Threshold Energy For Reaction Research Articles

EVALUATION OF 67Ga CROSS SECTIONS USING EXIFON CODE FOR MEDICAL APPLICATIONS

Radioisotopes play very important roles in nuclear medicine for imaging and therapeutic applications. In the present studies, model calculation of excitation function for production of 67Ga radioisotope was performed using the EXIFON code, a nuclear reaction cross sections theoretical model code, for the reaction 65Cu(α, 2n)67Ga. The work was performed in the incident alpha energy range of 0 - 40 MeV. Similarly, the Q-value software (interface) was used for the calculation of reaction threshold and Q-value energies of the reaction of interest and were respectively found to be 14.97 MeV and -14.10 MeV. The calculated excitation function has a peak value of 1025 mb around 25 MeV incident energy. The results from the EXIFON code were compared with the experimentally measured cross sections data retrieved from IAEA database, the EXFOR database, as well as the theoretical data from Talys code via its library, the TENDL-2019. Our results partially agree with the theoretical data from Talys code (via the TENDL-2019 library) within the investigated energy region. The results however overestimated the measured (experimental) data and only agree in shape of the excitation function. The present work does not consider the effect of shell structure during the execution of the EXIFON model code. This work could be of importance to the developers and users of nuclear reaction model codes for new developments and enhancements of the existing codes, as well as to serve as a rough guide for experimentalists during production of radioisotopes for nuclear medicine applications

Microcanonical Rate Constants for Unimolecular Reactions in the Low-Pressure Limit.

Low-pressure-limit microcanonical rate constants, κ0(E,J), describe the rate of activating bath gas collisions in a unimolecular reaction and are calculated here using classical trajectories and quantized thresholds for reaction. The resulting semiclassical rate constants are two-dimensional (in total energy E and total angular momentum J) and are intermediate in complexity between the four-dimensional state-to-state collisional energy and angular momentum transfer rate constant, R(E',J';E,J), and the highly averaged thermal rate constant, k0. Results are presented for CH4 (+M), C2Hx (+M), x = 3-6, and H2O (+M), where κ0(E,J) is shown generally to be a sensitive function of the bath gas, temperature, and initial state of the unimolecular reactant. Strong variations in κ0 with respect to E and J lead to complex trends in relative microcanonical bath gas efficiencies. This underlying complexity may complicate the search for simple explanations for observed trends in relative thermal bath gas efficiencies. A different measure of the microcanonical collision efficiency that describes the energy range of activating collisions is introduced that supports the empirical decomposition of collisional activation into separable translational-to-vibrational and rotational-to-vibrational activation mechanisms. The two mechanisms depend differently on mass, temperature, and the J-dependence of the threshold energy for reaction, with rotational-to-vibrational activation favored for heavier baths and for reactions with rigid transition states. Finally, κ0 is used to test the accuracy of several two-dimensional models for R that were proposed for use in master equation studies.

ON MECHANISMS OF OXYGEN INFLUENCE ON GAS-PHASE PARAMETERS AND SILICON REACTIVE-ION ETCHING KINETICS IN HBr + Cl2 + O2 PLASMA

The effects of both HBr/O2 and Cl2/O2 mixing ratios in HBr+Cl2+O2 gas mixture on plasma parameters, steady-state densities of active species and Si etching kinetics were studied under the typical conditions of reactive ion etching process: total gas pressure (p = 10 mTorr), input power (W = 500 W), bias power (Wdc = 200 W). The data on internal plasma parameters and plasma chemistry were obtained using a combination of Langmuir probe diagnostics and 0-dimensional (global) plasma modeling. It was found that the variation in HBr/O2 mixing ratio at constant Cl2 fraction in a feed gas is characterized by the stronger impact on the steady-state plasma composition through both electron-impact and atom-molecular reaction kinetics as well as allows one to obtain the wider change in the total halogen atom density. It was shown that changes in both HBr/O2 and Cl2/O2 mixing ratios toward O2-rich plasmas lowers the Si etching rate that exhibits no evident correlations with total halogen atom flux and ion energy flux. The model-based analysis of Si etching kinetics allowed one to conclude that the effective reaction probability for Si + Cl/Br heterogeneous reaction depends on the flux of oxidative species – oxygen atoms and OH radicals. The reasons may be 1) the oxidation of silicon resulting in higher reaction threshold energy; and 2) the decreasing fraction of free adsorption sites for Cl/Br atoms due to the oxidation of reaction products into the lower volatile SiBrxOy and SiClxOy compounds.

