2,753 research outputs found
Photoluminescence and Electroluminescence of Erbium and Neodymium Implanted Semiconductors
Low temperature photoluminescence (PL) and electroluminescence (EL) measurements were used to study the excitation of erbium- and neodymium-implanted GaAs and AlxGa1-xAs (x=0. 1, 0.3) pn-junctions. The rare-earth (RE) emissions were investigated as a function of ion dose, aluminum mole fraction, laser excitation power, and applied forward bias voltage for the implanted samples. Low temperature PL was also measured from Er doped silicon grown by the metalorganic chemical vapor-phase deposition (MOCVD) method using various growth parameters.. The MOCVD-grown Si samples were studied as a function of metalorganic source temperature, silane (SiH4) flow, growth time, and hydrogen presence during growth
Gas micro-well track imaging detectors for gamma-ray astronomy
We describe our program to develop gas micro-well detectors (MWDs) as three-dimensional charged particle trackers for use in advanced gamma-ray telescope concepts. A micro-well detector consists of an array of individual micro-patterned gas proportional counters opposite a planar drift electrode. The well anodes and cathodes may be connected in X and Y strips, respectively, to provide two-dimensional imaging. When combined with transient digitizer electronics, which record the time signature of the charge collected in the wells of each strip, full three-dimensional reconstruction of charged-particle tracks in large gas volumes is possible. Such detectors hold great promise for advanced Compton telescope (ACT) and advanced pair telescope (APT) concepts due to the very precise measurement of charged particle momenta that is possible (Compton recoil electrons and electron-positron pairs, respectively). We present preliminary lab results, including detector fabrication, prototype electronics, and initial detector testing. We also discuss applications to the ACT and APT mission concepts, based on GEANT3 and GEANT4 simulations
Funding New Business Ventures: Differences In Minority And Non-Minority Family-Owned Business Access To Start-Up Capital
Access to start-up capital is important in starting any business venture. In addition, the source of that start-up capital can sometimes affect the prospects for success in the new business venture. This study examines the differences in the source of start-up funds between male minority and male non-minority family business owners. The findings indicate that male minority family business owners more often use credit cards and gifts from family as a source of start-up funds whereas the male non-minority family business owners rely more often on bank loans. The authors propose a more extensive survey of a larger sample of family business owners in order to further identify differences in source of start-up capital between male minority and male non-minority family business owners
Gain Stabilization of a Submillimeter SIS Heterodyne Receiver
We have designed a system to stabilize the gain of a submillimeter heterodyne
receiver against thermal fluctuations of the mixing element. In the most
sensitive heterodyne receivers, the mixer is usually cooled to 4 K using a
closed-cycle cryocooler, which can introduce ~1% fluctuations in the physical
temperature of the receiver components. We compensate for the resulting mixer
conversion gain fluctuations by monitoring the physical temperature of the
mixer and adjusting the gain of the intermediate frequency (IF) amplifier that
immediately follows the mixer. This IF power stabilization scheme, developed
for use at the Submillimeter Array (SMA), a submillimeter interferometer
telescope on Mauna Kea in Hawaii, routinely achieves a receiver gain stability
of 1 part in 6,000 (rms to mean). This is an order of magnitude improvement
over the typical uncorrected stability of 1 part in a few hundred. Our gain
stabilization scheme is a useful addition to SIS heterodyne receivers that are
cooled using closed-cycle cryocoolers in which the 4 K temperature fluctuations
tend to be the leading cause of IF power fluctuations.Comment: 7 pages, 6 figures accepted to IEEE Transactions on Microwave Theory
and Technique
Place matters: but does local leadership?
The arrival of New Labour into Government witnessed the prominent re-emergence of place onto the policy agenda. This heralded a range of area-based-initiatives designed to both tackle neighbourhood forms of deprivation and to re-establish a sense of identity and connection between individuals and their local community. In terms of place-making, effective and inclusive participation, representation and leadership were all identified as prerequisites for the creation of sustainable communities . But how important is local leadership and strategic vision within local public service organisations in achieving the desired place-making outcomes? This paper examines the extent to which local leadership and strategic vision represents a significant factor in promoting higher levels of satisfaction, belonging, cohesion and participation across single tier councils in England. The ensuing empirical evidence raises significant questions not only about the importance of local leadership in place-making, but also the environmental and organizational factors that shape local places
Improving photoelectron counting and particle identification in scintillation detectors with Bayesian techniques
Many current and future dark matter and neutrino detectors are designed to measure scintillation light with a large array of photomultiplier tubes (PMTs). The energy resolution and particle identification capabilities of these detectors depend in part on the ability to accurately identify individual photoelectrons in PMT waveforms despite large variability in pulse amplitudes and pulse pileup. We describe a Bayesian technique that can identify the times of individual photoelectrons in a sampled PMT waveform without deconvolution, even when pileup is present. To demonstrate the technique, we apply it to the general problem of particle identification in single-phase liquid argon dark matter detectors. Using the output of the Bayesian photoelectron counting algorithm described in this paper, we construct several test statistics for rejection of backgrounds for dark matter searches in argon. Compared to simpler methods based on either observed charge or peak finding, the photoelectron counting technique improves both energy resolution and particle identification of low energy events in calibration data from the DEAP-1 detector and simulation of the larger MiniCLEAN dark matter detector
Understanding Accretion Outbursts in Massive Protostars through Maser Imaging
The bright maser emission produced by several molecular species at centimeter
to long millimeter wavelengths provides an essential tool for understanding the
process of massive star formation. Unimpeded by the high dust optical depths
that affect shorter wavelength observations, the high brightness temperature of
these emission lines offers a way to resolve accretion and outflow motions down
to scales below 1 au in deeply embedded Galactic star-forming regions at
kiloparsec distances. The recent identification of extraordinary accretion
outbursts in two high-mass protostars, both of which were heralded by maser
flares, has rapidly impacted the traditional view of massive protostellar
evolution, leading to new hydrodynamic simulations that can produce such
episodic outbursts. In order to understand how these massive protostars evolve
in response to such events, larger, more sensitive ground-based centimeter
wavelength interferometers are needed that can simultaneously image multiple
maser species in the molecular gas along with faint continuum from the central
ionized gas. Fiducial observations of a large sample of massive protostars will
be essential in order to pinpoint the progenitors of future accretion
outbursts, and to quantify the outburst-induced changes in their protostellar
photospheres and outflow and accretion structures. Knowledge gained from these
studies will have broader impact on the general topic of accretion onto massive
objects.Comment: Science white paper submitted to the Astro2020 Decadal Survey. arXiv
admin note: substantial text overlap with arXiv:1806.0698
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