arXiv subjects

G. G. Kiss

Publications and source records attributed to G. G. Kiss.

At least 55 records · Page 3Linked to original sources

arXiv2014

Alpha induced reaction cross section measurements on 162Er for the astrophysical gamma process

The cross sections of the 162Er(a,g,)166Yb and 162Er(a,n)165Yb reactions have been measured for the first time. The radiative alpha capture reaction cross section was measured from Ec.m. = 16.09 down to Ec.m. = 11.21 MeV, close to the astrophysically relevant region (which lies between 7.8 and 11.48 MeV at 3 GK stellar temperature). The 162Er(a,n)165Yb reaction was studied above the reaction threshold between Ec.m. = 12.19 and 16.09 MeV. The fact that the 162Er(a,g)166Yb cross sections were measured below the (a,n) threshold at first time in this mass region opens the opportunity to study directly the a-widths required for the determination of astrophysical reaction rates. The data clearly show that compound nucleus formation in this reaction proceeds differently than previously predicted.

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arXiv2013

Cross section and reaction rate of 92Mo(p,gamma)93Tc determined from thick target yield measurements

For the better understanding of the astrophysical gamma-process the experimental determination of low energy proton- and alpha-capture cross sections on heavy isotopes is required. The existing data for the 92Mo(p,gamma)93Tc reaction are contradictory and strong fluctuation of the cross section is observed which cannot be explained by the statistical model. In this paper a new determination of the 92Mo(p,gamma)93Tc and 98Mo(p,gamma)99mTc cross sections based on thick target yield measurements are presented and the results are compared with existing data and model calculations. Reaction rates of 92Mo(p,gamma)93Tc at temperatures relevant for the gamma-process are derived directly from the measured thick target yields. The obtained rates are a factor of 2 lower than the ones used in astrophysical network calculations. It is argued that in the case of fluctuating cross sections the thick target yield measurement can be more suited for a reliable reaction rate determination.

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arXiv2013

$^{110,116}$Cd($α,α$)$^{110,116}$Cd elastic scattering and systematic investigation of elastic $α$ scattering cross sections along the $Z$ = 48 isotopic and $N$ = 62 isotonic chains

The elastic scattering cross sections for the reactions $^{110,116}$Cd($α,α$)$^{110,116}$Cd at energies above and below the Coulomb barrier are presented to provide a sensitive test for the alpha-nucleus optical potential parameter sets. Additional constraints for the optical potential are taken from the analysis of elastic scattering excitation functions at backward angles which are available in literature. Moreover, the variation of the elastic alpha scattering cross sections along the $Z = 48$ isotopic and $N = 62$ isotonic chain is investigated by the study of the ratios of the of $^{106,110,116}$Cd($α,α$)$^{106,110,116}$Cd scattering cross sections at E$_{c.m.} \approx$ 15.6 and 18.8 MeV and the ratio of the $^{110}$Cd($α,α$)$^{110}$Cd and $^{112}$Sn($α,α$)$^{112}$Sn reaction cross sections at E$_{c.m.} \approx$ 18.8 MeV, respectively. These ratios are sensitive probes for the alpha-nucleus optical potential parameterizations. The potentials under study are a basic prerequisite for the prediction of $α$-induced reaction cross sections, e.g.\ for the calculation of stellar reaction rates in the astrophysical $p$- or $γ$-process.

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arXiv2013

High precision $^{113}$In($α,α$)$^{113}$In elastic scattering at energies around the Coulomb barrier for the astrophysical $γ$ process

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 has to be predicted in Hauser-Feshbach statistical model calculations using global optical potential parameterizations. While the nucleon+nucleus optical potential is fairly known, for the $α$+nucleus optical potential several different parameterizations exist and large deviations are found between the predictions calculated using different parameter sets. 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. Between 20$^{\circ}$ and 175$^{\circ}$ complete elastic alpha scattering angular distributions were measured on the $^{113}$In \textit{p} nucleus with high precision at E$_{c.m.}$ = 15.59 and 18.82 MeV. The elastic scattering cross sections of the $^{113}$In($α$,$α$)$^{113}$In 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. Predictions for the reaction cross sections of $^{113}$In($α,γ$)$^{117}$Sb and $^{113}$In($α$,n)$^{116}$Sb at astrophysically relevant energies were given using the global and local optical potential parameterizations.

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arXiv2013

Investigation of alpha-induced reactions on the p nucleus 168Yb

Cross sections for the 168Yb(alpha,gamma)172Hf and 168Yb(alpha,n)171$Hf reactions were measured by means of the activation method using alpha particles with energies between 12.9 MeV and 15.1 MeV. The spectroscopy of the gamma rays emitted by the reaction products was performed using three different HPGe detector types, namely clover-type high-purity germanium detectors, a low-energy photon spectrometer detector, and a coaxial high-purity germanium detector. The results were compared to Hauser-Feshbach statistical model calculations. Within certain assumptions, astrophysical conclusions could be drawn concerning the production of the p nucleus 168Yb. The data in this work can serve as a contribution to the current very fragmentary experimental data base for charged-particle induced reactions. In addition, the absolute intensity for nine gamma-ray transitions following the electron capture decay of 171Hf could be derived.

