Two isotopically-characterized targets of high-purity (99.84±0.03%) 235U-oxide and a neutron dosimetry package were exposed to a pulsed neutron irradiation using the Godiva critical assembly at the ...National Criticality Experiments Research Center. Following irradiation, the fissioned targets were counted with a broad-energy germanium detector. Estimates of the total fissions for each target were made using measurements of well-known fission products. The total fission values were then used to measure the fission yields of 21 fission products with half-lives between 636 and 119,700 seconds. Mean values of seventeen of the measured fission products fell within the 95% confidence level of the Evaluated Nuclear Data Files VII.1/b (ENDF/B-VII.1) using the larger of the two uncertainties: 84gBr, 89Rb, 91Sr, 93Y, 97Zr, 101Mo, 105Ru, 128Sn, 129Sb, 130gSb, 131Sb, 131mTe, 133I, 135I, 141Ba, 142Ba, and 142La. Seven isotopes were measured to better precision than is listed in the ENDF/B-VII.1 database: 89Rb, 93Y, 128Sn, 129Sb, 130gSb, 131mTe, and 133I. Four of the measured isotopes were found to be statistically different from the ENDF/B-VII.1 database: 94Y, 134Te, 134I, and 138Xe. Two of the isotopic analyses conducted in this paper fit the parent and daughter data simultaneously. The parent-daughter pairs measured in this way were the 134Te/I and 142Ba/La pairs. The fission yield results obtained in this way are unique, especially for 134I, because the parent and daughter half-lives are so close. Here, given the limited number of direct measurements of the 134Te/I pair from fast neutron induced fission and the seemingly high estimate for 134Te from previous work, it is likely that the value in this paper is more accurate; however, additional measurements of this pair are needed to validate this assertion.
We hypothesized that the reduction in toughness that accompanies elevated levels of cross-linking in ultrahigh-molecular weight polyethylene would lead to increased wear and damage in acetabular cups ...under conditions of impingement with the neck of the femoral component. Test cups were exposed to 2.8, 10, and 20 Mrad of gamma irradiation and were tested in a simulator, incorporating impingement during every wear cycle. Wear behavior was compared based on wear rate, observations of damage, and the number and morphology of wear particles. The 10-Mrad cups had the lowest rate (15.2 +/- 5.1 mg/Mc), followed by the 2.8-Mrad (40.1 +/- 4.2 mg/Mc) and 20-Mrad (67.9 +/- 24.2 mg/Mc) groups. The wear damage was most severe in the 20-Mrad cups, although all 3 groups showed pitting and delaminating. Wear rates and damage of highly cross-linked polyethylene cups were significantly higher in an impingement wear test than in previous simulator studies.
We present maps of 7.78 deg super(2) of the Lupus molecular cloud complex at 24, 70, and 160 mu m. They were made with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) ...instrument as part of the Spitzer Legacy Program "From Molecular Cores to Planet-Forming Disks" (c2d). The maps cover three separate regions in Lupus, denoted I, III, and IV. We discuss the c2d pipeline and how our data processing differs from it. We compare source counts in the three regions with two other data sets and predicted star counts from the Wainscoat model. This comparison shows the contribution from background galaxies in Lupus I. We also create two color-magnitude diagrams using the 2MASS and MIPS data. From these results, we can identify background galaxies and distinguish them from probable young stellar objects. The sources in our catalogs are classified based on their spectral energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample of young steilar object candidates. From 2MASS data, we create extinction maps for each region and note a strong correspondence between the extinction and the 160 mu m emission. The masses we derived in each Lupus cloud from our extinction maps are compared to masses estimated from super(13)CO and C super(18) O and found to be similar to our extinction masses in some regions, but significantly different in others. Finally, based on our color-magnitude diagrams, we selected 12 of our reddest candidate young stellar objects for individual discussion. Five of the 12 appear to be newly discovered YSOs.
