Reaction cross-section capture model

We are limited in this modeling process by the accuracy with which measurements can be made and by the accuracy of the fission yields and neutron reaction cross sections which are used to interpret the results. As an example consider the Nd- Nd fission product pair, which has been used as an indicator of thermal neutron fluence because the capture cross section for the former is large and for the latter is small. The thermal cross section for l53Nd has recently been listed as 325 ( 10) barns (20), and more recently as 266 barns (11). Using the 325-barn value we deduce an age of about 2 to 27T billion years from neodymium to uranium ratios in the Oklo reactors, while an age of about 1.8 billion years is obtained using the 266-barn figure. 

The study of artificial photosynthesis has been the subject of ongoing attention for many years now due to the need for sustainable energy resources. In natural photosynthesis a lightharvesting antenna system with a large optical cross-section (for example the LH2 complex) absorbs a photon that is funneled by energy transfer (ET) to the reaction centre [1-3]. Excellent candidates to mimic the natural antenna system are molecules that efficiently absorb light and are able to transfer the captured energy to other parts of the molecule. Molecules based on Zn and free-base porphyrins are examples of compounds that can be used as models for the LID complex [4]. 

The shape resonances have been described by Feshbach in elastic scattering cross-section for the processes of neutron capture and nuclear fission [7] in the cloudy crystal ball model of nuclear reactions. These scattering theory is dealing with configuration interaction in multi-channel processes involving states with different spatial locations. Therefore these resonances can be called also Feshbach shape resonances. These resonances are a clear well established manifestation of the non locality of quantum mechanics and appear in many fields of physics and chemistry [8,192] such as the molecular association and dissociation processes. 

B4++ He reaction. This collisional system has been investigated theoretically within the framework of the semiclassical close-coupling formalism using different model potential approaches [2,3] which lead to a discrepancy of about a factor 5 for the double capture cross section values. We have thus performed an alternative study of this system by means of a full molecular expansion method, focusing our attention on the double electron capture process. 

Figures 5A and 5B present the total reactive cross section for reaction N( D) + 0 2(X E ) —> O( P) -I- NO(X n) as a function of the initial relative translational energy [115] on its two lowest adiabatic surfaces (2 A and 1 A"). The Oo reactant is in its ground vib-rotational state in both cases. The cross sections have been calculated using the real wavepaeket [98] and the capture model approaches [112]. Figure 5A shows the total reactive cross section on the 2 A surface.

We shall develop next a single-channel model that captures the key features of a catalytic combustor. The catalytic materials are deposited on the walls of a monolithic structure comprising a bundle of identical parallel tubes. The combustor includes a fuel distributor providing a uniform fuel/air composition and temperature over the cross section of the combustor. Natural gas, typically >98% methane, is the fuel of choice for gas turbines. Therefore, we will neglect reactions of minor components and treat the system as a methane combustion reactor. The fuel/air mixture is lean, typically 1/25 molar, which corresponds to an adiabatic temperature rise of about 950°C and to a maximum outlet temperature of 1300°C for typical compressor discharge temperatures ( 350°C). Oxygen is present in large stoichiometric excess and thus only methane mass balances are needed to solve this problem. 

On the more nuclear physics side, and apart from the already mentioned 22Ne (a, n) 25Mg reaction, it is of substantial importance to predict reliably the rates of thousands of nucleon or a-particle radiative captures and of the inverse transformations. For a long time to come, almost all these data will have to be provided by theory, but experiments have to help constraining and improving the models as much as possible. Efforts have to be started in order to measure directly the cross sections of photoreactions near threshold. In addition, more experiments should have to be conducted at sub-Coulomb energies on radiative proton and a-particle captures. 

ABSTRACT. Calculation of the rate constant at several temperatures for the reaction +(2p) HCl X are presented. A quantum mechanical dynamical treatment of ion-dipole reactions which combines a rotationally adiabatic capture and centrifugal sudden approximation is used to obtain rotational state-selective cross sections and rate constants. Ah initio SCF (TZ2P) methods are employed to obtain the long- and short-range electronic potential energy surfaces. This study indicates the necessity to incorporate the multi-surface nature of open-shell systems. The spin-orbit interactions are treated within a semiquantitative model. Results fare better than previous calculations which used only classical electrostatic forces, and are in good agreement with CRESU and SIFT measurements at 27, 68, and 300 K. ... 

In hot-fusion reactions, the cross section for producing heavy-element nuclides is determined by the probability that the highly excited compound nucleus will avoid fission in the deexcitation process. Cold fusion near the reaction barrier is qualitatively different the formation of the compound nucleus comes about in two separate steps [105, 107]. The reacting nuclei come into contact, captured into a dinuclear configuration, which is separated from an equilibrated compound nucleus by a potential-energy barrier which is not reproduced by the one-dimensional Coulomb-barrier model [94, 95, 210, 219, 220]. This extra barrier diverts the trajectory of the reaction through multidimensional deformation space toward quasifission, making reseparation much more likely than complete fusion. 

For example, modeling a steel column with beam elements is not appropriate for predicting failure where local or lateral buckling may be predominant Shell or plate elements should be used instead, and the mesh should be line enough to capture the curvature part of the cross section where local or lateral buckling occurs, (iii) Basic features of the model, such as element connectivity, boundary conditions, and loads, should be checked by means of simple, linear, static, and modal analyses. Sum of support reactions can be used to check load application. Mode... 

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