arXiv subjects

F. Käppeler

Publications and source records attributed to F. Käppeler.

At least 37 records · Page 2Linked to original sources

arXiv2016

Integral measurement of the $^{12}$C(n,p)$^{12}$B reaction up to 10 GeV

The integral measurement of the $^{12}$C(n,p)$^{12}$B reaction was performed at the neutron time of flight facility n_TOF at CERN. The total number of $^{12}$B nuclei produced per neutron pulse of the n_TOF beam was determined using the activation technique in combination with a time of flight technique. The cross section is integrated over the n_TOF neutron energy spectrum from reaction threshold at 13.6 MeV to 10 GeV. Having been measured up to 1 GeV on basis of the $^{235}$U(n,f) reaction, the neutron energy spectrum above 200 MeV has been reevaluated due to the recent extension of the cross section reference for this particular reaction, which is otherwise considered a standard up to 200 MeV. The results from the dedicated GEANT4 simulations have been used to evaluate the neutron flux from 1 GeV up to 10 GeV. The experimental results related to the $^{12}$C(n,p)$^{12}$B reaction are compared with the evaluated cross sections from major libraries and with the predictions of different GEANT4 models, which mostly underestimate the $^{12}$B production. On the contrary, a good reproduction of the integral cross section derived from measurements is obtained with TALYS-1.6 calculations, with optimized parameters.

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arXiv2015

The $^{63}$Ni(n,$γ$) cross section measured with DANCE

The neutron capture cross section of the s-process branch nucleus $^{63}$Ni affects the abundances of other nuclei in its region, especially $^{63}$Cu and $^{64}$Zn. In order to determine the energy dependent neutron capture cross section in the astrophysical energy region, an experiment at the Los Alamos National Laboratory has been performed using the calorimetric 4$π$ BaF$_2$ array DANCE. The (n,$γ$) cross section of $^{63}$Ni has been determined relative to the well known $^{197}$Au standard with uncertainties below 15%. Various $^{63}$Ni resonances have been identified based on the Q-value. Furthermore, the s-process sensitivity of the new values was analyzed with the new network calculation tool NETZ.

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arXiv2015

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

We explore modifications to the current scenario for the slow neutron capture process in asymptotic giant branch (AGB) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ([Fe/H] ~ -1.2) and low initial mass (~ 1 - 1.5 Msun) in the Large and Small Magellanic Clouds. We calculated the stellar evolution and nucleosynthesis for a 1.3 Msun star with [Fe/H]=-1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the 13C-pocket and proton ingestion scenarios. No s-process models can fully reproduce the abundance patterns observed in the post-AGB stars. When the Pb production is lowered the abundances of the elements between Eu and Pb, such as Er, Yb, W, and Hf, are also lowered to below those observed. Neutron-capture processes with neutron densities intermediate between the s and the rapid neutron-capture processes may provide a solution to this problem and be a common occurrence in low-mass, low-metallicity AGB stars.

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arXiv2015

The thermal neutron capture cross section of the radioactive isotope $^{60}$Fe

50% of the heavy element abundances are produced via slow neutron capture reactions in different stellar scenarios. The underlying nucleosynthesis models need the input of neutron capture cross sections. One of the fundamental signatures for active nucleosynthesis in our galaxy is the observation of long-lived radioactive isotopes, such as $^{60}$Fe with a half-life of $2.60\times10^6$ yr. To reproduce this $γ$-activity in the universe, the nucleosynthesis of $^{60}$Fe has to be understood reliably. A $^{60}$Fe sample produced at the Paul-Scherrer-Institut was activated with thermal and epithermal neutrons at the research reactor at the Johannes Gutenberg-Universität Mainz. The thermal neutron capture cross section has been measured for the first time to $σ_{\text{th}}=0.226 \ (^{+0.044}_{-0.049})$ b. An upper limit of $σ_{\text{RI}} < 0.50$ b could be determined for the resonance integral. An extrapolation towards the astrophysicaly interesting energy regime between $kT$=10 keV and 100 keV illustrates that the s-wave part of the direct capture component can be neglected.

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arXiv2014

High accuracy determination of the $^{238}$U/$^{235}$U fission cross section ratio up to $\sim$1 GeV at n_TOF (CERN)

The $^{238}$U to $^{235}$U fission cross section ratio has been determined at n_TOF up to $\sim$1 GeV, with two different detection systems, in different geometrical configurations. A total of four datasets have been collected and compared. They are all consistent to each other within the relative systematic uncertainty of 3-4%. The data collected at n_TOF have been suitably combined to yield a unique fission cross section ratio as a function of the neutron energy. The result confirms current evaluations up to 200 MeV. A good agreement is also observed with theoretical calculations based on the INCL++/Gemini++ combination up to the highest measured energy. The n_TOF results may help solving a long-standing discrepancy between the two most important experimental dataset available so far above 20 MeV, while extending the neutron energy range for the first time up to $\sim$1 GeV.