Reactions with Exotic Nuclei at Near and Sub-barrier Energies

We will introduce some new results derived recently by the nuclear reaction group at China Institute of Atomic Energy, on the properties of the optical model potentials of neutron-halo 6He system and reaction mechanisms induced by proton-drip line nucleus 17F at energies around the Coulomb barrier. For the study of optical model potentials of exotic nuclear system, we proposed a novel method, i.e., the transfer reaction method. This method has been applied to extract the optical potentials of neutron-halo 6He+209Bi system by measuring the one-proton transfer reactions induced by 7Li on a 208Pb target. A complete picture of threshold anomaly behavior was obtained in the 6He+209Bi system for the first time, where a decreasing trend of the depth of the imaginary potential is observed in the deep sub-barrier region, and the reaction threshold energy is extracted. Moreover, results show thatthe dispersion relation is not applicable for this exotic nuclear system. Reaction mechanisms of proton-rich nuclear systems 17F+89Y and 58Ni were also studied at energies around the Coulomb barrier. Continuum discretized coupled-channels calculations indicate that the coupling effects of the continuum states of 17F+89Y is not significant. Thanks to the employment of a powerful ionization-chamber based detector array, the reaction products over a large Z in the systems of 17F+58Ni can be identified clearly. The data analysis is undergoing.

Etching Kinetics and Mechanisms of SiC Thin Films in F-, Cl- and Br-Based Plasma Chemistries

The comparative study of SiC and SiO2 etching kinetics as well as the evaluation of SiC etching mechanisms in CF4 + Ar, Cl2 + Ar and HBr + Ar gas mixtures in inductively coupled plasma reactor were carried out. For each binary gas system, the experiments (SiC and SiO2 etching rates measurements, plasma diagnostic by Langmuir probes and optical spectroscopy) were conducted with variable fractional compositions of a feed gas at constant input power (700 W), bias power (300 W) and overall gas pressure (6 mTorr). The 0-dimensional (global) models for CF4 + Ar, Cl2 + Ar and HBr + Ar plasmas provided the information on the steady-state densities and fluxes for halogen atoms as well as on the ion energy flux to the treated surface. It was found that the transition toward Ar-rich plasmas in all investigated gas systems: (1) provides the non-monotonic (with a maximum at ~ 50–60% Ar) SiC etching rate; (2) leads to the monotonic decrease in both halogen atom density and flux; and (3) results in increasing both ion energy flux and effective probability for SiC + F, Cl or Br reactions. It was shown that absolute differences in effective reaction probabilities are in agreement with corresponding ion energy fluxes and reaction threshold energies.

Supernova β− decay of nuclides 53Fe, 54Fe, 55Fe and 56Fe in strongly screened plasma

Electron screening has strong effects on electron energy and threshold energy of the beta decay reaction. In this paper, we study β− decay rates of some iron isotopes. The beta decay rates increase by about two orders of magnitude due to electron screening. Strongly screened beta decay rates due to Q-value correction are more than one order of magnitude higher than those without Q-value correction.

Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism

Reactions occurring at a carbon atom through the Walden inversion mechanism are one of the most important and useful classes of reactions in chemistry. Here we report an accurate theoretical study of the simplest reaction of that type: the H+CH4 substitution reaction and its isotope analogues. It is found that the reaction threshold versus collision energy is considerably higher than the barrier height. The reaction exhibits a strong normal secondary isotope effect on the cross-sections measured above the reaction threshold, and a small but reverse secondary kinetic isotope effect at room temperature. Detailed analysis reveals that the reaction proceeds along a path with a higher barrier height instead of the minimum-energy path because the umbrella angle of the non-reacting methyl group cannot change synchronously with the other reaction coordinates during the reaction due to insufficient energy transfer from the translational motion to the umbrella mode.

H + Li2: a typical exothermic reactive system and its time-dependent dynamics investigation

In this paper, the time-dependent wave-packet method is used to study the three-dimensional dynamical properties of the H+Li2 reactive system on its ground state potential energy surface. The reaction probabilities for J=0 with different vibrational quantum numbers v=0, 1, 2, 3 and for v=0 with different total rotational quantum numbers, integral cross sections and rate constants are calculated for collision energies in a range between 0 and 0.4 eV. The features of the reaction probabilities and reaction threshold energy are analyzed. The results show that the vibrational excitation has a certain inhibitory effect on the reaction process and the reaction threshold energy increases with the increase of J. These phenomena are associated with the type of the exothermic reaction of the reactive system. The influence of the collision energy on the integral cross sections and the effect of the temperature on reaction rate constants are also investigated.

Dynamically biased RRKM model of activated gas-surface reactivity: vibrational efficacy and rotation as a spectator in the dissociative chemisorption of CH4on Pt(111)

The reactivity of CH(4) impinging on a Pt(111) surface was examined using a precursor-mediated microcanonical trapping model of dissociative chemisorption wherein the effects of rotational and vibrational energy could be explored. Dissociative sticking coefficients for a diverse range of non-equilibrium effusive beam, supersonic beam, and eigenstate-resolved experiments were simulated and an average relative discrepancy between theory and experiment of better than 50% was achieved by treating molecular rotations and translation parallel to the surface as spectator degrees of freedom, and introducing a dynamically-biased vibrational efficacy. The model parameters are {E(0) = 57.9 kJ mol(-1), s = 2, η(v) = 0.40} where E(0) is the apparent threshold energy for reaction, s is the number of surface oscillators participating in energy exchange within each gas-surface collision complex formed, and η(v) is the mean vibrational efficacy for reaction relative to normal translational energy which figures in the assembly of the active exchangeable energy which is available to surmount the activation barrier to dissociative chemisorption. GGA-DFT electronic structure calculations provided vibrational frequencies for the transition state for dissociative chemisorption. The asymmetry of the rotational state populations in supersonic and effusive molecular beam experiments allowed kinetic analysis to establish that taking rotation as a spectator degree of freedom is a good approximation. Surface phonons, rather than the incident molecules, are calculated to play the dominant role in supplying the energy required to overcome the activation barrier for dissociative chemisorption under the thermal equilibrium conditions relevant to high pressure catalysis. Over the temperature range 300 K ≤T≤ 1000 K, the thermal dissociative sticking coefficient is predicted to be well described by S(T) = S(0) exp(-E(a)/RT) where S(0) = 0.62 and E(a) = 62.6 kJ mol(-1).

Correlations in Nuclear Interactions between E<sub>CM</sub>/<i>u</i> and Unexplained Experimental Observables

A new concept is introduced for the classification of ”unresolved problems” in the understanding of interactions in thick targets irradiated with relativistic ions: The centre-of-mass energy per nucleon of a hypothetical compound nucleus from a primary interaction, E(subscript CM)/u, is calculated and correlated with experimental observations in thick target irradiations. One observes in various reactions of relativistic primary ions with thick targets that there appears to be a threshold energy for reactions leading to ”unresolved problems” which lies around ECM/u~150 MeV. All ”unresolved problems” are exclusively observed above this threshold, whereas below this threshold no ”unresolved problems” are found. A similar threshold at 158±3 MeV exists for massive pion production in nuclear interactions. Hagedorn had proposed this threshold decades ago and it is known as the Hagedorn limit. In this paper we will only mention, but not elaborate on Hagedorn's theoretical concept any further. Some considerations will be presented and further studies in this field are suggested.

