1,732 research outputs found
Polymerization of Methyl Methacrylate by Heat-Catalyst and Gamma-Irradiation Methods
Methyl methacrylate (MMA) was bulk-polymerized with 0 to 4% crosslinker (ethylene glycol dimethacrylate, EGDM, and trimethylol propane trimethacrylate, TMPTM), initiated with 0.05 to 5% catalyst (Vazo) at 65-75 C or 0.1 to 1 Mrad/hr gamma radiation at 20 C. Heat-catalyzed MMA conversion to polymer vs. time was obtained directly from polymer mass, which indicated that about 90% conversion had occurred at the exothermic peak temperature. The time to the exothermic peak temperature was used to determine sample polymerization time. The over-all polymerization rate varied with the half-power of initiator concentration. An Arrhenius plot of the initiator-time data gave an activation energy of 18 kcal/mole. A log-log relationship was found between crosslinker concentration and polymerization time over the 65-75 C temperature and 0.1-0.4% initiator range. The crosslinkers were found equally efficient in reducing polymerization time. Peak exothermie temperature varied directly with time, irrespective of the initiator and crosslinker concentrations or bath temperature except as they affected time. In the irradiation tests, the crosslinkers exhibited different data fits: log-log with EGDM and semilog for TMPTM. The time-dose rate equation for uncrosslinked MMA was analogous to that for heat-catalyzed polymerization. Molecular weight of uncrosslinked PMMA was determined as a function of temperature and catalyst concentration, and dose rate. Similar molecular weights were obtained for heat-catalyzed polymerization at 65 C and gamma irradiation at 20 C for numerically the same initiator concentration (%) and does rate (Mrad/hr)
Corrugated Silicon Platelet Feed Horn Array for CMB Polarimetry at 150 GHz
Next generation cosmic microwave background (CMB) polarization anisotropy
measurements will feature focal plane arrays with more than 600 millimeter-wave
detectors. We make use of high-resolution photolithography and wafer-scale etch
tools to build planar arrays of corrugated platelet feeds in silicon with
highly symmetric beams, low cross-polarization and low side lobes. A compact
Au-plated corrugated Si feed designed for 150 GHz operation exhibited
performance equivalent to that of electroformed feeds: ~-0.2 dB insertion loss,
<-20 dB return loss from 120 GHz to 170 GHz, <-25 dB side lobes and <-23 dB
cross-polarization. We are currently fabricating a 50 mm diameter array with 84
horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol
telescopes. Our fabrication facilities permit arrays up to 150 mm in diameter.Comment: 12 pages; SPIE proceedings for Millimeter, Submillimeter, and
Far-Infrared Detectors and Instrumentation for Astronomy V (Conference 7741,
June 2010, San Diego, CA, USA
Realism, Objectivity, and Evaluation
I discuss Benacerraf's epistemological challenge for realism about areas like mathematics, metalogic, and modality, and describe the pluralist response to it. I explain why normative pluralism is peculiarly unsatisfactory, and use this explanation to formulate a radicalization of Moore's Open Question Argument. According to the argument, the facts -- even the normative facts -- fail to settle the practical questions at the center of our normative lives. One lesson is that the concepts of realism and objectivity, which are widely identified, are actually in tension
Small-Scale Rack Testing of Wood-Frame Shear Walls
A series of racking tests were performed with small-scale (1.2-m X 1.2-m) plywood and OSB assemblies as a means of assessing the potential of reduced assembly sizes in screening variables for subsequent full-size tests. The plywood materials and configurations included variations in stud spacing, nailing, panel thickness and number, and addition of gypsum board. OSB assemblies differed in panel orientation and nailing. The framing used was KD Select Structural to minimize variations in fastening. Both of the standard assemblies (400-mm stud spacing) were also exposed to high relative humidity and effects of green framing. There were clear statistical differences between most plywood configurations, but the most prominent were for center-stud framing, and 9-mm-thick panels. The addition of gypsum board gave higher maximum load and greater stiffness, but the increased variability precluded finding significant differences with the basecase. OSB was significantly lower than plywood in most results. Moisture effects were minimal except for a greater deformation of OSB to the maximum load. The effect of green framing for the bottom plate was minimal
Unified Universal Seesaw Models
A set of Grand Unified Theories based upon the gauge groups SU(5)_\L \times
SU(5)_\R, SO(10)_\L \times SO(10)_\R and SU(4)_\C \times SU(4)_\L \times
SU(4)_\R is explored. Several novel features distinguish these theories from
the well-known , and SU(4)_\C \times SU(2)_\L \times SU(2)_\R
models which they generalize. Firstly, Standard Model quarks and leptons are
accompanied by and mix with heavy SU(2)_\L \times SU(2)_\R singlet partners.
