Verwendung schneller Reaktorneutronen zur raschen und zerst?rungsfreien Spurenanalyse, insbesondere von Sauerstoff

Zusammenfassung Der intensive Fluß schneller Neutronen, wie er von Kernreaktoren geliefert wird, kann für eine rasche und zerstörungsfreie, aktivierungs-analytische Bestimmung einer Vielzahl von Elementen verwendet werden. Bei manchen Elementen läßt sich sogar bei der Aktivierung mit schnellen Reaktorneutronen eine im Vergleich zur Aktivierungsanalyse mit thermischen Neutronen tiefere Erfassungsgrenze erreichen. Werden die Proben in einem schnellen, halbautomatischen Rohrpostsystem zum Reaktorkern befördert, können auch sehr kurzlebige Radionuklide gemessen werden. Besonders Sauerstoff läßt sich über die Kernreaktion ¹⁶O(n, p)¹⁶N sehr empfindlich bestimmen, wobei die hochenergetischen -Quanten des 7,2 sec Stickstoff-16 gemessen werden. Es können so bis zu 10 ppm Sauerstoff in vielen Arten von Analysenproben erfaßt werden. Das halbautomatische Analysensystem, das mit Hilfe einer schnellen Rohrpost die Durchführung rascher und zerstörungsfreier Aktivierungs-analysen im Forschungsreaktor München erlaubt, wird beschrieben. Die Verwendung des Systems zur Sauerstoffanalyse und die damit zusammenhängenden Probleme werden genauer besprochen.

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Summary The intense fast-neutron fluxes, which are available in nuclear reactors, can be used for rapid and non-destructive activation analysis of many elements. Some elements have even superior detection limits for activation with fast reactor neutrons compared to reactor thermal-neutron activation. Short-lived radionuclides can also be measured using a fast pneumatic tube system. Oxygen can be determined by the nuclear reaction ¹⁶O(n, p)¹⁶N initiated by fast reactor neutrons counting the high-energy gamma rays of the 7.4 sec nitrogen-16. The method is suitable for determining as little as 10 ppm of oxygen in many types of analytical samples. The fast transfer system, which is used in the Munich research reactor for rapid and non-destructive activation analysis is described. The use of the transfer-system for oxygen activation analysis and problems associated with oxygen analysis are discussed.

     

Determination of the alkaline earths in glass

The alkaline-earth group was isolated, after removal of silica, by precipitation as sulfates from an alcoholic solution. Those sulfates were reduced and dissolved by treatment with hydriodic acid. The alkaline earths were separated from each other by ion-exchange chromatography. The results are in good agreement with the values reported by the National Bureau of Standards.     

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO₂ spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, ²A₁ and à ²B₂, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO₂ absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO₂ spectrum down to 11200 cm⁻¹ with a new Ti:sapphire ICLAS spectrometer. As a result 249 ²B₂ vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm⁻¹ energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the ²A₁ and à ²B₂ electronic states, we observed not only the a₁ vibrational levels of the à ²B₂ state but also the b₂ vibrational levels of the ²A₁ state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of ²B₂ vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm⁻¹, corresponds to the bending motion of the à ²B₂ state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10⁻¹² s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.     

Palladium-induced cyclizations for the synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans: studies of the C7-C22 core of amphidinolide K

[formula: see text] The diastereoselective synthesis of cis-2,5-disubstituted-3-methylenetetrahydrofurans via Pd(0)-catalyzed cyclization of 2-methylene-1,4-diols is described. Investigations into the scope of the reaction and its application toward the synthesis of amphidinolide K is reported.     

Determination of in-core power in low energy research reactors by measurement of16N and18F in the primary coolant

The measurement of¹⁶N and¹⁸F activity in the primary coolant of the JASON Argonaut reactor has been used to monitor in-core reactor power. The¹⁶N is produced by the¹⁶O(n, p)¹⁶N reaction and the 6.1 MeV photopeak was measured on-line using a BGO detector adjacent to the primary coolant circuit. These data provided a relative measure of power stability during steady state operation and a measure of linearity at different power levels. The¹⁸F is produced in the primary coolant by the¹⁸O(p, n)¹⁸F reaction and aliquots of primary coolant were sampled from the reactor dump tank for off-line radiochemical analysis. The¹⁸F was separated as trimethylfluorosilane and the activity was determined by measurement of the 0.511 MeV annihilation photopeak using a NaI(TI) detector. The measured¹⁸F activity was used to determine actual in-core reactor power using both ab-initio calculations and by comparison of results with a calibrated power reactor. The¹⁸F data also provided a method of nomalising the¹⁶N data for direct monitoring of in-core reactor power in JASON.     

Differential scanning calorimetry of semiconducting As-Se glasses containing Sb

The compositional dependence of the glass transition temperature (T _g), and the temperature of the maximum rate of crystallization (T _p) of As-Se glasses containing Sb has been measured using non-isothermal DSC. The variation of these characteristic temperatures is found useful in interpreting the structural changes in As-Sb-Se glasses. The role of Sb in these glasses and its effect on the crystallization characteristics is analyzed.     

THE EFFECTS OF PORPHYRIN STRUCTURE AND AGGREGATION STATE ON PHOTOSENSITIZED PROCESSES IN AQUEOUS AND MICELLAR MEDIA

The efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen-mechanism was studied in pH 7.4-buffered aqueous solutions and in aqueous dispersions of Triton X-100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso-tetra (4-sulfonatophenyl)porphine, meso-tetra(4-carboxyphenyl)porphine and meso-tetra)N,N,N-trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O₂(¹Δ_g) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen-consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle-incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O₂(¹Δ_g) generation. On the other hand, hematoporphyrin IX, its Zn²⁺-complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen-consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3-100μM range.     

Low temperature specific heat calculations for amorphous polystyrene

Numerous attempts have previously been made to explain low temperature specific heat anomalies in glasses. The potential value of low frequency Raman data in interpreting such results is demonstrated here: the density of states for the 60 cm^?1 side-group mode in amorphous polystyrene is calculated from Raman measurements. Using the harmonic approximation, the contribution to the specific heat from this side-group motion is calculated and added to the Debye term. The form of C/T³ between 1.5 and 4 K is shown to be reproduced, although the magnitude of C/T³ is only about 70% of experimental values. This discrepancy and the anomalous behaviour below 1.5 K is believed to the due to lower-lying vibrational states of the system, perhaps not accessible through Raman scattering experiments.     

Flow injection analysis : Part VI. The determination of phosphate and chloride in blood serum by dialysis and sample dilution

The rapid determination of chloride and inorganic phosphate in blood serum based on continuous flow spectrophotometry was adapted to Flow Injection Analysis by applying dialysis as well as differential dilution of the injected samples. In addition to the manual injection procedure, an automated microsampling unit was developed and used. By continuous dialysis, chloride could be determined reproducibly at a sampling rate of 125 samples per hour at sample volumes of 60 μl or less. A similar sampling rate and high precision were obtained for the determination of inorganic phosphate by using differential dilution.