Measurement of 76Se and 78Se (γ, n) Cross Sections

The (γ, n) cross sections of Se isotopes (76Se,78Se) were measured to supply fundamental data for estimating the inverse reaction cross section, i.e., the 79Se(n, γ)80Se cross section. The enriched samples and a reference 197Au sample were irradiated with laser-Compton scattering (LCS) γ-rays. The excitation function of each (γ, n) cross section was determined for the energy range from each near neutron separation energy to the threshold energy of (γ, 2n) reaction. The energy point corresponding to each cross section was deduced using the accurately determined energy distribution of LCS γ-rays. Systematic (γ, n) cross sections for Se isotopes including 80Se were compared with those calculated by using a statistical model calculation code TALYS.

A quasiclassical trajectory study of mode specific reaction rate enhancements in H + H2O (v1v2v3) → OH + H2

The quasiclassical trajectory method is used to determine the dependence of the cross section and threshold energy for reaction upon initial vibrational state for the reaction H + H2O(v1v2v3) OH + H2. A significant rate enhancement is found when any of the three modes or combinations thereof are excited, but the enhancement is larger when the symmetric stretch or bend modes are excited than when the asymmetric stretch is excited. Enhancement efficiencies (the ratio of threshold energy reduction to reagent excitation energy) vary from 60% for H2O(001) to ˜100% for H2O(020) and (100).

Hydrogen Atom Transfer Reactions of C2-, C4-, and C6-: Bond Dissociation Energies of Linear H−C2n- and H−C2n (n = 1, 2, 3)

The reactions of C2-, C4-, and C6- with D2O and ND3 and of C4- with CH3OH, CH4, and C2H6 have been investigated using guided ion beam tandem mass spectrometry. Hydrogen (or deuterium) atom transfer is the major product channel for each of the reactions. The reaction threshold energies for collisional activation are reported. Several of the reactions exhibit threshold energies in excess of the reaction endothermicity. Potential energy calculations using density functional theory show energy barriers for some of the reactions. Dynamic restrictions related to multiple wells along the reaction path may also contribute to elevated threshold energies. The results indicate that the reactions with D2O have the smallest excess threshold energies, which may therefore be used to derive lower limits on the C-H bond dissociation energies of the C2nH- and C2nH (n = 1-3) linear species. The experimental lower limits for the bond dissociation energies of the neutral radicals to linear products are D0(C2-H) >or= 460 +/- 15 kJ/mol, D0(C4-H) >or= 427 +/- 12 kJ/mol, and D0(C6-H) >or= 405 +/- 11 kJ/mol.

Theoretical study of the H + Br 2 and Mu + Br 2 reactions: A new ab initio potential energy surface and quantum dynamics calculations

A new global potential energy surface for the H + Br 2 → HBr + Br reaction has been developed at the MRCI+Q/aug-cc-pVTZ ab initio level of theory. It has been found that this level calculation gives a negative barrier slightly below the H + Br 2 reactant energy level. Time-dependent and time-independent quantum dynamics calculations have been performed on this potential energy surface. The calculated cumulative reaction probabilities for J = 0 around the reaction threshold energy region show a very similar behavior for both the H + Br 2 and Mu + Br 2 reaction, in qualitative disagreement with experimental measurements, where a positive activation energy was observed for the H + Br 2 reaction while a negative activation energy for the Mu + Br 2 reaction. Variational calculations of vibrational energy levels have also been performed to study dynamical vibrational bonding for the Br–Mu–Br molecule. It has been found that the system has several vibrational bonding states, whose wave functions are localized around the transition-state of the Br + MuBr′ → MuBr + Br′ exchange reaction, below the lowest Br + MuBr asymptotic energy level. This result qualitatively supports the recent experimental finding that a long-lived radical containing a Mu atom is produced in the Mu + Br 2 reaction.

