STARLIB: A Next-Generation Reaction-Rate Library for Nuclear Astrophysics

STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For the first time, this library provides the rate probability density at all temperature grid points for convenient implementation in models of stellar phenomena. The recommended rate and its associated uncertainties are also included. Currently, uncertainties are absent from all other rate libraries, and, although estimates have been attempted in previous evaluations and compilations, these are generally not based on rigorous statistical definitions. A common standard for deriving uncertainties is clearly warranted. STARLIB represents a first step in addressing this deficiency by providing a tabular, up-to-date database that supplies not only the rate and its uncertainty but also its distribution. Because a majority of rates are lognormally distributed, this allows the construction of rate probability densities from the columns of STARLIB. This structure is based on a recently suggested Monte Carlo method to calculate reaction rates, where uncertainties are rigorously defined. In STARLIB, experimental rates are supplemented with: (i) theoretical TALYS rates for reactions for which no experimental input is available, and (ii) laboratory and theoretical weak rates. STARLIB includes all types of reactions of astrophysical interest to Z = 83, such as (p,g), (p,a), (a,n), and corresponding reverse rates. Strong rates account for thermal target excitations. Here, we summarize our Monte Carlo formalism, introduce the library, compare methods of correcting rates for stellar environments, and discuss how to implement our library in Monte Carlo nucleosynthesis studies. We also present a method for accessing STARLIB on the Internet and outline updated Monte Carlo-based rates.Comment: Accepted for publication in the Astrophysical Journal Supplement Series; 96 pages, 22 figure

Gating sensory noise in a spiking subtractive LSTM

Spiking neural networks are being investigated both as biologically plausible models of neural computation and also as a potentially more efficient type of neural network. Recurrent neural networks in the form of networks of gating memory cells have been central in state-of-the-art solutions in problem domains that involve sequence recognition or generation. Here, we design an analog Long Short-Term Memory (LSTM) cell where its neurons can be substituted with efficient spiking neurons, where we use subtractive gating (following the subLSTM in [1]) instead of multiplicative gating. Subtractive gating allows for a less sensitive gating mechanism, critical when using spiking neurons. By using fast adapting spiking neurons with a smoothed Rectified Linear Unit (ReLU)-like effective activation function, we show that then an accurate conversion from an analog subLSTM to a continuous-time spiking subLSTM is possible. This architecture results in memory networks that compute very efficiently, with low average firing rates comparable to those in biological neurons, while operating in continuous time

Experimental evidence of a natural parity state in 26^{26}Mg and its impact to the production of neutrons for the s process

We have studied natural parity states in $^{26}$Mg via the $^{22}$Ne($^{6}$Li,d)$^{26}$Mg reaction. Our method significantly improves the energy resolution of previous experiments and, as a result, we report the observation of a natural parity state in $^{26}$Mg. Possible spin-parity assignments are suggested on the basis of published $\gamma$-ray decay experiments. The stellar rate of the $^{22}$Ne($\alpha$,$\gamma$)$^{26}$Mg reaction is reduced and may give rise to an increase in the production of s-process neutrons via the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction.Comment: Published in PR

Measurements of thorium and uranium daughters in radioenvironmental samples using γγ-coincidence spectrometry

We present the performance of a γγ-coincidence spectrometer for measuring the activities of thorium and uranium daughters in environmental samples. The spectrometer consists of two NaI(Tl) detectors facing each other inside a low-background passive shield. We present coincidence gating schemes for achieving the best signal-to-noise ratios, coincidence detection efficiencies, background levels, and minimum detectable activities. The spectrometer is simulated using Geant4 to correct sample efficiencies for self-absorption effects. The device is used to measure thorium and uranium daughter activities in Brazil nuts, potting mix, and magazine paper. Our results for Brazil nuts agree with some, but not all, previous measurements. Thorium or uranium daughter activities have previously not been reported for commercial potting mix. For magazine paper, our measured activities are lower than most previously determined values

Primordial Nucleosynthesis

Primordial nucleosynthesis, or Big-Bang Nucleosynthesis (BBN), is one of the three evidences for the Big-Bang model, together with the expansion of the Universe and the Cosmic Microwave Background. There is a good global agreement over a range of nine orders of magnitude between abundances of 4He, D, 3He and 7Li deduced from observations, and calculated in primordial nucleosynthesis. This comparison was used to determine the baryonic density of the Universe. For this purpose, it is now superseded by the analysis of the Cosmic Microwave Background (CMB) radiation anisotropies. However, there remain, a yet unexplained, discrepancy of a factor 3-5, between the calculated and observed lithium primordial abundances, that has not been reduced, neither by recent nuclear physics experiments, nor by new observations. We review here the nuclear physics aspects of BBN for the production of 4He, D, 3He and 7Li, but also 6Li, 9Be, 11B and up to CNO isotopes. These are, for instance, important for the initial composition of the matter at the origin of the first stars. Big-Bang nucleosynthesis, that has been used, to first constrain the baryonic density, and the number of neutrino families, remains, a valuable tool to probe the physics of the early Universe, like variation of "constants" or alternative theories of gravity.Comment: Invited Plenary Talk given at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS

Thermonuclear Reaction Rate of 23Mg(p,gamma)24$Al

Updated stellar rates for the reaction 23Mg(p,gamma)24Al are calculated by using all available experimental information on 24Al excitation energies. Proton and gamma-ray partial widths for astrophysically important resonances are derived from shell model calculations. Correspondences of experimentally observed 24Al levels with shell model states are based on application of the isobaric multiplet mass equation. Our new rates suggest that the 23Mg(p,gamma)24Al reaction influences the nucleosynthesis in the mass A>20 region during thermonuclear runaways on massive white dwarfs.Comment: 13 pages (uses Revtex) including 3 postscript figures (uses epsfig.sty), accepted for publication in Phys. Rev.

New stellar reaction rates for 25Mg(p,γ) 26Al and 25Al(p,γ) 26Si

Existing experimental proton stripping reaction data on 25Mg leading to threshold states in 26Al are reinvestigated and reanalyzed in a consistent and improved manner. We use unbound state form factors in the DWBA analysis of the measured deuteron angular distributions to deduce absolute rather than relative proton partial widths. For higher-lying resonances these values are compared to widths obtained from (p,γ) work. It is also shown that several of the unique Jπ values assigned previously to 26Al states are erroneous. This paper reports on a reanalysis of spins, parities, and isospins for 26Al states located at Ex<8.00 MeV. We deduce new stellar rates for the reaction 25Mg(p,γ)26Al and compare our results with previous values. Furthermore, shell-model calculations for the mass A = 26 system are performed. Theoretical excitation energies, Jπ values, γ-ray transition strengths, spectroscopic factors, and proton partial widths are compared to experimental data and new shell-model assignments of experimental states in 26Al are derived. We estimate Coulomb displacement energies of excited 26Mg and 26Si mirror states and present new analog assignments for T = 1 triplet states in A = 26 nuclei. Based on shell-model results and analog state information we present updated stellar rates for the 25Al(p,γ)26Si reaction

New measurement of the Eαlab =0.83 MeV resonance in Ne 22 (α,γ) Mg 26

The Eαlab=0.83 MeV resonance in the Ne22(α,γ)Mg26 reaction strongly impacts the reaction rates in the stellar temperature region crucial for the astrophysical s process. We report on a new measurement of the energy and strength of this resonance using techniques different from previous investigations. We use a blister-resistant Ne22-implanted target and employ γγ-coincidence detection techniques. We find values for the resonance energy and strength of Eαlab=835.2±3.0 keV and ωγ=(4.6±1.2)×10-5 eV, respectively. Our mean values are higher compared to previous values, although the results overlap within uncertainties. The uncertainty in the resonance energy has been significantly reduced. The spin-parity assignment, based on the present and previous work, is Jπ= (0+, 1-, 2+, 3-)

Hydrogen Burning of 29Si and Its Impact on Presolar Stardust Grains from Classical Novae

Presolar stardust grains found in primitive meteorites are believed to retain the isotopic composition of stellar outflows at the time of grain condensation. Therefore, laboratory measurements of their isotopic ratios represent sensitive probes for investigating open questions related to stellar evolution, stellar explosions, nucleosynthesis, mixing mechanisms, dust formation, and galactic chemical evolution. For a few selected presolar grains, classical novae have been discussed as a potential source. For SiC, silicate, and graphite presolar grains, the association is based on the observation of small N(12C)/N(13C) and N(14N)/N(15N) number abundance ratios compared to solar values, and abundance excesses in 30Si relative to 29Si, as previously predicted by models of classical novae. We report on a direct measurement of the 29Si(p,γ)30P reaction, which strongly impacts simulated δ 29Si values from classical novae. Our new experimental 29Si(p,γ)30P thermonuclear reaction rate differs from previous results by up to 50% in the classical nova temperature range (T = 100-400 MK), while the rate uncertainty is reduced by up to a factor of 3. Using our new reaction rate in Monte Carlo reaction network and hydrodynamic simulations of classical novae, we estimate δ 29Si values with much reduced uncertainties. Our results establish δ 29Si values measured in presolar grains as a sensitive probe for assessing their classical nova paternity. We also demonstrate that δ 30Si values from nova simulations are currently not a useful diagnostic tool unless the large uncertainty of the 30P(p,γ)31S reaction rate can be significantly reduced