Spectro-streak picosecond studies of intramolecular charge-transfer fluorescence

A spectro-streak photometer, an instrument for simultaneously measuring fluorescence intensity, time, and wavelength,I(t, ), with a single picosecond excitation pulse, has been constructed. Two typical and currently highly topical examples of mesurements are discussed. (1) the temporal development of the fluorescence form the intramolecular charge-transfer (ICT) state of the rigid aromatic compound 4,5-(1-methylindolino)3,4-naphthanthracene is studied in the protic solvent hexanol. (2) Propyl chain-linked pyrene/N,N-dimethylaniline is used as the model compound to study conformational changes associated with the transition from a contact ion pair to a sandwich exciplex.     

Induced radioactivity of LDEF materials and structural components

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.     

Structural aspects of antibody-antigen interaction revealed through small random peptide libraries

Summary Two small random peptide libraries, one composed of 4550 dodecapeptides and one of 8000 tripeptides, were synthesized in newly developed credit-card format miniPEPSCAN cards (miniPEPSCAN libraries). Each peptide was synthesized in a discrete well (455 peptides/card). The two miniPEPSCAN libraries were screened with three different monoclonal antibodies (Mabs). Two other random peptide libraries, expressed on the wall of bacteria (recombinant libraries) and composed of 10⁷ hexa- and octapeptides, were screened with the same three Mabs. The aim of this study was to compare the amino acid sequence of peptides selected from small and large pools of random peptides and, in this way, investigate the potential of small random peptide libraries. The screening of the two miniPEPSCAN libraries resulted in the identification of a surprisingly large number of antibody-binding peptides, while the screening of the large recombinant libraries, using the same Mabs, resulted in the identification of only a small number of peptides. The large number of peptides derived from the small random peptide libraries allowed the determination of consensus sequences. These consensus sequences could be related to small linear and nonlinear parts of the respective epitopes. The small number of peptides derived from the large random peptide libraries could only be related to linear epitopes that were previously mapped using small libraries of overlapping peptides covering the antigenic protein. Thus, with respect to the cost and speed of identifying peptides that resemble linear and nonlinear parts of epitopes, small diversity libraries based on synthetic peptides appear to be superior to large diversity libraries based on expression systems.Abbreviations ABTS 2,2-azino-di-3-ethylbenzthiazoline sulfonate - EGF epidermal growth factor - Mab monoclonal antibody - OD_ccd optical density obtained using CCD camera - RAMPO rabbit-antimouse peroxidase - SDS sodium dodecylsulfate - TGEV transmissible gastroenteritis virus     

Neutron fluences and dose equivalents measured with passive detectors on LDEF

Neutron fluences were measured on LDEF in the low energy (< 1 MeV) and high energy (> 1 MeV) ranges. The low energy detectors used the ⁶Li(n,)T reaction with Gd foil absorbers to separate thermal (< 0.2 eV) and resonance (0.2 eV−1 MeV) neutron response. High energy detectors contained sets of fission foils (¹⁸¹Ta, ²⁰⁹Bi, ²³²Th, ²³⁸U) with different neutron energy thresholds. The measured neutron fluences together with predicted spectral shapes were used to estimate neutron dose equivalents. The detectors were located in the A0015 and P0006 experiments at the west and Earth sides of LDEF under shielding varying from 1 to 19 g/cm².

Dose equivalent rates varied from 0.8 to 3.3 μSv/d for the low energy neutrons and from 160 to 390 μSv/d for the high energy neutrons. This compares with TLD measured absorbed dose rates in the range of 1000–3000 μGy/d near these locations and demonstrates that high energy neutrons contribute a significant fraction of the total dose equivalent in LEO.

Comparisons between measurements and calculations were made for high energy neutrons based on fission fragment tracks generated by fission foils at different shielding depths. A simple 1-D slab geometry was used in the calculations. Agreement between measurements and calculations depended on both shielding depth and threshold energy of the fission foils. Differences increased as both shielding and thereshold energy increased. The modeled proton/neutron spectra appeared deficient at high energies. A 3-D model of the experiments is needed to help resolve the differences.     

Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO(2) removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO(2) is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ(13)C = -28.8‰.     

Isotope anomalies of carbon, hydrogen and nitrogen in peat from the area of the tunguska cosmic body explosion (1908)

Abstract Peat profiles from the area of the Tunguska explosion epicentre indicate significant carbon and hydrogen isotopic effects which are clearly associated with the zone of the 1908 "catastrophe", and which cannot be attributed to any known terrestrial processes. We explain them with the presence of extraterrestrial matter similar to carbonaceous chondrites or, more probably, to cometary matter. Initial data on nitrogen content and its isotope composition are consistent with the assumption of acid rainfall following the passage and explosion of the Tunguska cosmic body, as is known to have occurred during the Cretaceous-Tertiary boundary.     

Radiotracer measurement of the volatilization of organic trace constituents from water

Abstract The mass-transfer coefficient for the volatilization of organic microconstituents from water can be determined by laboratory radiotracer experiments. Formulation and practical aspects are considered and illustrated by the example of a 5.10(-7) M solution of monochlorobenzene.     

Unification of electromagnetism and gravitation: The equations of electrodynamics derived from the Riemann tensor in general relativity

The equations of free-space electrodynamics are derived directly from the Riemann curvature tensor and the Bianchi identity of general relativity by contracting on two indices to give a novel antisymmetric Ricci tensor. Within a factore/h, this is the field-strength tensor G of free-space electrodynamics. The Bianchi identity for G describes free-space electrodynamics in a manner analogous to, but more general than, Maxwell's equations for electrodynamics, the critical difference being the existence in general and special relativity of the Evans-Vigier fieldB ⁽³⁾.     

Laser action below threshold inversion due to coherent population trapping

The possibility of lasing without inversion on the 1079.8 nm line of a HeNe laser is investigated. It is shown that a modified double- scheme can be realized by use of the 877.9 nm line for introducing Zeeman coherence. It is shown experimentally that Zeeman coherence can enforce laser action, even if the inversion is below threshold. A different choice of the polarization of the driving beam can result in suppression of laser action above threshold. The underlying mechanisms are examined. On the 611.8 nm line, laser action below threshold inversion is obtained with the driving beam tuned to 824.9 nm (up-conversion).