First principles quantum dynamics study reveals subtle resonance in polyatomic reaction: The case of F + CH 4 → HF + CH 3

First principles quantum dynamics calculation has been carried out to investigate a recently observed resonance feature in the F + CH 4 reaction by molecular beam experiment [W.C. Shiu, J.J. Lin, K.P. Liu, Phys. Rev. Lett., 92 (2004) 103201]. The generalized semi-rigid vibrating rotor target (GSVRT) method is employed to perform the quantum dynamics calculation on new ab initio potential energy surfaces (PES) constructed from the extensive high level ab initio calculations. A resonance near the reaction threshold energy is observed in both the calculated microscopic reaction probabilities and integral cross-sections on the ZYH2 PES. The calculated resonance on the ZYH2 PES is in good agreement with the experimental observation. This resonance feature is highly quantum mechanical and sensitive to the accuracy of ab initio energies.

Intramolecular Vibrational Energy Redistribution Involving the Torsion in CF3CH3: A Molecular Dynamics Study

Classical trajectory calculations on intramolecular vibrational energy redistribution (IVR) involving the torsion in 1,1,1-trifluoroethane (TFE) are reported. Two potential energy functions (PEFs) are used to describe the potential energy surface. The "full" PEF gives excellent agreement with the experimental vibrational frequencies. The "simple" PEF omits nondiagonal interaction terms, but still gives very good agreement with the experimental frequencies. The "simple" PEF is intended to minimize mode-mode coupling. Neither PEF includes the HF elimination reaction. Calculations are carried out both with nominal microcanonical selection of initial coordinates and momenta, and with a modified selection method that places controlled amounts of energy in the torsion. Total (classical) vibrational energies from 0.005 to 140 kcal mol(-1) are investigated. The calculated time constants describing energy flow out of the torsional mode are <10 ps for classical vibrational energies near the classical reaction threshold energy (approximately 75 kcal mol(-1)) and greater. It is found that the rate of decay from the torsion largely depends on the amount of energy in the other vibrational modes. Analysis using power spectra shows that the torsional mode in TFE is strongly coupled to the other vibrational modes. These results strongly suggest that vibrational energy in TFE will not be sequestered in the torsion for time periods greater than a few tens of picoseconds when the molecule has enough energy to react via HF elimination.

Effect of entrance channel topology on reaction dynamics: O( 3P) + CH 4 → CH 3 + OH

The generalized semirigid vibrating rotor target (GSVRT) method is employed to investigate O( 3P) + CH 4 → CH 3 + OH reaction on a global potential energy surface (PES): the C–T surface of Corchado et al. [J.C. Corchado, J. Espinosa-Garcia, O. Roberto-Neto, Y.-Y. Chuang, D.G. Truhlar, J. Phys. Chem. A 102, 4899, 1998]. The reaction probability and rate constant from the present 5D GSVRT dynamics calculation are consistent with those from the 4D SVRT calculation, indicating that the umbrella vibrational motion of CH 4 is not strongly coupled to the reaction coordinate on the C–T surface. The calculated reaction threshold energy on the C–T surface is considerably larger than that from the experimental measurement. Our study indicates that the entrance channel topology of the C–T PES is responsible for the higher reaction threshold energy obtained from the dynamics calculation. This is the main reason for the large discrepancy in reaction rate constant between transition state-like results and the dynamics calculation which explores global potential energy surface beyond transition state region.

Electron scattering on stored mononucleotide anions

Collisions between electrons and deprotonated mononucleotides, AMP and UMP, were studied at the electrostatic storage ring ELISA where the two anions were stored with a $1∕e$ lifetime of $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. It is found that the threshold energy for electron-induced reactions leading to production of neutral fragments is as high as about $12\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for both species. The high threshold is explained within the context of a classical reaction model, applicable for atomic anions, and a high electron-binding energy of about $5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the two mononucleotides. There is no evidence in the cross sections for formation of short-lived doubly charged anions.

