Pre-equilibrium Reaction Mechanism Research Articles

Collective enhancement in the exciton model

The pre-equilibrium reaction mechanism is considered in the context of the exciton model. A modification to the one-particle one-hole state density is studied which can be interpreted as a collective enhancement. The magnitude of the collective enhancement is set by simulating the Lawrence Livermore National Laboratory (LLNL) pulsed-spheres neutron-leakage spectra. The impact of the collective enhancement is explored in the context of the highly deformed actinide, 239-Pu. A consequence of this enhancement is the removal of fictitious levels in the Distorted-Wave Born Approximation often used in modern nuclear reaction codes.

Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis.

Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO2 reduction without loss of selectivity (i.e., H2 evolution). Fast pre-equilibrium metal-hydride formation, at 108 M-1 s-1, boosts the CO2 electroreduction to formate rate up to 296 s-1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis.

Preequilibrium strength in light heavy-ion induced reactions up to 7 MeV/nucleon

The crux of this article is to analyze the contribution of the preequilibrium reaction process to the production of residues in the low energy heavy-ion reactions. The production cross-sections of a few radionuclides produced through the 3n-emission channel have been measured from the 7Li-, 11B- and 12C-induced reactions in the 89Y and 93Nb targets within the energy range of ∼2.4–6.4 MeV/nucleon. The measured excitation functions have been analyzed by comparing them with the statistical nuclear reaction models, based on the equilibrium and preequilibrium reaction mechanisms, in the framework of ALICE91 and EMPIRE3.2.2. The result shows that the mixing of equilibrium and preequilibrium emission of light or cluster particles is essential for the better reproduction of the measured cross-section data. Variation of the preequilibrium fractions with derived energy parameters has also been demonstrated.

Li7 -induced reaction on Monat : A study of complete versus incomplete fusion

Several investigations on the complete-incomplete fusion (CF-ICF) dynamics of $\alpha$-cluster well-bound nuclei has been contemplated above the Coulomb barrier ($\sim$ 4-7 MeV/nucleon) in recent years. It is therefore expected to observe significant ICF over CF in the reactions induced by a weakly bound $\alpha$-cluster nucleus at slightly above the barrier. In order to investigate CF-ICF in the loosely bound system, $^{\text{7}}$Li beam was bombarded on the $^{\text{nat}}$Mo foils, separated by the aluminium (Al) catcher foils alternatively, within $\sim$ 3-6.5 MeV/nucleon. Evaporation residues produced in each foil were identified by the $\gamma$-ray spectrometry. Measured cross section data of the residues were compared with the theoretical model calculations based on the equilibrium (EQ) and preequilibrium (PEQ) reaction mechanisms. The experimental cross section of $^{\text{101m,100,99m,97}}$Rh, $^{\text{97,95}}$Ru, $^{\text{99m,96,95,94,93m+g}}$Tc and $^{\text{93m}}$Mo residues measured at various projectile energies were satisfactorily reproduced by the simplified coupled channel approach in comparison to single barrier penetration model calculation. Significant cross section enhancement in the $\alpha$-emitting channels was observed compared to EQ and PEQ model calculations throughout observed energy region. The ICF process over CF was analyzed by comparing with EMPIRE. The increment of the incomplete fusion fraction was observed with increasing projectile energies. Theoretical model calculations reveal that compound reaction mechanism is the major contributor to the production of residues. Theoretical evaluations substantiate the contribution of ICF over the CF in $\alpha$-emitting channels.

Investigation of the production of 68Ga using pre-equilibrium models

In this study, some nuclear reactions for the production of 68Ga radioisotope are investigated using pre-equilibrium nuclear reaction models. For this aim, by the pre-equilibrium reaction mechanisms, the excitation functions and emission spectra of some nuclear reactions for the production of 68Ga radioisotope are calculated. These calculations are performed in the ALICE/ASH and the TALYS 1.6 codes. The obtained results have been discussed and compared with the available experimental results.

Nuclear interaction dynamics in the 56Fe (α, xn) reaction

Neutron spectra and angular distributions in the 56Fe (α, xn) reaction are measured at α-particle energies of 12.5, 16.3, 18.3, 26.8, and 45.2 MeV. The measurements are performed using time-of-flight fast neutron spectrometers on pulsed accelerators. The measured data are analyzed in the context of equilibrium, preequilibrium, and direct nuclear reaction mechanisms. The contributions from equilibrium, preequilibrium, and direct mechanisms of neutron emission are studied over a wide range of α-particle energies.

