2,947 research outputs found
A review of the research in testing in secondary school general science from 1938-1952 including nine reviews of standardized tests
Thesis (Ed.M.)--Boston Universit
A simple high-sensitivity technique for purity analysis of xenon gas
We report on the development and performance of a high-sensitivity
purity-analysis technique for gaseous xenon. The gas is sampled at macroscopic
pressure from the system of interest using a UHV leak valve. The xenon present
in the sample is removed with a liquid-nitrogen cold trap, and the remaining
impurities are observed with a standard vacuum mass-spectroscopy device. Using
calibrated samples of xenon gas spiked with known levels of impurities, we find
that the minimum detectable levels of N2, O2, and methane are 1 ppb, 160 ppt,
and 60 ppt respectively. This represents an improvement of about a factor of
10,000 compared to measurements performed without a coldtrap.Comment: 20 pages, 5 figure
Faster k-Medoids Clustering: Improving the PAM, CLARA, and CLARANS Algorithms
Clustering non-Euclidean data is difficult, and one of the most used
algorithms besides hierarchical clustering is the popular algorithm
Partitioning Around Medoids (PAM), also simply referred to as k-medoids. In
Euclidean geometry the mean-as used in k-means-is a good estimator for the
cluster center, but this does not hold for arbitrary dissimilarities. PAM uses
the medoid instead, the object with the smallest dissimilarity to all others in
the cluster. This notion of centrality can be used with any (dis-)similarity,
and thus is of high relevance to many domains such as biology that require the
use of Jaccard, Gower, or more complex distances.
A key issue with PAM is its high run time cost. We propose modifications to
the PAM algorithm to achieve an O(k)-fold speedup in the second SWAP phase of
the algorithm, but will still find the same results as the original PAM
algorithm. If we slightly relax the choice of swaps performed (at comparable
quality), we can further accelerate the algorithm by performing up to k swaps
in each iteration. With the substantially faster SWAP, we can now also explore
alternative strategies for choosing the initial medoids. We also show how the
CLARA and CLARANS algorithms benefit from these modifications. It can easily be
combined with earlier approaches to use PAM and CLARA on big data (some of
which use PAM as a subroutine, hence can immediately benefit from these
improvements), where the performance with high k becomes increasingly
important.
In experiments on real data with k=100, we observed a 200-fold speedup
compared to the original PAM SWAP algorithm, making PAM applicable to larger
data sets as long as we can afford to compute a distance matrix, and in
particular to higher k (at k=2, the new SWAP was only 1.5 times faster, as the
speedup is expected to increase with k)
Study of a zirconium getter for purification of xenon gas
Oxygen, nitrogen and methane purification efficiencies for a common zirconium
getter are measured in 1050 Torr of xenon gas. Starting with impurity
concentrations near 10^{-6} g/g, the outlet impurity level is found to be less
than 120*10^{-12} g/g for O2 and less than 950*10^{-12} g/g for N2. For methane
we find residual contamination of the purified gas at concentrations varying
over three orders of magnitude, depending on the purifier temperature and the
gas flow rate. A slight reduction in the purifier's methane efficiency is
observed after 13 mg of this impurity has been absorbed, which we attribute to
partial exhaustion of the purifier's capacity for this species. We also find
that the purifier's ability to absorb N2 and methane can be extinguished long
before any decrease in O2 performance is observed, and slower flow rates should
be employed for xenon purification due to the cooling effect that the heavy gas
has on the getter.Comment: 14 pages, 5 figure
A Xenon Condenser with a Remote Liquid Storage Vessel
We describe the design and operation of a system for xenon liquefaction in
which the condenser is separated from the liquid storage vessel. The condenser
is cooled by a pulse tube cryocooler, while the vessel is cooled only by the
liquid xenon itself. This arrangement facilitates liquid particle detector
research by allowing easy access to the upper and lower flanges of the vessel.
We find that an external xenon gas pump is useful for increasing the rate at
which cooling power is delivered to the vessel, and we present measurements of
the power and efficiency of the apparatus.Comment: 22 pages, 7 figures Corrected typos in authors lis
BMQ
BMQ: Boston Medical Quarterly was published from 1950-1966 by the Boston University School of Medicine and the Massachusetts Memorial Hospitals
DHODH modulates transcriptional elongation in the neural crest and melanoma
Melanoma is a tumour of transformed melanocytes, which are originally derived from the embryonic neural crest. It is unknown to what extent the programs that regulate neural crest development interact with mutations in the BRAF oncogene, which is the most commonly mutated gene in human melanoma1. We have used zebrafish embryos to identify the initiating transcriptional events that occur on activation of human BRAF(V600E) (which encodes an amino acid substitution mutant of BRAF) in the neural crest lineage. Zebrafish embryos that are transgenic for mitfa:BRAF(V600E) and lack p53 (also known as tp53) have a gene signature that is enriched for markers of multipotent neural crest cells, and neural crest progenitors from these embryos fail to terminally differentiate. To determine whether these early transcriptional events are important for melanoma pathogenesis, we performed a chemical genetic screen to identify small-molecule suppressors of the neural crest lineage, which were then tested for their effects on melanoma. One class of compound, inhibitors of dihydroorotate dehydrogenase (DHODH), for example leflunomide, led to an almost complete abrogation of neural crest development in zebrafish and to a reduction in the self-renewal of mammalian neural crest stem cells. Leflunomide exerts these effects by inhibiting the transcriptional elongation of genes that are required for neural crest development and melanoma growth. When used alone or in combination with a specific inhibitor of the BRAF(V600E) oncogene, DHODH inhibition led to a marked decrease in melanoma growth both in vitro and in mouse xenograft studies. Taken together, these studies highlight developmental pathways in neural crest cells that have a direct bearing on melanoma formation
Thermal Resonance in Signal Transmission
We use temperature tuning to control signal propagation in simple
one-dimensional arrays of masses connected by hard anharmonic springs and with
no local potentials. In our numerical model a sustained signal is applied at
one site of a chain immersed in a thermal environment and the signal-to-noise
ratio is measured at each oscillator. We show that raising the temperature can
lead to enhanced signal propagation along the chain, resulting in thermal
resonance effects akin to the resonance observed in arrays of bistable systems.Comment: To appear in Phys. Rev.
Search for Neutrinoless Double-Beta Decay in Xe with EXO-200
We report on a search for neutrinoless double-beta decay of Xe with
EXO-200. No signal is observed for an exposure of 32.5 kg-yr, with a background
of ~1.5 x 10^{-3} /(kg yr keV) in the region of interest. This
sets a lower limit on the half-life of the neutrinoless double-beta decay
(Xe) > 1.6 x 10 yr (90% CL),
corresponding to effective Majorana masses of less than 140-380 meV, depending
on the matrix element calculation
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