Measurement of (α,n) reaction cross sections of erbium isotopes for testing astrophysical rate predictions

Date of Acceptance: 30/01/2015The γ-process in core-collapse and/or type Ia supernova explosions is thought to explain the origin of the majority of the so-called p nuclei (the 35 proton-rich isotopes between Se and Hg). Reaction rates for γ-process reaction network studies have to be predicted using Hauser-Feshbach statistical model calculations. Recent investigations have shown problems in the prediction of α-widths at astrophysical energies which are an essential input for the statistical model. It has an impact on the reliability of abundance predictions in the upper mass range of the p nuclei. With the measurement of the 164,166Er(α,n)167,169Yb reaction cross sections at energies close to the astrophysically relevant energy range we tested the recently suggested low energy modification of the α+nucleus optical potential in a mass region where γ-process calculations exhibit an underproduction of the p nuclei. Using the same optical potential for the α-width which was derived from combined 162Er(α,n) and 162Er(α,γ) measurement makes it plausible that a low-energy modification of the optical α+nucleus potential is needed.Peer reviewedFinal Accepted Versio

Parity-Dependence in the Nuclear Level Density

Astrophysical reaction rates are sensitive to the parity distribution at low excitation energies. We combine a formula for the energy-dependent parity distribution with a microscopic-macroscopic nuclear level density. This approach describes well the transition from low excitation energies, where a single parity dominates, to high excitations where the two densities are equal.Comment: 4 pages, 3 figures; contribution to Nuclei In The Cosmos VIII, to appear in Nucl. Phys.

Low-lying dipole response in the Relativistic Quasiparticle Time Blocking Approximation and its influence on neutron capture cross sections

We have computed dipole strength distributions for nickel and tin isotopes within the Relativistic Quasiparticle Time Blocking approximation (RQTBA). These calculations provide a good description of data, including the neutron-rich tin isotopes $^{130,132}$Sn. The resulting dipole strengths have been implemented in Hauser-Feshbach calculations of astrophysical neutron capture rates relevant for r-process nucleosynthesis studies. The RQTBA calculations show the presence of enhanced dipole strength at energies around the neutron threshold for neutron rich nuclei. The computed neutron capture rates are sensitive to the fine structure of the low lying dipole strength, which emphasizes the importance of a reliable knowledge of this excitation mode.Comment: 15 pages, 4 figures, Accepted in Nucl. Phys.

Neutron-induced astrophysical reaction rates for translead nuclei

Neutron-induced reaction rates, including fission, are calculated in the temperature range 1.d8 <T (K) < 1.d10 within the framework of the statistical model for targets with atomic number 83 < Z < 119 (from Po to Uuo) from the neutron to the proton drip-line. Four sets of rates have been calculated, utilizing - where possible - consistent nuclear data for neutron separation energies and fission barriers from Thomas-Fermi (TF), Extended Thomas-Fermi plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and Hartree-Fock-Bogolyubov (HFB) predictions. Tables of calculated values as well as analytic seven parameter fits in the standard REACLIB format are supplied. We also discuss the sensitivity of the rates to the input, aiming at a better understanding of the uncertainties introduced by the nuclear input.Comment: 14 pages, 10 figures, 2 tables in paper, 2 in Annex and online tables example

Assessment of novel binocular colour, motion and contrast tests in glaucoma

The effects of glaucoma on binocular visual sensitivity for the detection of various stimulus attributes are investigated at the fovea and in four paracentral retinal regions. The study employed a number of visual stimuli designed to isolate the processing of various stimulus attributes. We measured absolute contrast detection thresholds and functional contrast sensitivity by using Landolt ring stimuli. This psychophysical Landolt C-based contrast test of detection and gap discrimination allowed us to test parafoveally at 6 ° from fixation and foveally by employing interleaved testing locations. First-order motion perception was examined by using moving stimuli embedded in static luminance contrast noise. Red/green (RG) and yellow/blue (YB) colour thresholds were measured with the Colour Assessment and Diagnosis (CAD) test, which utilises random dynamic luminance contrast noise (± 45 %) to ensure that only colour and not luminance signals are available for target detection. Subjects were normal controls (n = 65) and glaucoma patients with binocular visual field defects (n = 15) classified based on their Humphrey Field Analyzer mean deviation (MD) scores. The impairment of visual function varied depending on the stimulus attribute and location tested. Progression of loss was noted for all tests as the degree of glaucoma increased. For subjects with mild glaucoma (MD −0.01 dB to −6.00 dB) significantly more data points fell outside the normal age-representative range for RG colour thresholds than for any other visual test, followed by motion thresholds. This was particularly the case for the parafoveal data compared with the foveal data. Thus, a multifaceted measure of binocular visual performance, incorporating RG colour and motion test at multiple locations, might provide a better index for comparison with quality of life measures in glaucoma

