Pre-equilibrium Model Research Articles

Fast-neutron induced pre-equilibrium reactions on 55Mn and 63,65Cu at energies up to 40 MeV

The experimental cross sections for all neutron-induced reactions on the stable isotopes of Mn and Cu at incident energies up to 40 MeV are compared with results of pre-equilibrium (PE) and statistical model calculations. Taking advantage of the large sensitivity of statistical model analysis to residual nuclei parameters, for energies up to 20 MeV, our work was then focused on assessing PE model assumptions by data analysis in the energy range 20–40 MeV. The PE model reliance on the nuclear potential finite-depth correction for description of particle–hole state densities (PSD), nuclear-shell effects within the PSD formula, and single-particle level density is thus discussed with regard to the whole data basis. The related variance of the model results has been found as large as the differences between the global and local model calculations, while the data basis available between 20 and 40 MeV for the Mn and Cu isotopes is however too scarce in this respect.

Preequilibrium model of photonucleon reactions that is based on Fermi gas densities

A combination of the exciton and evaporation models is used to describe photonucleon reactions induced in heavy and medium-mass nuclei by photons of energy in the range 7 ≤ Eγ ≤ 140 MeV. The formation of a giant dipole resonance and quasideuteron absorption are considered as two mechanisms that are responsible for the photoexcitation of a nucleus in the energy regions Eγ ≲ 20 MeV and Eγ ≳ 40 MeV, respectively. As is well known, the densities of particle-hole states are employed in the exciton model, and these quantities are calculated on the basis of the Fermi gas model. This makes it possible to take into account the effect of the energy dependence of single-particle and single-hole densities of states on the rate of emission and intranuclear processes. The model in question is applied to describing partial photonucleon cross sections for 119Sn, 140Ce, 181Ta, and 208Pb nuclei.

SEA Domain Autoproteolysis Accelerated by Conformational Strain: Energetic Aspects

A subclass of proteins with the SEA (sea urchin sperm protein, enterokinase, and agrin) domain fold exists as heterodimers generated by autoproteolytic cleavage within a characteristic G−1S+1VVV sequence. Autoproteolysis occurs by a nucleophilic attack of the serine hydroxyl on the vicinal glycine carbonyl followed by an N→O acyl shift and hydrolysis of the resulting ester. The reaction has been suggested to be accelerated by the straining of the scissile peptide bond upon protein folding. In an accompanying article, we report the mechanism; in this article, we provide further key evidence and account for the energetics of coupled protein folding and autoproteolysis. Cleavage of the GPR116 domain and that of the MUC1 SEA domain occur with half-life (t½) values of 12 and 18 min, respectively, with lowering of the free energy of the activation barrier by ∼10 kcal mol−1 compared with uncatalyzed hydrolysis. The free energies of unfolding of the GPR116 and MUC1 SEA domains were measured to ∼11 and ∼15 kcal mol−1, respectively, but ∼7 kcal mol−1 of conformational energy is partitioned as strain over the scissile peptide bond in the precursor to catalyze autoproteolysis by substrate destabilization. A straining energy of ∼7 kcal mol−1 was measured by using both a pre-equilibrium model to analyze stability and cleavage kinetics data obtained with the GPR116 SEA domain destabilized by core mutations or urea addition, as well as the difference in thermodynamic stabilities of the MUC1 SEA precursor mutant S1098A (with a G−1A+1VVV motif) and the wild-type protein. The results imply that cleavage by N→O acyl shift alone would proceed with a t½ of ∼2.3 years, which is too slow to be biochemically effective. A subsequent review of structural data on other self-cleaving proteins suggests that conformational strain of the scissile peptide bond may be a common mechanism of autoproteolysis.