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arXiv2013

Activation measurement of the 3He(a,g)7Be reaction cross section at high energies

The astrophysically important 3He(a,g)7Be reaction was studied at high energies where the available experimental data are in contradiction. A thin window 3He gas cell was used and the cross section was measured with the activation method. The obtained cross sections at energies between Ec.m. = 1.5 and 2.5 MeV are compared with the available data and theoretical calculations. The present results support the validity of the high energy cross section energy dependence observed by recent experiments.

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arXiv2012

Elastic alpha scattering experiments and the alpha-nucleus optical potential at low energies

High precision angular distribution data of ($α$,$α$) elastic scattering are presented for the nuclei $^{89}$Y, $^{92}$Mo, $^{106,110,116}$Cd, $^{112,124}$Sn, and $^{144}$Sm at energies around the Coulomb barrier. Such data with small experimental uncertainties over the full angular range (20-170 degrees) are the indispensable prerequisite for the extraction of local optical potentials and for the determination of the total reaction cross section $σ_{\rm{reac}}$. A systematic fitting procedure was applied to the presented experimental scattering data to obtain comprehensive local potential parameter sets which are composed of a real folding potential and an imaginary potential of Woods-Saxon surface type. The obtained potential parameters were used in turn to construct a new systematic $α$-nucleus potential with very few parameters. Although this new potential cannot reproduce the angular distributions with the same small deviations as the local potential, the new potential is able to predict the total reaction cross sections for all cases under study.

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arXiv2012

Relation between total cross sections from elastic scattering and alpha-induced reactions: the example of 64Zn

The total reaction cross section is related to the elastic scattering angular distribution by a basic quantum-mechanical relation. We present new experimental data for alpha-induced reaction cross sections on 64Zn which allow for the first time the experimental verification of this simple relation at low energies by comparison of the new experimental reaction data to the result obtained from 64Zn(alpha,alpha)64Zn elastic scattering.

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arXiv2012

Investigation of alpha-induced reactions on 127I for the astrophysical gamma process

(Shorten version of the PRC abstract) Alpha-induced reactions on 127I have been studied using the activation technique in order to provide cross section data for the modeling of the astrophysical gamma process. The relative intensity of the 536.1 keV gamma transition was measured precisely, its uncertainty was reduced from 13% to 4%. By measuring the yield of the characteristic X-rays, the cross sections of the 127I(alpha,gamma)131Cs reaction have been determined for the first time close to the astrophysically relevant energy region, at energies 9.50 < Ec.m. < 15.15$ MeV. The 127I(alpha,n)130Cs reaction was studied in the range 9.62 < Ec.m. < 15.15 MeV by measuring the yield of the 536.1 keV gamma-ray and at the lower part of this energy range by counting the characteristic X-rays. A comparison of the resulting cross sections to predictions of statistical model calculations confirmed the predictions of the astrophysically relevant averaged alpha width. Nevertheless, the newly derived stellar reaction rates at gamma process temperatures for 127I(alpha,gamma)131$Cs and its reverse reaction are factors 4-10 faster than previous calculations, due to improvements in the reaction model.

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arXiv2012

Astrophysical analysis of the measurement of (alpha,gamma) and (alpha,n) cross sections of 169Tm

Reaction cross sections of 169Tm(alpha,gamma)173Lu and 169Tm(alpha,n)172Lu have been measured in the energy range 12.6<=E_alpha<=17.5 MeV and 11.5<=E_alpha<=17.5 MeV, respectively, using the recently introduced method of combining activation with X-ray counting. Improved shielding allowed to measure the (alpha,gamma) to lower energy than previously possible. The combination of (alpha,gamma) and (alpha,n) data made it possible to study the energy dependence of the alpha width. While absolute value and energy dependence are perfectly reproduced by theory at energies above 14 MeV, the observed change in energy dependence at energies below 14 MeV requires a modification of the predicted alpha width. Using an effective, energy-dependent, local optical alpha+nucleus potential it is possible to reproduce the data but the astrophysical rate is still not well constrained at gamma-process temperatures. The additional uncertainty stemming from a possible modification of the compound formation cross section is discussed. Including the remaining uncertainties, the recommended range of astrophysical reaction rate values at 2 GK is higher than the previously used values by factors of 2-37.

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arXiv2010

High precision $^{89}$Y($α$,$α$)$^{89}$Y scattering at low energies

Elastic scattering cross sections of the $^{89}$Y($α$,$α$)$^{89}$Y reaction have been measured at energies E$_{c.m.}$ = 15.51 and 18.63 MeV. The high precision data for the semi-magic $N = 50$ nucleus $^{89}$Y are used to derive a local potential and to evaluate the predictions of global and regional $α$-nucleus potentials. The variation of the elastic alpha scattering cross sections along the $N = 50$ isotonic chain is investigated by a study of the ratios of angular distributions for $^{89}$Y($α$,$α$)$^{89}$Y and $^{92}$Mo($α$,$α$)$^{92}$Mo at E$_{c.m.} \approx$ 15.51 and 18.63 MeV. This ratio is a very sensitive probe at energies close to the Coulomb barrier, where scattering data alone is usually not enough to characterize the different potentials. Furthermore, $α$-cluster states in $^{93}$Nb = $^{89}$Y $\otimes$ $α$ are investigated.