Central and Peripheral Visual Function: Effects of Age and Disease

* The overall aim of this study was to assess how the processing of different stimulus attributes in human vision is affected by ageing and disease. Both foveal and paracentral regions of the retina were investigated, with emphasis on both pregeniculate and postgeniculate impairments. Isolation of different stimulus attributes and the assessment of visual performance were carried out using a series of Advanced Vision and Optometric Tests (AVOT) developed at City University. A total of 133 normal controls and 59 patients participated in the study. Contrast detection (CT) and contrast acuity (CA) thresholds were assessed using Landolt ring stimuli. First-order motion was examined using moving stimuli embedded in static luminance contrast noise. Red/green (RG) and yellow/blue (YB) colour sensitivity were investigated using dynamic luminance contrast noise, a technique that isolates the use of colour signals. The effects of ageing and loss of visual function caused by disease were examined at the fovea and at each of four paracentral locations. Visual performance was assessed to establish how ageing and disease affect the thresholds for detection of stimulus structure, motion and colour. For each of the AVOT tests, non-parametric limits were established based on data from normal subjects to allow differentiation of the effects of ageing and disease in the patient group. All attributes tested were influenced by ageing, degeneration and disease. The results demonstrate the benefots of parafoveal and peripheral testing. Some conditions are reflected in the periphery first, encroaching on foveal vision with progressing disease. Stimulus attributes tested parafoveally enabled discovery of disease earlier than at the foveal location. * Ageing affects contrast detection thresholds (CT) differently for foveal and parafoveal locations. Data were more variable for the foveal location across the sample, indicating larger intersubject differences compared to the parafovea. In general, the ageing effects on visual performance can be described by a weak linear upwards trend in threshold as age increases. Additionally, some presumed subclinical cases were included in the sample. These subjects exhibited no signs of abnormality on standard examination but had larger CT thresholds, especially in the older age groups. * The effect of age on contrast acuity thresholds (CA) was similar for foveal and parafoveal locations. The appropriate choice of target size scaling ensured similar results with no statistically significant differences between foveal and paracentral thresholds. Data were less variable at the fovea indicating larger intersubject differences at parafoveal locations across the sample. In general, the ageing effect was accounted for by a weak linear upwards trend of CA thresholds with age, although the thresholds for presumed subclinical cases in the older subject groups departed significantly from the linear trend and increased rapidly with age. * Motion sensitivity thresholds are influenced by age beyond the 5th decade. Statistical analysis revealed that motion data can be separated into a younger (20-49.9) and an older (50-79.9) age group. Within each of these age bands motion thresholds showed no statistically significant differences. Within each group, the thresholds were similar for the five locations tested, a finding facilitated by the larger target size employed paracentrally. * Chromatic sensitivity thresholds were influenced by age beyond the 6th decade. Statistical analysis revealed that RG and YB colour data can again be separated into a younger (20-59.9) and an older (60-79.9) age band. Within each of these age bands, age effects are not statistically signficant. Foveal thresholds were, however, statistically significantly different from parafoveal locations for both RG and YB discrimination. Ageing had a greater effect on YB thresholds than on RG thresholds. RG thresholds for subjects within the older age band were significantly larger with a similar increment for all five locations tested. * The following findings were established from studies in 36 patients with pregeniculate lesions (23 with Glaucoma, 6 retinal conditions, 6 optic nerve conditions and 1 chiasmal lesion): In general, loss was usually diffuse and corresponded with the location of the visual field defect. The majority of pregeniculate patients exhibited impairment of all functions tested: CT, CA and motion were all substantially impaired and chromatic discrimination was also affected symmetrically in one or both channels (RG, YB). This pattern of loss was present within the area identified by visual field loss, where visual attributes were often not seen at the phosphor limits of the display. Some pregeniculate patients also exhibited substantial loss of all visual functions in areas where perimetric loss was largely absent. In most pregeniculate patients all quadrants revealed similar loss. In some patients the least affected quadrant exhibited normal CT or motion thresholds, but CA and colour vision were always affected to some degree. In pregeniculate patients, loss in both CT and CA was a marker of profound loss in both colour and motion. Chromatic sensitivity loss was always symmetric, frequently the RG channel was more affected than YB. The YB channel was affected more in patients with early glaucoma with more advanced disease, the RG channel was a®ected most, and finally further progression of the disease resulted in large thresholds limited by the phosphor limits of the visual display. * In 23 patients with postgeniculate lesions: the majority of those with pre-striate damage exhibited loss of all tested visual functions with symmetric chromatic impairment. Some pre-striate lesions were associated only with CA and colour loss, and in these cases the chromatic loss was symmetric and affected either one or both channels. Striate or extra-striate lesions tended to exhibit loss of CT, CA, motion and colour vision within the area identified by visual field testing. More specific loss tended to be associated with less impaired areas that were often normal on perimetric testing. Some striate or extra-striate lesions were only associated with CA and colour loss. In such cases chromatic loss was asymmetric for one or more colour categories. When striate or extra-striate lesions were also accompanied by underlying pre-striate damage, chromatic sensitivity loss was always symmetric and sometimes accompanied by CT loss. Motion was affected least in postgeniculate conditions and was always correlated with the area of densest visual field loss. The findings from this study show that the loss of chromatic sensitivity in cerebral achromatopsia varies considerably with location in the visual field. The same subject can exhibit loss of chromatic sensitivity that is either colour channel or colour category specific and such losses often affect only restricted areas of the visual field. * The findings from this investigation show how ageing processes affect the most important aspects of visual performance and provide the statistical limits needed to differentiate ageing effects from disease. The study also reveals how specific, localised damage to visual pathways can produce selective loss of visual function and how the latter varies with retinal topography. The observed variation in the processing of the same stimulus attribute with retinal location as well as the differences measured for different stimulus attributes at the same location illustrate the importance of the testing paradigm employed to reveal early onset of disease