Roles of Supernova Ejecta in Nucleosynthesis of the Light Elements Li, Be, and B

Explosions of Type Ic supernovae (SNe Ic) are investigated using a relativistic hydrodynamic code to study the role of their outermost layers of the ejecta in light-element nucleosynthesis through spallation reactions as a possible mechanism of the ‘‘primary’’ process. We have confirmed that the energy distribution of the outermost layers with a mass fraction of only 0.001% follows the empirical formula proposed by previous work when the explosion is furious. In such explosions, a significant fraction of the ejecta (>0.1% in mass) have energy greater than the threshold energy for spallation reactions. On the other hand, we find that the outermost layers of ejecta become more energetic than the empirical formula would predict when the explosion energy per unit ejecta mass is smaller than � 1:3;10 51 ergs M � 1 � . As a consequence, it is necessary to numerically calculate explosions to estimate light-element yields from SNe Ic. The usage of the empirical formula would overestimate the yields by a factor of k3 for energetic explosions, such as SN 1998bw, and underestimate the yields by a similar factor for less energetic explosions, such as SN 1994I. The yields of the light elements Li, Be, and B (LiBeB) from SNe Ic are estimated by solving the transfer equation of cosmic rays originated from ejecta of SNe Ic and compared with observations. The abundance ratios Be/O and B/O produced by each of our SNe Ic models are consistent with those of metal-poor stars. The total amounts of these elements estimated from observations indicate that energetic SNe Ic, such as SN 1998bw, could be candidates for a production site of Be and B in the Galactic halo only when the fraction of this type out of all the SNe was more than a factor of 100 higher than the value estimated from current observational data. This primary mechanism would predict that there are stars significantly deficient in light elements that were formed from the interstellar medium not affected by SNe Ic. Since this has no support from current observations, other primary mechanisms, such as the light-element formation in superbubbles, are needed for other types of SNe. The observed abundance pattern of all elements including heavy elements in metal-poor stars suggests that these two mechanisms should have supplied similar amounts of Be and B. Our calculations show that SNe Ic can not produce an appreciable amount of Li. Subject heading gs: cosmic rays — nuclear reactions, nucleosynthesis, abundances — relativity — shock waves — supernovae: general

Experimental kerma coefficients of biologically important materials at neutron energies below 75 MeV

The present work summarizes our results already published on cross sections and partial kerma coefficients for hydrogen, carbon, and oxygen and then applies them for determining experimental partial and total kerma coefficients of composite biologically important materials. Double-differential cross sections for light-charged particle production (proton, deuteron, triton, and alpha particle) induced by fast neutrons on hydrogen, carbon, and oxygen have been experimentally measured at several incident energies from 25 to 75 MeV. The measurements covered the laboratory angular range 20 degrees to 160 degrees and were extended to very forward and very backward angles by using a reliable extrapolation procedure. Energy-differential, angle-differential, and total production cross sections were derived from the measured data. The experimental methods and data reduction procedures are briefly presented here. The experimental cross sections were compared to existing data in the literature for nucleon-induced reactions and against prediction of nuclear models. Partial and total elemental kerma coefficients were deduced on the basis of the measured cross sections. Procedures for extrapolating the partial kerma coefficients down to the reaction threshold energies for each of the measured ejectile species have been applied to carbon and oxygen. A simple-to-use analytical formula to describe the experimental hydrogen kerma coefficients was proposed which provides the recoil kerma coefficients in the incident neutron energy range 0.3 to 100 MeV. The present article reports for the first time experimental partial kerma coefficients for composite materials of biological interest. Resulting total kerma coefficients are compared to theoretical predictions and to other experimental data.