Russian exhibits a rich pattern of phonological vowel reduction, by which some vowel contrasts are neutralized in unstressed syllables. Recent work in phonology suggests a mechanism by which phonetic ...vowel reduction--compression of the overall vowel space due to target undershoot--might lead to patterns like Russian. Presenting acoustic data from 9 speakers of Russian, we use Euclidean distance measures, measures of F1-F0 and F2-F1, and Bayesian classification to provide a basic picture of how the overall vowel space, as well as the distribution of vowels, change as stress is reduced. We are particularly interested in whether contraction of the vowel space in unstressed positions is primarily due to raising, and in whether contrasting pairs of vowels are evenly spaced within and across contexts. Our results provide qualified support for the first hypothesis, but largely do not support the hypothesis of equal spacing, in particular across contexts. Of additional interest, we find that some impressionistically described neutralizations are incomplete.
We report infrared spectroscopic observations of HD 105, a nearby (640 pc) and relatively young (630 Myr) G0 star with excess infrared continuum emission, which has been modeled as arising from an ...optically thin circumstellar dust disk with an inner hole of size 13 AU. We have used the high spectral resolution mode of the Infrared Spectrometer (IRS) on the Spitzer Space Telescope to search for gas emission lines from the disk. The observations reported here provide upper limits to the fluxes of H sub(2) S(0) 28 km, H sub(2) S(1) 17 km, H sub(2) S(2) 12 km, Fe II 26 km, Si II 35 km, and S I 25 km infrared emission lines. The H sub(2) line upper limits place direct constraints on the mass of warm molecular gas in the disk: M(H sub(2)) < 4.6, 3.8 x 10 super(-2), and 3.0 x 10 super(-3) M sub(J) at T = 50, 100, and 200 K, respectively. We also compare the line flux upper limits to predictions from detailed thermal/chemical models of various gas distributions in the disk. These comparisons indicate that if the gas distribution has an inner hole with radius r sub(i,gas), the surface density at that inner radius is limited to values ranging from 3 g cm super(-2) at r sub(i,gas) = 0.5 AU to 0.1 g cm super(-2) at r sub(i,gas) = 5-20 AU. These values are considerably below the value for a minimum mass solar nebula, and suggest that less than 1 Jupiter mass (M sub(J)) of gas (at any temperature) exists in the 1-40 AU planet-forming region. Therefore, it is unlikely that there is sufficient gas for gas giant planet formation to occur in HD 105 at this time.
Far-Infrared Imaging of NGC 55 Engelbracht, C. W; Gordon, K. D; Bendo, G. J ...
The Astrophysical journal. Supplement series,
09/2004, Letnik:
154, Številka:
1
Journal Article
Formation of the vertebrate neuromuscular junction (NMJ) takes place in a stereotypic pattern in which nerves terminate at select sarcolemmal sites often localized to the central region of the muscle ...fibers. Several lines of evidence indicate that the muscle fibers may initiate postsynaptic differentiation independent of the ingrowing nerves. For example, nascent acetylcholine receptors (AChRs) are pre-patterned at select regions of the muscle during the initial stage of neuromuscular synaptogenesis. It is not clear how these pre-patterned AChR clusters are assembled, and to what extent they contribute to pre- and post-synaptic differentiation during development. Here, we show that genetic deletion of the AChR gamma-subunit gene in mice leads to an absence of pre-patterned AChR clusters during initial stages of neuromuscular synaptogenesis. The absence of pre-patterned AChR clusters was associated with excessive nerve branching, increased motoneuron survival, as well as aberrant distribution of acetylcholinesterase (AChE) and rapsyn. However, clustering of muscle specific kinase (MuSK) proceeded normally in the gamma-null muscles. AChR clusters emerged at later stages owing to the expression of the AChR epsilon-subunit, but these delayed AChR clusters were broadly distributed and appeared at lower level compared with the wild-type muscles. Interestingly, despite the abnormal pattern, synaptic vesicle proteins were progressively accumulated at individual nerve terminals, and neuromuscular synapses were ultimately established in gamma-null muscles. These results demonstrate that the gamma-subunit is required for the formation of pre-patterned AChR clusters, which in turn play an essential role in determining the subsequent pattern of neuromuscular synaptogenesis.
PDF
