Experimental determination of the 3^3He(α\alpha,γ\gamma)7^7Be reaction cross section above the 7^7Be proton separation threshold

The $^3$He($\alpha$,$\gamma$)$^7$Be reaction plays a major role both in the BBN producing the majority of the primordial $^7$Li, and in the pp-chain, where it is the branching point. As a few-nucleon system, this reaction is often used to validate ab-initio theoretical calculations and/or test R-matrix theory and code implementations. For the latter, experimental data in an extended energy range is of crucial importance to test the fit and extrapolation capabilities of the different codes. The $^3$He($\alpha$,$\gamma$)$^7$Be reaction cross section has been measured by several groups up to the first resonance ($E_{c.m.} \approx 3$ MeV) in the reaction. However, only one dataset exists above the $^7$Be proton separation threshold measured in a narrow energy range ($E_{c.m.} = 4.0-4.4$ MeV). In this work we extend the available experimental capture cross section database to the energy range of known $^7$Be levels. The activation method was used. The experiment was performed using a thin-window gas cell with two high-purity Al foils as entrance and exit windows. The activity of the $^7$Be nuclei implanted in the exit/catcher foil was measured by detecting the yield of the emitted $\gamma$~rays using shielded high-purity germanium detectors. New experimental $^3$He($\alpha$,$\gamma$)$^7$Be reaction cross section data were obtained for the first time in the $E_{c.m.}=4.3-8.3$ MeV energy region. The new dataset with about 0.2 MeV step covers the energy range of known levels and particle separation thresholds. No prominent structures are observer around the $^7$Be levels. The reaction cross section is slowly increasing with increasing energy. Above the $^6$Li$+p_1$ threshold, a decrease starts in the cross section trend. The overall structure of the cross section suggest a broad resonance peaking around $E_x=7.5$ MeV $^7$Be excitation energy, with a width of 8 MeV.Comment: Accepted for publication in PR

Consistency and diversity of spike dynamics in the neurons of bed nucleus of Stria Terminalis of the rat: a dynamic clamp study

Neurons display a high degree of variability and diversity in the expression and regulation of their voltage-dependent ionic channels. Under low level of synaptic background a number of physiologically distinct cell types can be identified in most brain areas that display different responses to standard forms of intracellular current stimulation. Nevertheless, it is not well understood how biophysically different neurons process synaptic inputs in natural conditions, i.e., when experiencing intense synaptic bombardment in vivo. While distinct cell types might process synaptic inputs into different patterns of action potentials representing specific "motifs'' of network activity, standard methods of electrophysiology are not well suited to resolve such questions. In the current paper we performed dynamic clamp experiments with simulated synaptic inputs that were presented to three types of neurons in the juxtacapsular bed nucleus of stria terminalis (jcBNST) of the rat. Our analysis on the temporal structure of firing showed that the three types of jcBNST neurons did not produce qualitatively different spike responses under identical patterns of input. However, we observed consistent, cell type dependent variations in the fine structure of firing, at the level of single spikes. At the millisecond resolution structure of firing we found high degree of diversity across the entire spectrum of neurons irrespective of their type. Additionally, we identified a new cell type with intrinsic oscillatory properties that produced a rhythmic and regular firing under synaptic stimulation that distinguishes it from the previously described jcBNST cell types. Our findings suggest a sophisticated, cell type dependent regulation of spike dynamics of neurons when experiencing a complex synaptic background. The high degree of their dynamical diversity has implications to their cooperative dynamics and synchronization

High precision 113In(α,α)113In elastic scattering at energies near the Coulomb barrier for the astrophysical γ process

Background: The γ process in supernova explosions is thought to explain the origin of proton-rich isotopes between Se and Hg, the so-called p nuclei. The majority of the reaction rates for γ process reaction network studies have to be predicted in Hauser-Feshbach statistical model calculations using global optical potential parametrizations. While the nucleon+nucleus optical potential is fairly well known, for the α+nucleus optical potential several different parametrizations exist and large deviations are found between the predictions calculated using different parameter sets.Purpose: By the measurement of elastic α-scattering angular distributions at energies around the Coulomb barrier a comprehensive test for the different global α+nucleus optical potential parameter sets is provided. Methods: Between 20∘ and 175∘ complete elastic alpha scattering angular distributions were measured on the 113In p nucleus with high precision at Ec.m.=15.59 and 18.82 MeV. Results: The elastic scattering cross sections of the 113In(α,α)113In reaction were measured for the first time at energies close to the astrophysically relevant energy region. The high precision experimental data were used to evaluate the predictions of the recent global and regional α+nucleus optical potentials. Parameters for a local α+nucleus optical potential were derived from the measured angular distributions. Conclusions: Predictions for the reaction cross sections of 113In(α,γ)117Sb and 113In(α,n)116Sb at astrophysically relevant energies were given using the global and local optical potential parametrizations.Peer reviewe

