The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH₃OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH₃O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.     

Effect of dose on the radiolysis of gaseous ethylene

Abstract

It is shown that a recently-introduced, computational technique makes possible, for the first time, the theoretical treatment of the simultaneous production of reacting species and their diffusion-limited reactions. The approach is illustrated with radiation-damage examples, but the technique should be applicable, as well. to problems in photo- and radiation-chemistry, in luminescence, and in precipitation-type reactions.     

Choice of conditions of an electrical discharge for generating chemically active particles for the decomposition of impurities in water

An electrical discharge between a liquid surface and an electrode positioned above it is considered. A second electrode, which delivers the output from a high-voltage source, is located at the bottom of the vessel containing the liquid. The conditions that must be met by the electrical discharge in order to efficiently initiate reactions in the liquid phase are analyzed. Under these conditions the number of active particles generated by the discharge turns out to depend on the concentration in the liquid of the substance with which the active particles interact. It is shown that for a corona or spark discharge the reactions can occur in a liquid layer 10–20 mm thick and that for specific reactions there exists an optimum value of the electric field at which the energy expenditures on the initiation of the reaction will be minimum. Zh. Tekh. Fiz. 69, 58–63 (January 1999)     

Generation of neutrons in giant upward atmospheric discharges

It has been shown that the efficiency of the reactions of the fusion of deuterons in the atmosphere is very low; therefore, nuclear fusion cannot be responsible for the generation of neutrons by lightning. The generation of neutrons in thunderstorm fields is attributed to photonuclear reactions in giant upward atmospheric discharges over thunderclouds.     

Wronskian analysis of resonance tunnelling reactions

A Wronskian formalism is developed for resonance tunnelling reactions. It is shown how the S matrix can be written in terms of Wronskians between solutions of the Schrödinger equation representing incoming and outgoing waves. The method is an adaptation of the Jost function approach to elastic two-body scattering. Formal expressions are derived for the imaginary parts of the energy eigenvalues that arise from the application of complex boundary conditions used in a previous semiclassical analysis. When the imaginary parts are small, so that the resonance is sharp, the S matrix can be written in a Breit-Wigner resonance form. The theory unifies and extends the semiclassical analysis of resonance tunnelling reactions given previously.     

Enzymic processes as a mechanism of biological magnetoreception

It is shown that enzymatic reactions with electron transfer can play the role of magnetosensitive stages of intracellular processes. The system of von Neumann-Liouville equations for the spin density matrices of the participants of ion-radical enzymatic processes in cells was solved using the steady-state approximation. Within the framework of the Δg mechanism of spin conversion of ion-radical pairs of the substrates of enzymatic reactions, the magnetic-field dependences of the rates of enzymatic processes are calculated. Magnetic field effects can be used as evidence for the ion-radical mechanism of enzymatic reactions and as a means of determining their kinetic characteristics.     

Reactions on 209Bi induced by intermediate energy protons and the effect of direct reactions

Excitation functions and ranges of recoil nuclei in the ²⁰⁹Bi(p, 3n)²⁰⁷Po, ²⁰⁹Bi(p, 4n) ²⁰⁶Po and ²⁰⁹Bi(p, p3n)²⁰⁶Bi reactions have been measured for incident energies from 18 MeV up to 52 MeV. It has been found that the recoil ranges in (p, p3n) reactions are rather shorter than those in (p, 4n), and that beyond E_p = 40 MeV the high energy tail of the excitation function for (p, p3n) is roughly flat, in contrast to the decreasing tail for (p, 4n). A theoretical analysis of the excitation functions and of the nuclear recoil ranges has been made. It has been found that in (p, p3n) reactions the direct process plays a very important part in the reaction mechanism. It is also found that the reaction mechanisms of (p, 3n) and (p, 4n) reactions should be interpreted by means of an admixture of the equilibrium compound process and the pre-equilibrium decay process at bombarding energies up to 40 MeV and 50 MeV, respectively, and that the direct process seems to appear at bombarding energies higher than these respective energies.     

Application of <Emphasis Type="Italic">T</Emphasis>-Invariance Conditions in Selecting Mechanisms of Nuclear Reactions,Decays, and Fission

T-invariance conditions for the differential cross sections of multiparticle multistep nuclear reactions are found with allowance for spin orientations of particles in the initial channels of such reactions. It is shown that the asymmetry coefficients for different T-parities in the differential cross sections for original and time-reversed reactions are expressed in terms of unified scalar (pseudoscalar) functions that depend of the 3-momenta and spins of particles involved in the initial and final channels of the reactions under analysis. It is also shown that knowledge of the aforementioned functions for the asymmetries under analysis in the original reaction makes it possible to reconstruct the respective functions for the analogous asymmetries in the time-reversed reaction without studying it experimentally. By considering the example of T-even and T-odd asymmetries in reactions where oriented nuclei undergo binary and ternary fission induced by cold polarized neutrons, it is demonstrated that the T-invariance conditions in question can be used to select mechanisms behind the appearance of the above asymmetries—in particular, mechanisms associated with the presence of T-noninvariant interactions.     

