Non-statistical Processes Research Articles

Application of Tube Dynamics to Non-Statistical Reaction Processes

A technique based on dynamical systems theory is introduced for the computation of lifetime distributions and rates of chemical reactions and scattering phenomena, even in systems that exhibit non-statistical behavior. In particular, we merge invariant manifold tube dynamics with Monte Carlo volume determination for accurate rate calculations. This methodology is applied to a three-degree-of-freedom model problem and some ideas on how it might be extended to higher-degree-of-freedom systems are presented.

Photodestruction of adenosine 5'-monophosphate (AMP) nucleotide ions in vacuo: statistical versus nonstatistical processes.

The lifetimes of both deprotonated adenosine 5(')-monophosphate (AMP) and protonated AMP, after 266-nm photon absorption and intramolecular vibrational redistribution, were determined to be 16 micros based on a newly developed heavy-ion storage ring technique. Protonation of the adenine nucleobase seriously affects the fragmentation mechanism: the major part of the photoexcited anions fragment in a statistical (ergodic) process, whereas nearly all the cations (>99%) fragment in a nonergodic process. In solution at natural pH the AMP anion is prevalent and photoproduct formation is therefore less important as the time for vibrational cooling is of the order of picoseconds.

Reaction mechanism leading to angular momentum saturation in the 28Si+ 12C reaction

Evaporation residues and evaporated light particles were measured for the 28Si+ 12C reaction to separate out the contribution of statistical and non-statistical processes involved in the decay of a hot rotating compound nucleus. Saturation of angular momentum l cr for a mass-40 system is discussed and compared with the established theoretical predictions to understand the reaction mechanism responsible for it. The role of very asymmetric channels in the dynamical evolution of a fused system is also discussed.

Competition between adsorbate fragmentation and desorption on laser-heated surfaces

We present a theoretical study of the consequences of rapid laser heating of a model adsorbate–substrate system. The adsorbate in this study contains one weak bond and a number of strong bonds. The impetus for our theoretical model comes from the desorption/dissociation experiments on labile organic molecules adsorbed on heated surfaces. Among the numerous interesting results of these experiments are nonstatistical processes where depending on the heating rate, stronger bonds break before the weaker ones. We model these processes by means of a stochastic trajectory approach using the generalized Langevin method. Results are given for model polyatomics of different lengths and configurations as well as different heating rates. The strengths of the weak bonds inside the molecule are changed and reactivities of adsorbates in parallel and perpendicular adsorption geometries are contrasted. The branching ratio between desorption and dissociation is found to depend sensitively on a number of these variables. We also find that small changes in the relative bond strengths of the weak bonds can change the reactivity from statistical to nonstatistical.

Neutron decay of the giant resonance region in 124Sn and especially of the giant monopole resonance

The decay of the isoscalar giant monopole and quadrupole resonances and of the underlying continuum in 124Sn has been studied with the 124Sn(α, α'n) 123Sn reaction. The data are compared with calculations based on the statistical model to extract the non-statistical decay component. The giant monopole resonance (GMR) decay mode is predominantly statistical; non-statistical processes account for 15% of the decay. Possible evidence for a pre-equilibrium decay branch of the GMR between 5% and 10% was found. The non-statistical fraction of the decay of the GMR decreases with increasing excitation energy. The contribution of neutron knock-out was found to be in qualitative agreement with PWIA calculations.

Strong nonstatistical effects in neutron capture at the 2p size resonance region.

This paper investigates the nonstatistical effects in radiative neutron capture cross sections for nuclides at the 2p size resonance region of neutron strength function, specifically /sup 27/Al(n,..gamma..) and /sup 28/Si(n,..gamma..) reactions. The neutron energies studied range from thermal up to 2 MeV. The calculated results show that in comparison with medium and heavy nuclei, the nonstatistical fractions in the total capture cross sections for the two lighter nuclides are extremely large. In the thermal energy region, the fractions are about 50%. In the resonance energy region, the averaged contributions from the nonstatistical processes are considerably larger than that from the statistical mechanism. The physical significance of the results was briefly discussed. It was also indicated that this conclusion is qualitatively consistent with the fact that the measured capture data show significant correlations in both nuclides.