Solving the stellar 62Ni problem with AMS

An accurate knowledge of the neutron capture cross sections of 62,63Ni is crucial since both isotopes take key positions which affect the whole reaction flow in the weak s process up to A=90. No experimental value for the 63Ni(n,gamma) cross section exists so far, and until recently the experimental values for 62Ni(n,gamma) at stellar temperatures (kT=30 keV) ranged between 12 and 37 mb. This latter discrepancy could now be solved by two activations with following AMS using the GAMS setup at the Munich tandem accelerator which are also in perfect agreement with a recent time-of-flight measurement. The resulting (preliminary) Maxwellian cross section at kT=30 keV was determined to be 30keV = 23.4 +/- 4.6 mb. Additionally, we have measured the 64Ni(gamma,n)63Ni cross section close to threshold. Photoactivations at 13.5 MeV, 11.4 MeV and 10.3 MeV were carried out with the ELBE accelerator at Forschungszentrum Dresden-Rossendorf. A first AMS measurement of the sample activated at 13.5 MeV revealed a cross section smaller by more than a factor of 2 compared to NON-SMOKER predictions.Comment: Proceedings of the 11th International Conference on Accelerator Mass Spectrometry in Rome, Sept. 14-19, 2008; to be published in Nucl. Instr. Meth.

The Path to Improved Reaction Rates for Astrophysics

This review focuses on nuclear reactions in astrophysics and, more specifically, on reactions with light ions (nucleons and alpha particles) proceeding via the strong interaction. It is intended to present the basic definitions essential for studies in nuclear astrophysics, to point out the differences between nuclear reactions taking place in stars and in a terrestrial laboratory, and to illustrate some of the challenges to be faced in theoretical and experimental studies of those reactions. The discussion revolves around the relevant quantities for astrophysics, which are the astrophysical reaction rates. The sensitivity of the reaction rates to the uncertainties in the prediction of various nuclear properties is explored and some guidelines for experimentalists are also provided.Comment: 100 pages, 33 figures, 1 table; accepted for publication in Int. J. Mod. Phys. E (scheduled for February 2011 issue); the formatting here differs in that it includes a table of contents and numbered paragraphs 5.4.2.1-5.4.2.10; v2: updated references; v3: typos fixed; v4: final typo fix, content similar to published version

Cross-section measurement of the Ba 130 (p,γ) La 131 reaction for γ -process nucleosynthesis

Background: Deviations between experimental data of charged-particle-induced reactions and calculations within the statistical model are frequently found. An extended data base is needed to address the uncertainties regarding the nuclear-physics input parameters in order to understand the nucleosynthesis of the neutron-deficient p nuclei. Purpose: A measurement of total cross-section values of the Ba130(p,γ)La131 reaction at low proton energies allows a stringent test of statistical model predictions with different proton+nucleus optical model potentials. Since no experimental data are available for proton-capture reactions in this mass region around A ≈130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials. Method: The total reaction cross-section values were measured by means of the activation method. After the irradiation with protons, the reaction yield was determined by use of γ-ray spectroscopy using two clover-type high-purity germanium detectors. In total, cross-section values for eight different proton energies could be determined in the energy range between 3.6 MeV ≤Ep≤ 5.0 MeV, thus, inside the astrophysically relevant energy region. Results: The measured cross-section values were compared to Hauser-Feshbach calculations using the statistical model codes TALYS and SMARAGD with different proton+nucleus optical model potentials. With the semimicroscopic JLM proton+nucleus optical model potential used in the SMARAGD code, the absolute cross-section values are reproduced well, but the energy dependence is too steep at the lowest energies. The best description is given by a TALYS calculation using the semimicroscopic Bauge proton+nucleus optical model potential using a constant renormalization factor. Conclusions: The statistical model calculation using the Bauge semimicroscopic proton+nucleus optical model potential deviates by a constant factor of 2.1 from the experimental data. Using this model, an experimentally supported stellar reaction rate for proton capture on the p nucleus Ba130 was calculated. At astrophysical temperatures, an increase in the stellar reaction rate of 68% compared to rates obtained from the widely used NON-SMOKER code is found. This measurement extends the scarce experimental data base for charged-particle-induced reactions, which can be helpful to derive a more globally applicable proton+nucleus optical model potential.Peer reviewedFinal Accepted Versio