Calculation of Neutron Cross Sections on 93Nb for JENDL-4

Nuclear data of 93Nb in the Japanese Evaluated Nuclear Data Library JENDL-3.3 were reexamined in the incident neutron energy range from 10keV to 20MeV toward JENDL-4. Cross sections, differential and double-differential cross sections were calculated on the basis of nuclear reaction models such as the spherical optical model, the distorted wave Born approximation, the Kalbach preequilibrium model, and the Hauser-Feshbach statistical model. It was found that experimental neutron emission spectra were reproduced well by the present calculation compared with the JENDL-3.3 evaluations for emission energies > 1 MeV. Also, calculated cross sections were almost consistent with the available experimental data for total, elastic and inelastic scattering, (n, γ), (n, 3n), (n, Xp), (n, Xd), (n, t), (n, α), (n, nα), and (n, Xα) reactions. An appreciable change from the JENDL-3.3 evaluation was not seen for the (n,2n) cross section, which was about 20 percent larger than the experiments for incident energies ≤ 14 MeV.

EMPIRE: Nuclear Reaction Model Code System for Data Evaluation

EMPIRE is a modular system of nuclear reaction codes, comprising various nuclear models, and designed for calculations over a broad range of energies and incident particles. A projectile can be a neutron, proton, any ion (including heavy-ions) or a photon. The energy range extends from the beginning of the unresolved resonance region for neutron-induced reactions (∽ keV) and goes up to several hundred MeV for heavy-ion induced reactions. The code accounts for the major nuclear reaction mechanisms, including direct, pre-equilibrium and compound nucleus ones. Direct reactions are described by a generalized optical model (ECIS03) or by the simplified coupled-channels approach (CCFUS). The pre-equilibrium mechanism can be treated by a deformation dependent multi-step direct (ORION + TRISTAN) model, by a NVWY multi-step compound one or by either a pre-equilibrium exciton model with cluster emission (PCROSS) or by another with full angular momentum coupling (DEGAS). Finally, the compound nucleus decay is described by the full featured Hauser-Feshbach model with γ-cascade and width-fluctuations. Advanced treatment of the fission channel takes into account transmission through a multiple-humped fission barrier with absorption in the wells. The fission probability is derived in the WKB approximation within the optical model of fission. Several options for nuclear level densities include the EMPIRE-specific approach, which accounts for the effects of the dynamic deformation of a fast rotating nucleus, the classical Gilbert-Cameron approach and pre-calculated tables obtained with a microscopic model based on HFB single-particle level schemes with collective enhancement. A comprehensive library of input parameters covers nuclear masses, optical model parameters, ground state deformations, discrete levels and decay schemes, level densities, fission barriers, moments of inertia and γ-ray strength functions. The results can be converted into ENDF-6 formatted files using the accompanying code EMPEND and completed with neutron resonances extracted from the existing evaluations. The package contains the full EXFOR (CSISRS) library of experimental reaction data that are automatically retrieved during the calculations. Publication quality graphs can be obtained using the powerful and flexible plotting package ZVView. The graphic user interface, written in Tcl/Tk, provides for easy operation of the system. This paper describes the capabilities of the code, outlines physical models and indicates parameter libraries used by EMPIRE to predict reaction cross sections and spectra, mainly for nucleon-induced reactions. Selected applications of EMPIRE are discussed, the most important being an extensive use of the code in evaluations of neutron reactions for the new US library ENDF/B-VII.0. Future extensions of the system are outlined, including neutron resonance module as well as capabilities of generating covariances, using both KALMAN and Monte-Carlo methods, that are still being advanced and refined.

