Comparison of the yields of high-energy gamma-rays withE γ≧10 MeV measured in the reactionsnatSn+40Ar and68Zn+64Zn

The spectra of theKX-rays of reaction products were observed in coincidence with high energy gamma-rays (E _γ≧10 MeV) in the^natSn+⁴⁰Ar (E _Ar=300 MeV) and⁶⁸Zn +⁶⁴Zn (E _Zn=290 MeV) reactions. Two groups ofKX-rays can be clearly identified in the spectra. The first one corresponds to the evaporation residue of the compound nucleus and the second one to the target-like products of the reaction. The spectra of gamma transitions gated by theKX-rays of the evaporation residues were measured in both reactions. The relative yields and the positions of the broad shoulders observed at an energyE _γ≧10 MeV in these spectra, differ drastically in the asymmetric and the symmetric systems. The ratio between the yields of gamma-rays emitted at angles of 0° and 90° to the beam direction was determined in the^natSn+⁴⁰Ar reaction. It was found to be greater than unity only atE _γ=(10–14) MeV. This experimental evidence may indicate the emission of high energy gamma-rays from the dinuclear system prior to the formation of the compound nucleus.     

Study of fusion and nucleon transfer channels in the <Superscript>197</Superscript>Au + <Superscript>6</Superscript>He reaction in an energy range of <Superscript>6</Superscript>He to 20 Mev/A

New data on the cross sections of reactions occurring during the interaction of ⁶He nuclei with ¹⁹⁷Au at energies of ⁶He from 40 to 120 MeV are presented. The experiments were performed in the ACCULINNA secondary beam separator of the Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. To identify the reaction products, the activation method for measuring the gamma activity of the target assembly of thin foils was used. The excitation functions for fusion reactions involving the evaporation of up to ten neutrons from the compound nucleus as well as reactions with the emission of charged particles and nucleon transfer in the investigated energy region are obtained. Data analysis was carried out using two codes: ALICE-MP and NRV. The cross sections for the (⁶He, xn) reactions occurring through the compound nucleus are mostly in agreement with the results of model calculations based on the statistical approach. It is shown that, up to the energies of 114 MeV, the cross-section drop in the complete fusion reactions is negligible. The experimental excitation functions of reactions leading to the formation of isotopes of mercury and gold (transfer reaction) indicate that the main contribution to their formation is made by direct processes and that evaporation reactions (⁶He, pxn) and (⁶He, αxn) play a minor role, as is evidenced by a comparison of the measured cross sections with the calculation results.     

Investigating the nuclear fusion and nucleon transfer channels in the 197Au + 6He reaction at 6He energies up to 20 MeV/A

New data on the cross sections of reactions that proceed during the interaction of ⁶He and ¹⁹⁷Au nuclei in the ⁶He energy range of 40 to 120 MeV are reported. The experiments were performed using the secondary beam of the ACCULINNA separator at the Flerov Laboratory of Nuclear Reactions (FLNR), JINR. Reaction products are identified by means of activation method to measure the gamma activity of a thin-foil target assembly. Excitation functions for ⁶He fusion reactions with subsequent emission of up to 10 neutrons from the compound nucleus are measured. Cross sections for reactions with emission of charged particles and nucleon transfer were also measured. The experimental cross sections for the (⁶He, xn) reactions that proceed via the formation of a compound nucleus agree in general with calculations using models that involve the statistical approach. It is shown that the complete fusion reaction cross section drops slightly up to an energy of 114 MeV. The experimental excitation functions for the reactions resulting in the formation of mercury and gold isotopes indicate that the main contribution to their formation comes from direct processes, while the evaporation reactions (⁶He, pxn) and (⁶He, αxn) are of minor importance.     

Temperature-programmed oxidation of equilibrium fluid catalytic cracking catalysts

Characterization of coke on equilibrium, fluid catalytic cracking (FCC) catalysts contaminated with metals was investigated using temperature-programmed oxidation (TPO). TPO spectra of spent equilibrium catalysts from cracking of sour imported heavy gas oil (SIHGO) were deconvoluted into four peaks (Peak K, L, M and N). The four peaks were assigned to different types of coke on the catalyst. Peak L in the TPO spectrum was assigned to the 'contaminant' coke in the vicinity of metals. The amount of contaminant coke (Peak L) correlates with metal-contaminant concentration. The size of Peak L which is related to amount of contaminant coke decreased significantly for the spent highly contaminated catalyst pretreated with hydrogen and methane prior to cracking reactions as compared to the non-pretreated catalysts. Since both hydrogen and methane pretreatment can reduce oxidation state of the vanadium that present at high concentrations on the equilibrium catalysts the decrease in the amount of contaminant-coke represented by Peak L was explained by the reduction of the oxidation state of vanadium. Less contaminant coke was produced after the equilibrium catalysts were pretreated using hydrogen and methane gases since reduced vanadium has lower dehydrogenation activity compared to oxidized vanadium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Response of the cation-sensitive glass electrodes to alkyl-substituted ammonium ions

Summary The response of cation-sensitive glass electrodes to alkyl-substituted ammonium ions has been evaluated. Increasing ion size results in decreasing electrode selectivity, progressively greater deviations from the Nernst slope, and increasing equilibration times. Direct potentiometric measurements should be suited to the estimation of smaller alkyl-substituted ammonium ions in the presence of multivalent cations.

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Zusammenfassung In der vorliegenden Untersuchung wurde das Ansprechen von kation-empfindlichen Glaselektroden auf alkylsubstituierte Ammoniumionen geprüft. Mit steigender Größe des Ions konnte eine Abnahme der Selektivität der Elektrode, zunehmende Abweichungen von der Nernstschen Gleichung und eine wachsende Zeitdauer für die Einstellung des Gleichgewichts beobachtet werden. Es ergab sich, daß die kation-empfindliche Glaselektrode zur Bestimmung kleinerer alkylsubstituierter Ammoniumionen in Gegenwart mehrwertiger Kationen verwendet werden kann.

     

A direct approach to study radiative emission from triplet excitations in molecular semiconductors and conjugated polymers

Using the recently discovered time-dependent spin-orbit-photon interaction operator and first order perturbation theory, the rate of spontaneous emission from triplet excitations is derived within the two-level approximation for organic molecular solids and conjugated polymers. The calculated rates and corresponding radiative lifetimes agree very well with the known experimental results. Present results are compared with those obtained through the traditional approach of the second order perturbation theory in some molecular crystals and found to be in better agreement with experiments.     

Some numerical studies of the evolution of generalized parton distributions

We study the evolution behavior of generalized parton distributions at small longitudinal momentum fraction. Particular attention is paid to the ratio of a generalized parton distribution and its forward limit, to the mixing between quarks and gluons, and to the dependence on the squared momentum transfer t.     

The decay of the neutron–rich nucleus 216Bi

The decay of the neutron–rich isotope ²¹⁶Bi, produced by proton–induced spallation at the PS Booster–ISOLDE facility, was investigated by β-γγ, αγ coincidence and spectrum-multiscaling measurements. A new method for reducing isobaric contamination enabled to cover the unknown region “east” of ²⁰⁸Pb for the isobaric chain A=216. The half-life of the β decay of ²¹⁶Bi was found as T_1/2= 135 ± 5 s. Its decay scheme was extended and the possible shell model configurations are proposed. Received: 13 July 1999 / Revised version: 22 September 1999