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arXiv2010

Determining reaction cross sections via characteristic X-ray detection: α-induced reactions on 169Tm for the astrophysical γ -process

The cross sections of the 169Tm(α, γ)173Lu and 169Tm(α,n)172Lu reactions have been measured first time using a new method, by detecting the characteristic X-ray radiation following the electron capturedecay of 172,173Lu. Despite the relatively long half-life of the reaction products (T1/2 = 500 and 6.7 days, respectively) it was possible to measure the cross section of the 169Tm(α,γ)173Lu reaction close to the Gamow window (T9 = 3.5 GK), between Ec.m. = 13.16 and 17.08 MeV. The 169Tm(α,n)172Lu reaction cross section was measured from Ec.m. = 11.21 MeV up to Ec.m. = 17.08 MeV. The experimental results have been compared to theoretical predictions

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arXiv2010

Total reaction cross sections from elastic alpha-nucleus scattering angular distributions around the Coulomb barrier

The total reaction cross section $σ_{\rm{reac}}$ is a valuable measure for the prediction of $α$-induced reaction cross sections within the statistical model and for the comparison of scattering of tightly bound projectiles to weakly bound and exotic projectiles. Here we provide the total reaction cross sections $σ_{\rm{reac}}$ derived from our previously published angular distributions of $α$-elastic scattering on $^{89}$Y, $^{92}$Mo, $^{112,124}$Sn, and $^{144}$Sm at energies around the Coulomb barrier.

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arXiv2010

Alpha-induced reaction cross section measurements on 151Eu for the astrophysical gamma-process

In order to extend the experimental database relevant for the astrophysical gamma-process towards the unexplored heavier mass region, the cross sections of the 151Eu(alpha,gamma)155Tb and 151Eu(alpha,n)154Tb reactions have been measured at low energies between 12 and 17 MeV using the activation technique. The results are compared with the predictions of statistical model calculations and it is found that the calculations overestimate the cross sections by about a factor of two. A sensitivity analysis shows that this discrepancy is caused by the inadequate description of the alpha+nucleus channel. A factor of two reduction of the reaction rate of 151Eu(alpha,gamma)155Tb in gamma-process network calculations with respect to theoretical rates using the optical potential by McFadden and Satchler (1966) is recommended.

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arXiv2009

Alpha-induced reactions for the astrophysical p-process: the case of 151Eu

The cross sections of 151Eu(alpha,gamma)155Tb and 151Eu(alpha,n)154Tb reactions have been measured with the activation method. Some aspects of the measurement are presented here to illustrate the requirements of experimental techniques needed to obtain nuclear data for the astrophysical p-process nucleosynthesis. Preliminary cross section results are also presented and compared with the predictions of statistical model calculations.

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arXiv2009

New High-Precision Measurement of the Reaction Rate of the 18O(p,alpha)15N Reaction via THM

The 18O(p,alpha)15N reaction rate has been extracted by means of the Trojan-Horse method. For the first time the contribution of the 20-keV peak has been directly evaluated, giving a value about 35% larger than previously estimated. The present approach has allowed to improve the accuracy of a factor 8.5, as it is based on the measured strength instead of educated guesses or spectroscopic measurements. The contribution of the 90-keV resonance has been determined as well, which turned out to be of negligible importance to astrophysics.

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arXiv2009

Odd p isotope 113In: Measurement of alpha-induced reactions

One of the few p nuclei with an odd number of protons is 113In. Reaction cross sections of 113In(alpha,gamma)117Sb and 113In(alpha,n)116Sb have been measured with the activation method at center-of-mass energies between 8.66 and 13.64 MeV, close to the astrophysically relevant energy range. The experiments were carried out at the cyclotron accelerator of ATOMKI. The activities were determined by off-line detection of the decay gamma rays with a HPGe detector. Measured cross sections and astrophysical S factor results are presented and compared with statistical model calculations using three different alpha+nucleus potentials. The comparison indicates that the standard rates used in the majority of network calculations for these reactions were too fast due to the energy dependence of the optical alpha potential at low energy.

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arXiv2009

Suppression of the stellar enhancement factor and the reaction 85Rb(p,n)85Sr

It is shown that a Coulomb suppression of the stellar enhancement factor occurs in many endothermic reactions at and far from stability. Contrary to common assumptions, reaction measurements for astrophysics with minimal impact of stellar enhancement should be preferably performed for those reactions instead of their reverses, despite of their negative reaction Q-value. As a demonstration, the cross section of the astrophysically relevant 85Rb(p,n)85Sr reaction has been measured by activation between 2.16<=E_{c.m.}<= 3.96 MeV and the astrophysical reaction rates at p-process temperatures for (p,n) as well as (n,p) are directly inferred from the data. Additionally, our results confirm a previously derived modification of a global optical proton potential. The presented arguments are also relevant for other alpha- and proton-induced reactions in the p-, rp-, and nu-p-processes.

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