Site-specific Binding Affinities within the H2B Tail Domain Indicate Specific Effects of Lysine Acetylation

Acetylation of specific lysines within the core histone tail domains plays a critical role in regulating chromatin-based activities. However, the structures and interactions of the tail domains and the molecular mechanisms by which acetylation directly alters chromatin structures are not well understood. To address these issues we developed a chemical method to quantitatively determine binding affinities of specific regions within the individual tail domains in model chromatin complexes. Examinations of specific sites within the H2B tail domain indicate that this tail contains distinct structural elements and binds within nucleosomes with affinities that would reduce the activity of tail-binding proteins 10-50-fold from that deduced from peptide binding studies. Moreover, we find that mutations mimicking lysine acetylation do not cause a global weakening of tail-DNA interactions but rather the results suggest that acetylation leads to a much more subtle and specific alteration in tail interactions than has been assumed. In addition, we provide evidence that acetylation at specific sites in the tail is not additive with several events resulting in similar, localized changes in tail binding.

Nuclear theory for high-energy nuclear reactions of biomedical relevance

A full description of all possible nuclear reactions that take place in a macroscopic device can only be accomplished with a nuclear model code in combination with key experimental data. To address this issue, the authors demonstrate some of the capabilities of TALYS, a nuclear reaction program which simulates nuclear reactions that involve neutrons, gamma rays, protons, deuterons, tritons, helions and alpha particles, in the 1 keV to 200 MeV energy range. A suite of nuclear reaction models has been implemented into a single code system, enabling to evaluate basically all nuclear reactions beyond the resonance range. The main nuclear models used, such as newly developed optical models, various compound nucleus, fission, gamma-ray strength, level density and pre-equilibrium models, all driven by a comprehensive database of nuclear structure parameters have been briefly mentioned. The predictive power of the code is demonstrated by comparing calculated results with a diverse set of experimental observables. The aim is to show that TALYS represents a robust computational approach that covers the whole path from fundamental nuclear reaction models to the creation of complete data libraries for nuclear applications.

Systematics of (p,α) (p,nα), and (p,np) reaction cross-sections

Semi-empirical systematics of (p,α) (p,nα), and (p,np) reaction cross-sections were obtained at various incident proton energies from 17.9 to 28.5 MeV. Systematics are based on analytical formulas derived using the pre-equilibrium exciton model, evaporation model and semi-empirical mass formula. Parameters of systematics were fitted to the data obtained from the analysis of available measured cross-sections for (p,α) (p,nα), and (p,np) reactions.

The radiation damage database: Section on helium cross section

A radiation damage database with emphasis on spallation interactions is described. Currently, the database contains damage energy, displacement, helium, and hydrogen cross sections for 23 elemental targets irradiated by proton and neutron projectiles up to 3.2GeV. In this paper, the focus is on proton-induced helium cross sections, but it is shown that for high energies (above about 500MeV) proton- and neutron-induced helium cross sections are almost equal. The cross section calculations were run on the Cascade–Exciton Model code (no options) and also on the Bertini code with three nuclear level-density models and multistage pre-equilibrium model on and off. Calculation and experimental results are compared. For various targets, we tried to determine which code and options give best agreement with experiment. In some cases, such determinations are uncertain, partly because of limited and conflicting experimental information and partly perhaps because of the need for modifications in the codes.

Semi-empirical systematics of (n, 3He) reaction cross-section at 14.6 and 20 MeV

A new semi-empirical formula for the evaluation of (n, 3He) reaction cross-sections at the energy of 14.6 and 20 MeV is presented. Formula was derived using the analytical expression for the evaluation of the 3He spectrum within the frame of pre-equilibrium exciton model. The systematics obtained is compared with the empirical formula for the (n, 3He) reaction cross-section.

Spallation Radiation Damage Calculations and Database: Cross-Section Discrepancies between the Codes

Abstract A radiation damage database at NCSU is described that contains, at present, cross sections for damage energy, displacements, helium, and hydrogen. For neutrons and protons at 20-3200 MeV energies, the targets include 23 target elements from Mg to U and eight practical alloys. The underlying calculations employ Bertini, ISABEL, and CEM2k intranuclear cascade models as provided within MCNPX, three level-density formulations (GCCI, HETC, and Jülich), and multistage pre-equilibrium model (MPM) on and off. The database also includes radiation damage cross sections for lower-energy neutrons and protons, as obtained from LA150, SPECTER, ENDF, and SRIM. Examples of the database contents are provided with regard to damage energy and displacement cross sections for neutrons on Al, Fe, and W. Also, neutron flux and displacement rate calculations are described for neutrons on Fe and the 316 stainless steel mercury container vessel at SNS. In addition, discrepancies between the codes are discussed.