The resulting fermion mass matrices are seesaw in form. Discrete parity
symmetries render the determinants of these mass matrices real and eliminate CP
violating gauge terms. The unified seesaw models consequently provide a
possible resolution to the strong CP problem. Secondly, \sinsq at the
unification scale is numerically smaller than the experimentally measured
scale value. The weak angle must therefore increase as it evolves down in
energy. Finally, proton decay is suppressed by small seesaw mixing factors in
all these theories.Comment: 22 pages with 2 figures not included but available upon request,
CALT-68-185
Sub-dekahertz ultraviolet spectroscopy of 199Hg+
Using a laser that is frequency-locked to a Fabry-Perot etalon of high
finesse and stability, we probe the 5d10 6s 2S_1/2 (F=0) - 5d9 6s 2D_5/2 (F=2)
Delta-m_F = 0 electric-quadrupole transition of a single laser-cooled 199Hg+
ion stored in a cryogenic radio-frequency ion trap. We observe
Fourier-transform limited linewidths as narrow as 6.7 Hz at 282 nm (1.06 X
10^15 Hz), yielding a line Q = 1.6 X 10^14. We perform a preliminary
measurement of the 5d9 6s2 2D_5/2 electric-quadrupole shift due to interaction
with the static fields of the trap, and discuss the implications for future
trapped-ion optical frequency standards.Comment: 4 pages, 4 figures, submitted for publicatio
Supernova pointing with low- and high-energy neutrino detectors
A future galactic SN can be located several hours before the optical
explosion through the MeV-neutrino burst, exploiting the directionality of
--scattering in a water Cherenkov detector such as Super-Kamiokande. We
study the statistical efficiency of different methods for extracting the SN
direction and identify a simple approach that is nearly optimal, yet
independent of the exact SN neutrino spectra. We use this method to quantify
the increase in the pointing accuracy by the addition of gadolinium to water,
which tags neutrons from the inverse beta decay background. We also study the
dependence of the pointing accuracy on neutrino mixing scenarios and initial
spectra. We find that in the ``worst case'' scenario the pointing accuracy is
at 95% C.L. in the absence of tagging, which improves to
with a tagging efficiency of 95%. At a megaton detector, this accuracy can be
as good as . A TeV-neutrino burst is also expected to be emitted
contemporaneously with the SN optical explosion, which may locate the SN to
within a few tenths of a degree at a future km high-energy neutrino
telescope. If the SN is not seen in the electromagnetic spectrum, locating it
in the sky through neutrinos is crucial for identifying the Earth matter
effects on SN neutrino oscillations.Comment: 13 pages, 7 figures, Revtex4 format. The final version to be
published in Phys. Rev. D. A few points in the original text are clarifie
Single Production in Collisions at the NLC
Single production in collisions at the NLC can be used to
probe the Majorana nature of the heavy neutrinos present in the Left-Right
Symmetric Model below the kinematic threshold for their direct production. For
colliders in the TeV range, typical cross sections of order
are obtained, depending on the specific choice of model parameters.
Backgrounds arising from Standard Model processes are shown to be small. This
analysis greatly extends the kinematic range of previous studies wherein the
production of an on-shell, like-sign pair of 's at the NLC was considered.Comment: 13pp, 3 figures (available on request), LaTex, SLAC-PUB-647
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