Investigation of α -induced reactions on Sb isotopes relevant to the astrophysical γ process

This document is the Accepted Manuscript version of the following article: Z. Korkulu, et al, ‘Investigation of α-induced reactions on Sb isotopes relevant to the astrophysical γ process’, Physical Review C, Vol. 97(4): 045803, April 2018, available online at DOI: https://doi.org/10.1103/PhysRevC.97.045803 © 2018 American Physical Society.Background: The reaction rates used in γ-process nucleosynthesis network calculations are mostly derived from theoretical, statistical model cross sections. Experimental data is scarce for charged particle reactions at astrophysical, low energies. Where experimental (α,γ) data exists, it is often strongly overestimated by Hauser-Feshbach statistical model calculations. Further experimental α-capture cross sections in the intermediate and heavy mass region are necessary to test theoretical models and to gain understanding of heavy element nucleosynthesis in the astrophysical γ process. Purpose: The aim of the present work is to measure the Sb121(α,γ)I125, Sb121(α,n)I124, and Sb123(α,n)I126 reaction cross sections. These measurements are important tests of astrophysical reaction rate predictions and extend the experimental database required for an improved understanding of p-isotope production. Method: The α-induced reactions on natural and enriched antimony targets were investigated using the activation technique. The (α,γ) cross sections of Sb121 were measured and are reported for the first time. To determine the cross section of the Sb121(α,γ)I125, Sb121(α,n)I124, and Sb123(α,n)I126 reactions, the yields of γ rays following the β decay of the reaction products were measured. For the measurement of the lowest cross sections, the characteristic x rays were counted with a low-energy photon spectrometer detector. Results: The cross section of the Sb121(α,γ)I125, Sb121(α,n)I124, and Sb123(α,n)I126 reactions were measured with high precision in an energy range between 9.74 and 15.48 MeV, close to the astrophysically relevant energy window. The results are compared with the predictions of statistical model calculations. The (α,n) data show that the α widths are predicted well for these reactions. The (α,γ) results are overestimated by the calculations but this is because of the applied neutron and γ widths. Conclusions: Relevant for the astrophysical reaction rate is the α width used in the calculations. While for other reactions the α widths seem to have been overestimated and their energy dependence was not described well in the measured energy range, this is not the case for the reactions studied here. The result is consistent with the proposal that additional reaction channels, such as Coulomb excitation, may have led to the discrepancies found in other reactions.Peer reviewe

Autonomous Bursting in a Homoclinic System

A continuous train of irregularly spaced spikes, peculiar of homoclinic chaos, transforms into clusters of regularly spaced spikes, with quiescent periods in between (bursting regime), by feeding back a low frequency portion of the dynamical output. Such autonomous bursting results to be extremely robust against noise; we provide experimental evidence of it in a CO2 laser with feedback. The phenomen here presented display qualitative analogies with bursting phenomena in neurons.Comment: Submitted to Phys. Rev. Lett., 14 pages, 5 figure

Synchronous Behavior of Two Coupled Electronic Neurons

We report on experimental studies of synchronization phenomena in a pair of analog electronic neurons (ENs). The ENs were designed to reproduce the observed membrane voltage oscillations of isolated biological neurons from the stomatogastric ganglion of the California spiny lobster Panulirus interruptus. The ENs are simple analog circuits which integrate four dimensional differential equations representing fast and slow subcellular mechanisms that produce the characteristic regular/chaotic spiking-bursting behavior of these cells. In this paper we study their dynamical behavior as we couple them in the same configurations as we have done for their counterpart biological neurons. The interconnections we use for these neural oscillators are both direct electrical connections and excitatory and inhibitory chemical connections: each realized by analog circuitry and suggested by biological examples. We provide here quantitative evidence that the ENs and the biological neurons behave similarly when coupled in the same manner. They each display well defined bifurcations in their mutual synchronization and regularization. We report briefly on an experiment on coupled biological neurons and four dimensional ENs which provides further ground for testing the validity of our numerical and electronic models of individual neural behavior. Our experiments as a whole present interesting new examples of regularization and synchronization in coupled nonlinear oscillators.Comment: 26 pages, 10 figure