Nuclear-level densities around Z = 50 from neutron evaporation spectra in (p, n) reactions

Neutron excitation functions, spectra, and angular distributions in the (p, n) reactions on the isotopes 116Sn, 118Sn, 122Sn, and 124Sn were measured in the proton-energy range 7–11 MeV. The measurements were performed by the time-of-flight method with the aid of a fast-neutron spectrometer at the EGP-15 pulsed tandem accelerator of the Institute of Physics and Power Engineering (Obninsk). A high resolution (about 0.6 ns/m) and a high stability of the time-of-flight spectrometer made it possible to identify reliably low-lying levels along with the continuous section of the neutron spectra. The data obtained in this way were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed with the aid of the precise Hauser-Feshbach formalism of statistical theory. The nuclear-level densities in the isotopes 116Sb, 118Sb, 122Sb, and 124Sb were determined, along with their energy dependences and model parameters. In the excitation-energy range 0–2 MeV, the energy dependence of the nuclear-level densities exhibits a structure that is associated with the shell inhomogeneities of the spectrum of single-particle states near filled shells. The isotopic dependence of the nuclear-level density is discovered and explained. It is also shown that the data obtained here for the nuclear-level density differ markedly from the predictions of model systematics of nuclear-level densities.

Neutron displacement cross-sections for tantalum and tungsten at energies up to 1 GeV

The neutron displacement cross-section has been evaluated for tantalum and tungsten at energies from 10 −5 eV up to 1 GeV. The nuclear optical model, the intranuclear cascade model combined with the pre-equilibrium and evaporation models were used for the calculations. The number of defects produced by recoil atoms nuclei in materials was calculated by the Norgett, Robinson, Torrens model and by the approach combining calculations using the binary collision approximation model and the results of the molecular dynamics simulation. The numerical calculations were done using the NJOY code, the ECIS96 code, the MCNPX code and the IOTA code.

Analysis of n [formula omitted]232Th interaction up to 200 MeV

Observed fission cross section data are reproduced up to E n ∼ 200 MeV within Hauser–Feshbach pre-equilibrium model. Symmetric and asymmetric fission mode partitioning of the observed fission cross section is predicted. Fission barriers of neutron-deficient Th nuclides are compared with the microscopic and macroscopic–microscopic calculations. Nucleon optical model potential for 232Th is obtained. Rigid rotator coupled-channel model reproduces experimental neutron total cross section data up to 200 MeV.

Preequilibrium reactions with complex particle channels

Investigations of nucleon induced reactions at incident energies of $14--90\phantom{\rule{0.3em}{0ex}}\text{MeV}$ resulting in the emission of complex particles ($A=2--4$) have provided insights which complement those previously obtained from ($N,\mathit{xN}$) reactions. The description of the preequilibrium energy spectra required modifications to an earlier phenomenological model for direct pickup reactions. This model supplements the usual exciton preequilibrium model. Work on complex particle induced reactions confirms some of these results, extends them to include stripping and exchange reactions, and provides evidence for a projectile dependence of the average effective matrix elements for the residual interactions in the exciton model. A full description of reactions with complex projectiles will require the inclusion of a realistic breakup component and the resulting reduction of the cross section available for the exciton model calculations. Reactions with complex particles in the entrance and/or exit channels have provided indirect evidence for the amount of surface peaking of the initial target-projectile interaction. A summary of additional data needed to help resolve remaining questions is presented.