Cross-section calculations of proton induced (p,n) and (p,2n) reactions for production of diagnostic 67Ga, 81Rb, 111In, 123,124I, 123Cs and 123Xe radioisotopes

Investigation of pre-equilibrium (PEQ) and equilibrium (EQ) effects on proton induced reactions for production of radioisotopes are very important. Therefore, in this study, we have calculated the PEQ and EQ cross-sections for 67Zn(p,n)67Ga, 68Zn(p,2n)67Ga, 82Kr(p,2n)81Rb, 111Cd(p,n)111In, 112Cd(p,2n)111In, 123Te(p,n)123I, 124Te(p,2n)123I, 124Te(p,n)124I and 124Xe(p,2n)123Cs reactions for production diagnostic radioisotopes. Calculations have been performed by using the hybrid model, geometry dependent hybrid model and full exciton model of PEQ reaction mechanism with 1–40 MeV proton incident energy. We have also investigated the EQ effects on these reactions using the Weisskopf–Ewing model in the same energy range. The excitation functions including the PEQ and EQ effects on these reactions are evaluated by using the ALICE/ASH (2006) and the TALYS 1.4 (2011) codes. Our results have shown that using these codes is suitable for production diagnostic isotopes mentioned above. To obtain excitation functions for producing the diagnostic radioisotopes the PEQ mechanism has been found more dominant than that of the EQ. The results are discussed and compared with the available experimental data.

Pre-equilibrium and equilibrium calculations of (n, p) reactions on 32S, 64Zn, 67Zn, 89Y, 90Zr and 153Eu targets used for production of 32P, 64Cu, 67Cu, 89Sr, 90Y and 153Sm therapeutic radionuclides

In this study, by using the pre-equilibrium and equilibrium reaction mechanisms, the cross-sections of (n, p) reactions on 32S, 64Zn, 67Zn, 89Y, 90Zr and 153Eu targets have been calculated. The excitation functions of (n, p) reactions on 32S, 64Zn, 67Zn, 89Y, 90Zr and 153Eu targets by using the hybrid model, the geometry dependent hybrid model and the exciton model of pre-equilibrium reaction mechanism at the energies in the range 1–20MeV have been investigated. To investigate the equilibrium effects, the Weisskopf–Ewing model is used. The proton emission spectra of (n, xp) reactions by 14.6, 14.8 and 14.8MeV neutron incident energies, respectively, on 32S, 89Y and 90Zr targets are also investigated using the same models. The obtained results have been discussed and compared with the experimental results.

Alpha Induced Reaction Cross Section Calculations of Tantalum Nucleus

The fusion energy is attractive as an energy source because the fusion will not produce CO2 or SO2 and so fusion will not contribute to environmental problems, such as particulate pollution and excessive CO2 in the atmosphere. The fusion reaction does not produce radioactive nuclides and it is not self-sustaining, as is a fission reaction when a critical mass of fissionable material is assembled. Since the fusion reaction is easily and quickly quenched the primary sources of heat to drive such an accident are heat from radioactive decay and heat from chemical reactions. Both the magnitude and time dependence of the generation of heat from radioactive decay can be controlled by proper selection and design of materials. Tantalum is one of the candidate materials for the first wall of fusion reactors and for component parts of irradiation chambers. Accurate experimental cross-section data of alpha induced reactions on Tantalum are also of great importance for thermonuclear reaction rate determinations since the models used in the study of stellar nucleosynthesis are strongly dependent on these rates (Santos et al. in J Phys G 26:301, 2000). In this study, neutron-production cross sections for target nuclei 181Ta have been investigated up to 100 MeV alpha energy. The excitation functions for (α, xn) reactions (x = 1, 2, 3) have been calculated by pre-equilibrium reaction mechanism. And also neutron emission spectra for 181Ta (α, xn) reactions at 26.8 and 45.2 MeV have been calculated. The mean free path multiplier parameters has been investigated. The pre-equilibrium results have been calculated by using the hybrid model, the geometry dependent hybrid (GDH) model. Calculation results have been also compared with the available measurements in literature.