GRS: A statistical and non-statistical model code for calculations of cross sections and gamma-ray spectra

Abstract This paper briefs the non-statistical processes in neutron radiative capture reactions such as potential-valence captures, direct-semidirect captures, and captures in the compound elastic and inelastic channels, their physical significances and calculational expressions. Then a code GRS to calculate gamma-ray production data resulting from neutron induced reactions as well as relevant cross sections is described. In addition to multistep Hauser-Feshbach model, special attention has been paid to the calculation of higher energy gamma-rays, mainly produced by the non-statistical processes.

Gamma rays from fast neutron capture in silicon and sulphur

Gamma-ray spectra from neutron capture in natural samples of silicon and sulphur have been recorded at several neutron energies between 3.2 and 15 MeV. Time-of-flight techniques have been used to improve the signal-to-background ratio and the γ-radiation has been detected by a large NaI(Tl) scintillator. Cross sections have been determined for transitions to individual (or groups of) levels in the final nuclei. Calculations based on the direct-semidirect model show that this model fails to describe the observed energy dependence of the cross sections in the low-energy range of the giant dipole resonance. The inclusion of the compound-nucleus capture process gives a conclusive improvement in this energy region. A discrepancy in magnitude between observed and calculated cross sections for the reaction 32S(n, γ) 33S still persists. Furthermore, the experimental results for 28Si(n, γ) 29Si show considerable fluctuations which might be attributed to other non-statistical capture processes.

The 24Mg( α, α') 24Mg reaction; statistical analysis

The 24Mg( α, α') 24Mg reaction to the 1.37 MeV state has been studied over an α-energy range of 9.625–13.825 MeV, in ≈40 keV steps, and over a 25°–160° angular interval. These cross sections have been analysed in terms of statistical theory and a number of deviations from its predictions are found. These deviations point to the importance of non-statistical processes, such as intermediate structure, to the α-scattering. The average compound nuclear width o 1 62±7 keV is found for 28Si over the 18–22 MeV excitation.

Radiative capture mechanisms in heavy nuclei

The known cross section data for radiative capture in heavy nuclei of 5–50 MeV nucleons and composite projectiles are summarized and the probable reaction mechanisms involved are discussed. Almost all the (α, γ) and (d, γ) results are shown to be in reasonable agreement with statistical theory predictions, especially when corrections for multiple γ-ray cascades and angular momentum effects are included. In particular, the variation of over two orders of magnitude in the reported (α, γ) maximum cross-sections are correctly predicted by statistical theory. To allow for the different nuclear penetrabilities of the various projectiles, each of the experimental capture cross section values has been divided by the appropriate optical model total reaction cross-section. The results provide strong support for the proposal that nucleon capture proceeds predominantly by direct or collective reaction mechanisms. Evidence that non-statistical processes make important contributions in the case of 3He ion capture is also presented. It is concluded that there is no current evidence for mechanisms other than compound nucleos formation and decay in the radiative capture of deuterons and α-particles and that the projectile neutron/proton ratio has a marked effect on the probability of radiative capture.

Hyperon and kaon production by 16 GeV/c negative pions on protons

Strange particle production by 16 GeV/c negative pions in hydrogen has been studied. 60 000 pictures were taken with the CERN 32 cm hydrogen bubble chamber. The cross sections found for associated and K-K pair production are σyk= 1.32±0.15 mb, σk-k=2.21±0.25 mb. The angular distributions of the Λ0 and Σ+ hyperons show a pronounced backward peaking in the pion-proton c.m.s. The backward peak is less pronounced for Σ(−. The ratio Σ+/Σ− is 1.3 ±0.3 at production. The neutral kaons are significantly peaked forward. The average longitudinal momentum in the c.m.s. is 0.26±0.05 GeV/c along the direction of the incident pion. The average transverse momentum is small, ∼ 0.5 GeV/c, and possibly independent of the kind of particle produced. Our results indicate a higher mean value for Σ± than for Λ0 and K0. The average pion multiplicities are: for Y + K production 4.9 ±0.1, for K + -K production 4.0±0.1. The kinetic energy distributions have been compared with the predictions of the statistical theory. The contribution of nonstatistical processes is important. Different estimates give 30% as a lower limit.