Assessment of neutron- and proton-induced nuclear interaction and ionization models in Geant4 for Simulating single event effects

This paper examines the performance of the Geant4 radiation transport toolkit for the simulation of energy deposition from proton- and neutron-nuclear interactions in silicon microelectronics. The results show that for large (/spl sim/300 /spl mu/m) to small (/spl sim/0.5 /spl mu/m) feature-size devices, the nucleon-nuclear and electromagnetic interaction models within the toolkit provide energy deposition spectra and single event upset rate predictions that are in good agreement with experimental data. The new Binary Cascade and Classical Cascade models, together with the nuclear pre-equilibrium model in Geant4, do not significantly differ in the results they produce. For small feature-size devices, it is shown that it is necessary to consider the effects of ionization by particles produced by nuclear interactions several micrometers above the sensitive volume.

A global pre-equilibrium analysis from 7 to 200 MeV based on the optical model potential

We present a pre-equilibrium model for nucleons with incident energies from 7 to 200 MeV, for nuclides in the mass range A ⩾ 24 . This is accomplished by a new global approach for the two-component exciton model which, together with the complementary compound and direct reaction mechanisms, enables a description of continuum energy spectra over the whole outgoing energy range. We develop new forms for the internal transition rates with collision probabilities based on a recent optical model potential. To connect with conventional semi-classical analyses, we derive from this approach a new energy-dependent form for the average squared matrix element M 2 . Both implementations are shown to remove problems encountered when predicting measurements with older parameterizations that apply in more restricted energy ranges. Surface effects which depend on the type of projectile and the target mass are included. Another feature necessary for the large energy range of our analysis is the generalization of multiple pre-equilibrium processes up to any order of particle emission. To constrain our parameterization as much as possible and to assess the predictive power of our model, we have compared it with the complete experimental data collection of ( n , x n ) , ( n , x p ) , ( p , x n ) and ( p , x p ) spectra. As a result, remaining shortcomings of our model, as well as mutually inconsistent experimental data sets are identified.

Evidence of shell effects in the excitation function of0.136‐MeVisomeric state populated in theNb93(n,2n)Nbm92reaction

Calculation for the excitation function of $0.136\text{\penalty1000-\hskip0pt}\mathrm{MeV}$ $({2}^{+})$ isomeric state in $^{92}\mathrm{Nb}$ has been carried out using Hauser-Feshbach and pre-equilibrium nuclear reaction models in $10--20\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ energy range. A satisfactory agreement between the calculation and measurements requires the suppression of contribution from the negative-parity doublet consisting of the $0.226\text{\penalty1000-\hskip0pt}\mathrm{MeV}$ $({2}^{\ensuremath{-}})$ and $0.390\text{\penalty1000-\hskip0pt}\mathrm{MeV}$ $({3}^{\ensuremath{-}})$ states in $^{92}\mathrm{Nb}$. This can be explained on the basis of highly retarded gamma transitions of higher energy states to the negative-parity doublet on account of its different shell model configuration compared to the shell model configuration of positive-parity states.

α-Particle induced reactions on rhodium

Excitation functions for103Rh (α,xn);x = 1–4 and103Rh (α,αxn);x = 1–3 reactions were measured up to 50 MeV bombarding energy using stacked foil activation technique and high purity germanium (HPGe) γ-ray spectroscopy method. The experimental results were compared with calculations considering equilibrium as well as pre-equilibrium hybrid model of Blann (ALICE/90). It is found that the initial exciton configurationn0 = 4(4p0h) gives fairly good agreements for (α,xn) reactions. There seems to be indication of direct inelastic scattering effects in (α,αxn)-type of reactions.

238U–nucleon–nucleus optical potential up to 200 MeV

Nucleon optical model potential for 238U is obtained. Rigid rotator coupled channel model reproduces experimental neutron total cross section data up to 200 MeV and produce the reaction cross section energy dependence in the range up to 200 MeV, suitable for the Hauser–Feshbach and pre-equilibrium model calculations of neutron- and proton-induced fission cross sections.