Cross Sections Calculations of (d, t) Nuclear Reactions up to 50 MeV

In nuclear fusion reactions two light atomic nuclei fuse together to form a heavier nucleus. Fusion power is the power generated by nuclear fusion processes. In contrast with fission power, the fusion reaction processes does not produce radioactive nuclides. The fusion will not produce CO2 or SO2. So the fusion energy will not contribute to environmental problems such as particulate pollution and excessive CO2 in the atmosphere. Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However, the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production is still believed to be unlikely before 2050. Although there have been significant research and development studies on the inertial and magnetic fusion reactor technology, there is still a long way to go to penetrate commercial fusion reactors to the energy market. In the fusion reactor, tritium self-sufficiency must be maintained for a commercial power plant. Therefore, for self-sustaining (D–T) fusion driver tritium breeding ratio should be greater than 1.05. Working out the systematics of (d, t) nuclear reaction cross sections is of great importance for the definition of the excitation function character for the given reaction taking place on various nuclei at different energies. Since the experimental data of charged particle induced reactions are scarce, self-consistent calculation and analyses using nuclear theoretical models are very important. In this study, (d, t) cross sections for target nuclei 19F, 50Cr, 54Fe, 58Ni, 75As, 89Y, 90Zr, 107Ag, 127I, 197Au and 238U have been investigated up to 50 MeV deuteron energy. The excitation functions for (d, t) reactions have been calculated by pre-equilibrium reaction mechanism. Calculation results have been also compared with the available measurements in literature.

Excitation functions and isotopic effects in (n,p) reactions for stable iron isotopes from reaction threshold to 20 MeV

The excitation functions for (n,p) reactions from reaction threshold to 20MeV on four stable Iron isotopes viz; 54Fe, 56Fe, 57Fe and 58Fe were calculated using Talys-1.2 nuclear model code. This essentially involves fitting a set of global parameters. An excellent agreement between the calculated and experimental data is obtained with minimal effort on parameter fitting and only one free parameter called ‘shell damping factor’ has been adjusted. This is very important for the validation of nuclear model approaches. The systematic decrease in (n,p) cross-sections with increasing neutron number in reactions induced by neutrons on isotopes of iron is explained in terms of the pre-equilibrium model. The compound nucleus and pre-equilibrium reaction mechanism as well as the isotopic effects were also studied extensively.

Probable nuclear reactions to produce proton rich rhenium radionuclides

The crux of the present work is to explore the various channels leading to the production of proton rich rhenium radionuclides, 181–186Re, for different applications. The possible production routes encompass both light and heavy ion induced reactions up to a maximum 100 MeV projectile energy starting from threshold. The nuclear reaction model codes ALICE91 and PACE-II were employed in this endeavour. Excitation functions of the rhenium radionuclides have been calculated using the aforesaid nuclear reaction model codes and compared with the measured data wherever available. The contributions of preequilibrium and equilibrium reaction mechanisms to the total reaction cross section were analysed. For the first time, this study talks about the possibility of light-heavy ion (6,7Li and 9Be) induced production of proton rich rhenium radionuclides.

Study of the pre-equilibrium mean free path constant for light and heavy nuclei for (n, xn) reactions

Using pre-equilibrium reaction mechanisms, the neutron-emission spectra produced by (n,xn) reactions for some target (A=16∼238) nuclei have been calculated for incident neutron energy in the range of 15–26MeV. From light nuclei to heavy nuclei, the multiple pre-equilibrium mean free path constants from internal transition have been investigated. The obtained results are discussed and compared with the available experimental data.

Excitation functions of (n,p) reactions for stable isotopes of Cr, Fe and Ni from threshold to 20 MeV

The excitation functions of (n,p) reactions for the stable isotopes of Cr, Fe and Ni were calculated using EMPIRE 3.0 β1 (ARCOLE) from threshold to 20 MeV. The cross-sections are calculated using full featured Hauser–Feshbach statistical model with pre-equilibrium effects by invoking DEGAS option in the code. A good agreement between the calculated and experimental data (EXFOR data base) as well as evaluated data is obtained with option of neutron and proton potentials by Koning (Global) and HFB parity dependent nuclear level density. This is an important step to the validation of nuclear model with superior predictive power. The compound nucleus and pre-equilibrium reaction mechanisms as well as the isotopic effects were also studied.

