Muonium as a shallow center in GaN

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.Comment: 4 figures, to be published in Phys. Rev. Letter

Response of Carcinogen-Altered Mouse Epidermal Cells to Phorbol Ester Tumor Promoters and Calcium

Primary cultures of mouse epidermal cells are induced to terminally differentiate when extracellular calcium levels are increased to more than 0.1mM After carcinogen treatment, cellular foci can be selected that resist this calcium signal to terminally differentiate Calcium causes these foci to stratify, however, in contrast to normal epidermis, DNA- synthesizing cells in these foci are found in the suprabasal cell layers as well as in basal cells Cell lines derived from these foci may be considered to be putative initiated cells Three of these cell lines, designated 308, D, and F, have been characterized for their response to calcium and phorbol ester tumor promoters. The formation of cornified cells and the activity of epidermal transglutaminase were utilized as markers of epidermal differentiation. Neither calcium nor the tumor promoter 12-O-tetradecanoylphorbol-13- acetate (TPA) increased transglutaminase activity or cornification of any of the 3 lines Proliferation was estimated by the [3H]thymidine labeling index, by incorporation of [3H]thymidine into DNA, and by a clonal growth assay. Unlike primary normal cultures, rising the calcium level of the medium did not markedly reduce the rate of proliferation of any of the 3 cell lines. in 2 of the lines, line 308 and line D, proliferation increased in response to TPA exposure. in line F, [3H]thymidine incorporation in confluent cultures was inhibited by TRA, while in cells plated at clonal densities, TPA was cytotoxic at doses of 5 ng/ml or higher. If these calcium-resistant epidermal cell lines correspond to initiated cells, their lack of sensitivity to the induction of terminal differentiation by TPA could account for their growth relative to normal cells. Those lines that also respond to stimulation of proliferation by TPA to a greater extent than normal cells would have a further growth advantage

COMPTEL upper limits for Seyfert galaxies

The gamma‐ray emission of Seyfert galaxies has fallen far short of pre‐GRO expectations. No single object of this class has been detected by either COMPTEL or EGRET, and OSSE has detected only a fraction of the Seyferts expected. To derive a more stringent upper limit to the emission from these objects in the energy ranges 0.75 to 1 and 1 to 3 MeV, we have summed a large number of COMPTEL observations acquired during Phase 1 of the GRO mission. From a total of 47 observations of 23 individual X‐ray selected Seyfert galaxies, we derive preliminary upper limits of 8×10−8 photons/(cm2 s keV) in the 0.75‐1 MeV band and 1×10−8 photons/(cm2 s keV) in the 1‐3 MeV band

COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission

The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have been analyzed. During the first two years PKS 0528+134 was most significantly detected at energies above 3 MeV. During the last year there is only evidence for the quasar at energies below 3 MeV indicating a spectral change. The time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well represented by a power-law shape. Spectra collected from different observational periods reveal different power-law shapes: a hard state during flaring observations reported by EGRET, and a soft state otherwise. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During low intensisty gamma-ray phases no spectral break is obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray flaring phases however, the combined COMPTEL and EGRET data require a spectral bending at MeV-energies. By fitting broken power-law functions the best-fit values for the break in photon index range between 0.6 and 1.7, and for the break energy between ~5 MeV and ~20 MeV. Because the flux values measured by COMPTEL below 3 MeV in both states are roughly equal, the observations would be consistent with an additional spectral component showing up during gamma-ray flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a high-energy electron-positron population with a low-energy cutoff in their bulk Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for gamma-ray flaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies.Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33 (1997

COMPTEL Observations of AGN at MeV-Energies

The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy in the MeV-band.Comment: 4 pages, 2 figures including 1 color plot, to appear in the Proceedings of the 3rd INTEGRAL Workshop "The Extreme Universe", held in Taormina, Italy, 14-18 September 199

Pancreatic islets communicate with lymphoid tissues via exocytosis of insulin peptides.

Tissue-specific autoimmunity occurs when selected antigens presented by susceptible alleles of the major histocompatibility complex are recognized by T cells. However, the reason why certain specific self-antigens dominate the response and are indispensable for triggering autoreactivity is unclear. Spontaneous presentation of insulin is essential for initiating autoimmune type 1 diabetes in non-obese diabetic mice1,2. A major set of pathogenic CD4 T cells specifically recognizes the 12-20 segment of the insulin B-chain (B:12-20), an epitope that is generated from direct presentation of insulin peptides by antigen-presenting cells3,4. These T cells do not respond to antigen-presenting cells that have taken up insulin that, after processing, leads to presentation of a different segment representing a one-residue shift, B:13-214. CD4 T cells that recognize B:12-20 escape negative selection in the thymus and cause diabetes, whereas those that recognize B:13-21 have only a minor role in autoimmunity3-5. Although presentation of B:12-20 is evident in the islets3,6, insulin-specific germinal centres can be formed in various lymphoid tissues, suggesting that insulin presentation is widespread7,8. Here we use live imaging to document the distribution of insulin recognition by CD4 T cells throughout various lymph nodes. Furthermore, we identify catabolized insulin peptide fragments containing defined pathogenic epitopes in β-cell granules from mice and humans. Upon glucose challenge, these fragments are released into the circulation and are recognized by CD4 T cells, leading to an activation state that results in transcriptional reprogramming and enhanced diabetogenicity. Therefore, a tissue such as pancreatic islets, by releasing catabolized products, imposes a constant threat to self-tolerance. These findings reveal a self-recognition pathway underlying a primary autoantigen and provide a foundation for assessing antigenic targets that precipitate pathogenic outcomes by systemically sensitizing lymphoid tissues

COMPTEL upper limits for the 56Co γ-rays from SN1998bu

The type Ia supernova SN 1998bu in M96 was observed by COMPTEL for a total of 88 days starting 17 days after the detection of the SN. A special mode improving the low-energy sensitivity was invoked. We obtained images in the 847 keV and 1238 keV lines of 56Co using an improved point-spread function for the low-energies. We do not detect SN1998bu. Sensitive upper limits at both energies constrain the standard supernova model for this event

Improvement in measurement accuracy for hybrid scanner

The capability to provide dense three-dimensional (3D) data (point clouds) at high speed and at high accuracy has made terrestrial laser scanners (TLS) widely used for many purposes especially for documentation, management and analysis. However, similar to other 3D sensors, proper understanding regarding the error sources is necessary to ensure high quality data. A procedure known as calibration is employed to evaluate these errors. This process is crucial for TLS in order to make it suitable for accurate 3D applications (e.g. industrial measurement, reverse engineering and monitoring). Two calibration procedures available for TLS: 1) component, and 2) system calibration. The requirements of special laboratories and tools which are not affordable by most TLS users have become principle drawback for component calibration. In contrast, system calibration only requires a room with appropriate targets. By employing optimal network configuration, this study has performed system calibration through self-calibration for Leica ScanStation C10 scanner. A laboratory with dimensions of 15.5m x 9m x 3m and 138 well-distributed planar targets were used to derive four calibration parameters. Statistical analysis (e.g. t-test) has shown that only two calculated parameters, the constant rangefinder offset error (0.7mm) and the vertical circle index error (-45.4inch were significant for the calibrated scanner. Photogrammetric technique was utilised to calibrate the 3D test points at the calibration field. By using the test points, the residual pattern of raw data and self-calibration results were plotted into the graph to visually demonstrate the improvement in accuracy for Leica ScanStation C10 scanner