Synthetic popularity reflects chemical reactivity

Large and diverse databases of chemical reactions contain statistically significant information about the propensities of molecules to undergo specific chemical transformations. It is shown that this information can be quantified to reflect reaction thermodynamics/kinetics and can be used to construct primitive (yet accurate) reactivity indices from the counts of reported reactions involving molecules/molecular positions of interest. These indices correlate with frontier orbital (FO) populations or Hammett σ and ρ parameters for a range of reactions involving aromatic substrates. These findings suggest that large chemical databases are not only a historical repository of chemical knowledge but also tools with which one can make useful chemical predictions. Copyright © 2009 John Wiley & Sons, Ltd.     

Transfer and fusion reactions of unstable nuclei

The transfer and fusion reactions are studied for reactions between a stable and an unstable nucleus with neutron skin, taking as examples the reactions ^16,28O+⁴⁰Ca. The two-dimensional time-dependent Hartree-Fock method is used. It is shown that, in such reactions, the nucleon transfer is enhanced enormously for both protons and neutrons. The neutron skin does not enhance the fusion cross section contrary to the usual expectation.     

On the light amplification in the reactions of atoms with condensing gases

Results of the experimental investigations of the reactions of oxygen or hydrogen atoms with gaseous NO, which is rapidly cooling in the supersonic jet, are discussed. It is shown that the intense chemiluminescence arises in the reaction of atoms with cluster droplets (NO)_n. The possibility is shown of light amplification in photorecombination reactions of atoms with condensing gases. Such heterophase reactions have a variety of advantages in laser applications compared with reactions in the homogeneous gaseous phase.     

Role of (<Emphasis Type="Italic">n,xn</Emphasis>) reactions in ADS,IAEA-benchmark and the Dubna Cascade Code

Dubna Cascade Code (version-2004) has been used for the Monte Carlo simulation of the 1500 MW_t accelerator driven sub-critical system (ADS) with ²³³U + ²³²Th fuel using the IAEA benchmark. Neutron spectrum, cross-section of (n, xn) reactions, isotopic yield, heat spectra etc. are simulated. Many of these results that help in understanding the IAEA benchmark are presented. It is revealed that the code predicts the proton beam current required for the 1500 MW_t ADS for K _eff = 0.98 to be 11.6 mA. Radial distribution of heat is fairly in agreement with other codes like the EA-MC and it needs nearly 1% less enrichment than given by other codes. This may be because the code takes care of the role of larger order of the (n, xn) reactions. It is emphasized that there is a strong need to study (n, xn) reactions both theoretically and experimentally for better design. This talk is dedicated to the fond memory of late Professor V S Barashenkov, JINR, Dubna     

Distribution function of fusion reaction products and entropy evolution

One of the outcomes of nuclear reactions is that reaction products have at birth distribution functions far from Maxwellian. What role do those distribution functions play in the evolution of the entropy of the system? The purpose of this work is to show the effect of the distribution functions of reactant and reaction products on the entropy of a system undergoing DD nuclear fusion reactions. This analysis is conducted with the help of the H-theorem, in the framework of kinetic theory. It will be found that at the onset of this reaction, generalized system entropy decreases markedly.     

Asymmetries of various <Emphasis Type="Italic">P</Emphasis>- and <Emphasis Type="Italic">T</Emphasis>-parities in angular distributions of products of cold-polarized-neutron-induced binary and ternary fission of oriented nuclei and <Emphasis Type="Italic">T</Emphasis>-invariance