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arXiv2014

Measurement of the $^{12}$C($n,p$)$^{12}$B cross section at n_TOF (CERN) by in-beam activation analysis

The integral cross section of the $^{12}$C($n,p$)$^{12}$B reaction has been determined for the first time in the neutron energy range from threshold to several GeV at the n_TOF facility at CERN. The measurement relies on the activation technique, with the $β$-decay of $^{12}$B measured over a period of four half-lives within the same neutron bunch in which the reaction occurs. The results indicate that model predictions, used in a variety of applications, are mostly inadequate. The value of the integral cross section reported here can be used as a benchmark for verifying or tuning model calculations.

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arXiv2014

GEANT4 simulation of the neutron background of the C$_6$D$_6$ set-up for capture studies at n_TOF

The neutron sensitivity of the C$_6$D$_6$ detector setup used at n_TOF for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n_TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a $^\mathrm{nat}$C sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured $^\mathrm{nat}$C yield has been discovered, which prevents the use of $^\mathrm{nat}$C data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two different samples are demonstrating the important role of accurate simulations of the neutron background in capture cross section measurements.

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arXiv2014

s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Source in the 13C-Pocket?

Slow neutron captures at A$\gtrsim$ 85 are mainly guaranteed by the reaction 13C($α$,n)16O in AGB stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass ($\lesssim 10^{-3}$ M$_{\odot}$), but models with rotation suggest that in such tiny layers excessive 14N hampers s-processing. Furthermore, s-element abundances in Galaxies require 13C-rich layers substantially extended in mass ($\gtrsim 4 \times 10^{-3}$ M$_{\odot}$). We therefore present new calculations aiming at clarifying those issues and at understanding if the solar composition helps to constrain the 13C "pocket" extension. We show: i) that mixing "from bottom to top" (like in magnetic buoyancy or other forced mechanisms) can form a 13C reservoir substantially larger than assumed so far, covering most of the He-rich layers; ii) that stellar models at a fixed metallicity, based on this idea reproduce the main s-component as accurately as before; iii) that they make nuclear contributions from unknown nucleosynthesis processes (LEPP) unnecessary, against common assumptions. These models also avoid problems of mixing at the envelope border and fulfil requirements from C-star luminosities. They yield a large production of nuclei below A = 100, so that 86,87Sr may be fully synthesized by AGB stars, while 88Sr, 89Y and 94Zr are contributed more efficiently than before. We finally suggest tests suitable to say a final word on the extension of the 13C pocket.

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arXiv2014

Neutron Reactions in Astrophysics

The quest for the origin of matter in the Universe had been the subject of philosophical and theological debates over the history of mankind, but quantitative answers could be found only by the scientific achievements of the last century. A first important step on this way was the development of spectral analysis by Kirchhoff and Bunsen in the middle of the 19$^{\rm th}$ century, which provided first insight in the chemical composition of the sun and the stars. The energy source of the stars and the related processes of nucleosynthesis, however, could be revealed only with the discoveries of nuclear physics. A final breakthrough came eventually with the compilation of elemental and isotopic abundances in the solar system, which are reflecting the various nucleosynthetic processes in detail. This review is focusing on the mass region above iron, where the formation of the elements is dominated by neutron capture, mainly in the slow ($s$) and rapid ($r$) processes. Following a brief historic account and a sketch of the relevant astrophysical models, emphasis is put on the nuclear physics input, where status and perspectives of experimental approaches are presented in some detail, complemented by the indispensable role of theory.

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arXiv2014

$^{62}$Ni($n,γ$) and $^{63}$Ni($n,γ$) cross sections measured at n_TOF/CERN

The cross section of the $^{62}$Ni($n,γ$) reaction was measured with the time-of-flight technique at the neutron time-of-flight facility n_TOF at CERN. Capture kernels of 42 resonances were analyzed up to 200~keV neutron energy and Maxwellian averaged cross sections (MACS) from $kT=5-100$ keV were calculated. With a total uncertainty of 4.5%, the stellar cross section is in excellent agreement with the the KADoNiS compilation at $kT=30$ keV, while being systematically lower up to a factor of 1.6 at higher stellar temperatures. The cross section of the $^{63}$Ni($n,γ$) reaction was measured for the first time at n_TOF. We determined unresolved cross sections from 10 to 270 keV with a systematic uncertainty of 17%. These results provide fundamental constraints on $s$-process production of heavier species, especially the production of Cu in massive stars, which serve as the dominant source of Cu in the solar system.

