSPs were discovered, eight of which are in
binary systems, including two eclipsing redbacks and one system, PSR
J15262744, that appears to have a white dwarf companion in an unusually
compact 5 hr orbit. We obtained phase-connected timing solutions for two of
these MSPs, enabling the detection of gamma-ray pulsations in the Fermi-LAT
data. A follow-up search for continuous gravitational waves from PSR
J15262744 in Advanced LIGO data using the resulting Fermi-LAT timing
ephemeris yielded no detection, but sets an upper limit on the neutron star
ellipticity of . We also detected X-ray emission from the
redback PSR J18036707 in data from the first eROSITA all-sky survey, likely
due to emission from an intra-binary shock.Comment: 17 pages, 8 figures, accepted for publication in MNRA
GPU-based ultra fast dose calculation using a finite pencil beam model
Online adaptive radiation therapy (ART) is an attractive concept that
promises the ability to deliver an optimal treatment in response to the
inter-fraction variability in patient anatomy. However, it has yet to be
realized due to technical limitations. Fast dose deposit coefficient
calculation is a critical component of the online planning process that is
required for plan optimization of intensity modulated radiation therapy (IMRT).
Computer graphics processing units (GPUs) are well-suited to provide the
requisite fast performance for the data-parallel nature of dose calculation. In
this work, we develop a dose calculation engine based on a finite-size pencil
beam (FSPB) algorithm and a GPU parallel computing framework. The developed
framework can accommodate any FSPB model. We test our implementation on a case
of a water phantom and a case of a prostate cancer patient with varying beamlet
and voxel sizes. All testing scenarios achieved speedup ranging from 200~400
times when using a NVIDIA Tesla C1060 card in comparison with a 2.27GHz Intel
Xeon CPU. The computational time for calculating dose deposition coefficients
for a 9-field prostate IMRT plan with this new framework is less than 1 second.
This indicates that the GPU-based FSPB algorithm is well-suited for online
re-planning for adaptive radiotherapy.Comment: submitted Physics in Medicine and Biolog
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