A SYSTEMATIC STUDY OF EQUILIBRIUM AND PRE-EQUILIBRIUM (p, n) REACTION MECHANISMS CROSS-SECTIONS AT INTERMEDIATE ENERGIES FOR 25 < A < 240 TARGET NUCLEI

In this present study, by using equilibrium and pre-equilibrium reaction mechanisms, the (p, n) cross-section values for some target (A = 25–240) nuclei were investigated in a range of 10–40 MeV incident energy. The excitation functions for pre-equilibrium calculations were newly calculated by using hybrid model, geometry dependent hybrid model, full-exciton model, and cascade exciton model. The reaction equilibrium component was calculated with a traditional compound nucleus model developed by Weisskopf–Ewing. Calculation results were compared with the available excitation functions measurements in literature.

Investigation of Nucleon Mean Free Path Dependence in Tritium Emission Spectra Produced by Proton Induced Reactions at 62 MeV

This study aims on the effect of nucleon mean free path on differential cross sections of tritium-emission spectra for (p, 3H) reactions on 27Al, 54, 56Fe, 89Y, 120Sn, 197Au, 208Pb and 209Bi induced by 62 MeV protons in the pre-equilibrium reaction mechanisms. The pre-equilibrium calculations were carried out by using the geometry dependent hybrid model. The calculated results are compared with experimental data existing in literature and found to be in good agreement. Furthermore, using different multiplication factor for the mean free path in the calculation changes the differential cross sections of tritium emission spectra.

Dynamics of nuclear interaction in the reaction 56Fe(α, xn)

The spectra and angular distributions of neutrons from the reaction 56Fe(α, xn) were measured at the alpha-particle energies of 12.3, 16.3, 18.3, 26.8, and 45.2 MeV. The measurements were performed with time-of-flight fast-neutron spectrometers at pulsed accelerators of charged particles. The measured data were analyzed within models of equilibrium, preequilibrium, and direct reaction mechanisms. The calculations were performed by using an exact formalism of Hauser-Feshbach statistical theory. The required nuclear-level densities in nickel isotopes excited in this reaction were determined on the basis of new experimental data on low-lying levels, neutron resonances, and spectra of evaporated neutrons. The contributions of equilibrium, preequilibrium, and direct neutron emission were studied over a broad range of alpha-particle energies.

Excitation functions and isotopic effects in (n, p) reactions for stable nickel isotopes from reaction threshold to 20 MeV

The excitation function for (n, p) reactions from reaction threshold to 20 MeV on five nickel isotopes viz; 58Ni, 60Ni, 61Ni, 62Ni and 64Ni were calculated using Talys-1.0 nuclear model code involving the fixed set of global parameters. A good agreement between the calculated and measured data is obtained with minimum effort on parameter fitting and only one free parameter called ‘Shell damping factor’. This is of importance to the validation of nuclear model approaches with increased predictive power. The systematic decrease in (n, p) cross-sections with increasing neutron number in reactions induced by neutrons on isotopes of nickel is explained in terms of the proton separation energy and the pre-equilibrium model. The compound nucleus and pre-equilibrium reaction mechanism as well as the isotopic effects were also studied.

Equilibrium and pre-equilibrium calculations of neutron production in medium–heavy targets irradiated by protons up to 100 MeV

In this study, by using equilibrium and pre-equilibrium reaction mechanisms, the (p,xn) cross-sections of the spallation neutron target nuclei 50,52,53,54Cr and 63,65Cu were investigated for incident proton energies up to 100 MeV. The excitation functions for pre-equilibrium reaction mechanisms were calculated by using a hybrid model, a geometry dependent hybrid model, the full exciton model and the cascade exciton model. Equilibrium effects were calculated according to the Weisskopf–Ewing model. The calculation results were also compared with the measurements in literature.

Non-equilibrium emission of neutrons in α-particle induced reactions with holmium

The stacked-foil activation technique followed by off line high-purity Ge γ-ray spectroscopy was used for the measurement of the excitation functions for energies up to 50 MeV for the 165Ho(α, 2n) 167Tm, 165Ho(α, 3n) 166Tm, and 165Ho(α, 4n) 165Tm reactions. The measured excitation functions were compared with theoretical predications considering equilibrium as well as pre-equilibrium reaction mechanisms according to the geometry-dependent hybrid (GDH) model of Blann using computer code ALICE-91. The high-energy parts of the excitation functions are due to the pre-equilibrium reaction mechanism, while the low-energy parts are mediated by compound-nucleus decay. It was found that the compound-nucleus-decay mechanism alone is unable to explain the experimental trend of our data. The pre-equilibrium fraction was also calculated. PACS Nos.: 22.55.–e, 27.60.+j