Dynamical principles in neuroscience

Dynamical modeling of neural systems and brain functions has a history of success over the last half century. This includes, for example, the explanation and prediction of some features of neural rhythmic behaviors. Many interesting dynamical models of learning and memory based on physiological experiments have been suggested over the last two decades. Dynamical models even of consciousness now exist. Usually these models and results are based on traditional approaches and paradigms of nonlinear dynamics including dynamical chaos. Neural systems are, however, an unusual subject for nonlinear dynamics for several reasons: (i) Even the simplest neural network, with only a few neurons and synaptic connections, has an enormous number of variables and control parameters. These make neural systems adaptive and flexible, and are critical to their biological function. (ii) In contrast to traditional physical systems described by well-known basic principles, first principles governing the dynamics of neural systems are unknown. (iii) Many different neural systems exhibit similar dynamics despite having different architectures and different levels of complexity. (iv) The network architecture and connection strengths are usually not known in detail and therefore the dynamical analysis must, in some sense, be probabilistic. (v) Since nervous systems are able to organize behavior based on sensory inputs, the dynamical modeling of these systems has to explain the transformation of temporal information into combinatorial or combinatorial-temporal codes, and vice versa, for memory and recognition. In this review these problems are discussed in the context of addressing the stimulating questions: What can neuroscience learn from nonlinear dynamics, and what can nonlinear dynamics learn from neuroscience?This work was supported by NSF Grant No. NSF/EIA-0130708, and Grant No. PHY 0414174; NIH Grant No. 1 R01 NS50945 and Grant No. NS40110; MEC BFI2003-07276, and Fundación BBVA

Test of statistical model cross section calculations for α -induced reactions on Ag 107 at energies of astrophysical interest

Background: Astrophysical reaction rates, which are mostly derived from theoretical cross sections, are necessary input to nuclear reaction network simulations for studying the origin of p nuclei. Past experiments have found a considerable difference between theoretical and experimental cross sections in some cases, especially for (α,γ) reactions at low energy. Therefore, it is important to experimentally test theoretical cross section predictions at low, astrophysically relevant energies. Purpose: The aim is to measure reaction cross sections of Ag107(α,γ)In111 and Ag107(α,n)In110 at low energies in order to extend the experimental database for astrophysical reactions involving α particles towards lower mass numbers. Reaction rate predictions are very sensitive to the optical model parameters and this introduces a large uncertainty into theoretical rates involving α particles at low energy. We have also used Hauser-Feshbach statistical model calculations to study the origin of possible discrepancies between prediction and data. Method: An activation technique has been used to measure the reaction cross sections at effective center of mass energies between 7.79 MeV and 12.50 MeV. Isomeric and ground state cross sections of the (α,n) reaction were determined separately. Results: The measured cross sections were found to be lower than theoretical predictions for the (α,γ) reaction. Varying the calculated averaged widths in the Hauser-Feshbach model, it became evident that the data for the (α,γ) and (α,n) reactions can only be simultaneously reproduced when rescaling the ratio of γ to neutron width and using an energy-dependent imaginary part in the optical α+Ag107 potential. Conclusions: The new data extend the range of measured charged-particle cross sections for astrophysical applications to lower mass numbers and lower energies. The modifications in the model predictions required to reproduce the present data are fully consistent with what was found in previous investigations. Thus, our results confirm the previously suggested energy-dependent modification of the optical α+nucleus potential.Peer reviewedFinal Accepted Versio

Approaching the Gamow Window with Stored Ions : Direct Measurement of Xe 124 (p,γ) in the ESR Storage Ring

© 2019 American Physical Society. All rights reserved.We report the first measurement of low-energy proton-capture cross sections of Xe124 in a heavy-ion storage ring. Xe12454+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The Cs125 reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.Peer reviewedFinal Published versio

Cross section of α\alpha-induced reactions on 197^{197}Au at sub-Coulomb energies

Statistical model calculations have to be used for the determination of reaction rates in large-scale reaction networks for heavy-element nucleosynthesis. A basic ingredient of such a calculation is the a-nucleus optical model potential. Several different parameter sets are available in literature, but their predictions of a-induced reaction rates vary widely, sometimes even exceeding one order of magnitude. This paper presents the result of a-induced reaction cross-section measurements on gold which could be carried out for the first time very close to the astrophysically relevant energy region. The new experimental data are used to test statistical model predictions and to constrain the a-nucleus optical model potential. For the measurements the activation technique was used. The cross section of the (a,n) and (a,2n) reactions was determined from g-ray counting, while that of the radiative capture was determined via X-ray counting. The cross section of the reactions was measured below E$_a=20.0$~MeV. In the case of the $^{197}$Au(a,2n)$^{199}$Tl reaction down to 17.5~MeV with 0.5-MeV steps, reaching closer to the reaction threshold than ever before. The cross section of $^{197}$Au(a,n)$^{200}$Tl and $^{197}$Au(a,g)$^{201}$Tl was measured down to E$_a=13.6$ and 14.0~MeV, respectively, with 0.5-MeV steps above the (a,2n) reaction threshold and with 1.0-MeV steps below that. The new dataset is in agreement with the available values from the literature, but is more precise and extends towards lower energies. Two orders of magnitude lower cross sections were successfully measured than in previous experiments which used g-ray counting only, thus providing experimental data at lower energies than ever before. The new precision dataset allows us to find the best-fit a-nucleus optical model potential and to predict cross sections in the Gamow window with smaller uncertainties.Comment: Accepted for publication in Phys. Rev.