It is shown that a quantum system whose Hamiltonian is independent of time is T -invariant if this Hamiltonian contains only those terms that do not change sign upon time reversal. It is also shown that the coincidence of the amplitudes for multistep direct and statistical nuclear reactions with the timereversed amplitudes for the reactions being studied is a condition that ensures the T -invariance of the amplitudes in question, the transition from the original amplitudes to their time-reversed counterparts being accomplished, first, upon introducing the inverse-reactionmatrices T instead of the original-reaction matrix T and, second, upon replacing the wave functions for the initial, final, and intermediate states of the system by the respective time-reversed functions. It is found that the T -even (T -odd) asymmetries in cross sections for nuclear reactions stem from the interference between the amplitudes characterizing these reactions and having identical (opposite) T -parities. It is shown that the T -invariance condition for the above T -even (T -odd) asymmetries is related to the conservation of (change in) the sign of these asymmetries upon going over from original to inverse nuclear reactions. Mechanisms underlying the appearance of possible T -even and T-odd asymmetries in the cross sections for the cold-polarizedneutron- induced binary and ternary fission of oriented target nuclei are analyzed for the case of employing T -invariant Hamiltonians for the systems under study. It is also shown that the asymmetries in question satisfy the T -invariance condition if the reactions being considered have a sequential multistep statistical character. It is concluded that T -invariance is violated in the limiting case where, in ternary nuclear fission, the emission of a light third particle froma fissile compound nucleus formed upon incident-neutron capture by a target nucleus and its separation to two fission fragments are simultaneous events.     

Cross-section energy dependence of planar and non-planar two-body hypercharge-exchange reactions

The energy dependence of total cross sections for twenty-three two-body hypercharge-exchange reactions is studied. It is found that the planar reactions have in general a less steep fall-off with increasing energy than the non-planar reactions. Explanations of the difference in terms of kinematics and dynamics (baryon exchange and t-channel exchange effects) are discussed and found highly improbable. Implications of the effects observed are discussed.     

Corrections to the generalized vector dominance due to diffractive ρ<Subscript>3</Subscript> production

The idea of the vector dominance is still in use in various analyses of the experimental data of photon–hadron reactions. It makes sense, therefore, to recast results of microscopic calculations of such reactions in this language. Here we present the diffractive DIS ρ₃ production as a specific correction to generalized vector dominance. We perform a coupled channel analysis of spin–orbital excitations in diffractive photoproduction and reiterate the point that ρ₃ in diffractive DIS will be sensitive to a novel aspect of diffraction.     

Quantum nature of ROT and TRI asymmetries in the ternary fission of nuclei

Effects of T-odd asymmetry in ternary-nuclear-fission reactions induced by polarized cold neutrons are considered within quantum theory. It is shown that the asymmetry coefficient can be expressed in terms of experimental angular distributions of third particles in reactions induced by unpolarized neutrons. The explicit form of this coefficient makes it possible to explain the difference in the magnitudes and signs of the TRI and ROT effects observed experimentally for different targets.     

Study of pairing deformations by means of two-particle transfer reactions

The macroscopic approach for two-particle transfer reactions is applied in normal spherical nuclei and pairing deformation parameters β_p are estimated. For Ni isotopes the macroscopic approach works reasonably well while for the lead region strong energy dependence of theβ _p values has been observed. It is shown that alpha-transfer reactions can also be treated within this formalism as couplings of the two-neutron and two-proton channels.     

Systematics of fragment observables

Multifragmentation is observed in many reaction types: light-ion-induced reactions at large incident energies (in the GeV region), central heavy-ion collisions from 30 to 100MeV/u, and peripheral heavy-ion collisions between 30 and 1000MeV/u or above. When nucleus-nucleus collisions are considered, another entrance channel parameter is the corresponding mass asymmetry. The first question which is addressed in this contribution is: do we observe similar reactions in each case? Multifragmentation may be related to a phase transition of nuclear matter. Some others features indicate that dynamical features are dominant. It is a priori possible that the underlying mechanisms are different in proton- and nucleus-induced reactions, in central and in peripheral collisions, at limited and at large bombarding energies. In order to see to what extent they can reflect similar behaviour, it is useful to compare the results of various reactions. The observables can be the fragment multiplicity, the mass distributions or the kinematical properties. In this contribution, we are looking for such general features. We will limit the discussion to the observations themselves, rather than the interpretation, which is the subject of numerous entries in this volume. The experimental results indicate that multifragmentation exhibits at the same time universal and entrance-channel-dependent properties.     

Mass-split dependence of the pre- and post-scission neutron multiplicities for fission of 251Es

Pre- and post-scission neutron multiplicities have been measured as a function of fission mass-split for the reactions of 105 and 120 MeV ¹⁹F with ²³²Th. The post-scission multiplicities show no evidence for persistence of the sawtooth yield observed for spontaneous fission of ²⁵²Cf; the dependence on mass-split is not consistent with the random neck rupture model. The pre-scission multiplicities show no evidence of quasi-fission for asymmetric mass-splits. It is proposed that pre-scission neutron multiplicity measurements should be a useful tool in studying quasi-fission produced by reactions with heavier projectiles.