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arXiv2014

Galactic Chemical Evolution and solar s-process abundances: dependence on the 13C-pocket structure

We study the s-process abundances (A > 90) at the epoch of the solar-system formation. AGB yields are computed with an updated neutron capture network and updated initial solar abundances. We confirm our previous results obtained with a Galactic Chemical Evolution (GCE) model: (i) as suggested by the s-process spread observed in disk stars and in presolar meteoritic SiC grains, a weighted average of s-process strengths is needed to reproduce the solar s-distribution of isotopes with A > 130; (ii) an additional contribution (of about 25%) is required in order to represent the solar s-process abundances of isotopes from A = 90 to 130. Furthermore, we investigate the effect of different internal structures of the 13C-pocket, which may affect the efficiency of the 13C(a, n)16O reaction, the major neutron source of the s-process. First, keeping the same 13C profile adopted so far, we modify by a factor of two the mass involved in the pocket; second, we assume a flat 13C profile in the pocket, and we test again the effects of the variation of the mass of the pocket. We find that GCE s-predictions at the epoch of the solar-system formation marginally depend on the size and shape of the 13C-pocket once a different weighted range of 13C-pocket strengths is assumed. We ascertain that, independently of the internal structure of the 13C-pocket, the missing solar-system s-process contribution in the range from A = 90 to 130 remains essentially the same.

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arXiv2014

Experimental neutron capture data of $^{58}$Ni from the CERN n_TOF facility

The $^{58}$Ni $(n,γ)$ cross section has been measured at the neutron time of flight facility n_TOF at CERN, in the energy range from 27 meV up to 400 keV. In total, 51 resonances have been analyzed up to 122 keV. Maxwellian averaged cross sections (MACS) have been calculated for stellar temperatures of kT$=$5-100 keV with uncertainties of less than 6%, showing fair agreement with recent experimental and evaluated data up to kT = 50 keV. The MACS extracted in the present work at 30 keV is 34.2$\pm$0.6$_\mathrm{stat}\pm$1.8$_\mathrm{sys}$ mb, in agreement with latest results and evaluations, but 12% lower relative to the recent KADoNIS compilation of astrophysical cross sections. When included in models of the s-process nucleosynthesis in massive stars, this change results in a 60% increase of the abundance of $^{58}$Ni, with a negligible propagation on heavier isotopes. The reason is that, using both the old or the new MACS, 58Ni is efficiently depleted by neutron captures.

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arXiv2013

A 4p BaF2 detector for (n,g) cross section measurements at a spallation neutron source

The quest for improved neutron capture cross sections for advanced reactor concepts, transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis has motivated new experimental efforts based on modern techniques. Recent measurements in the keV region have shown that a 4p BaF2 detector represents an accurate and versatile instrument for such studies. The present work deals with the potential of such a 4p BaF2 detector in combination with spallation neutron sources, which offer large neutron fluxes over a wide energy range. Detailed Monte Carlo simulations with the GEANT package have been performed to investigate the critical backgrounds at a spallation facility, to optimize the detector design, and to discuss alternative solutions.

physics.ins-det↗

arXiv2013

Measurement of the 92,93,94,100Mo(g,n) reactions by Coulomb Dissociation

The Coulomb Dissociation (CD) cross sections of the stable isotopes 92,94,100Mo and of the unstable isotope 93Mo were measured at the LAND/R3B setup at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Experimental data on these isotopes may help to explain the problem of the underproduction of 92,94Mo and 96,98Ru in the models of p-process nucleosynthesis. The CD cross sections obtained for the stable Mo isotopes are in good agreement with experiments performed with real photons, thus validating the method of Coulomb Dissociation. The result for the reaction 93Mo(g,n) is especially important since the corresponding cross section has not been measured before. A preliminary integral Coulomb Dissociation cross section of the 94Mo(g,n) reaction is presented. Further analysis will complete the experimental database for the (g,n) production chain of the p-isotopes of molybdenum.

astro-ph.IM↗

arXiv2013

Neutron activation of natural zinc samples at kT = 25 keV

The neutron-capture cross sections of 64Zn, 68Zn, and 70Zn have been measured with the activation technique in a quasistellar neutron spectrum corresponding to a thermal energy of kT = 25 keV. By a series of repeated irradiations with different experimental conditions, an uncertainty of 3% could be achieved for the 64Zn(n,g)65Zn cross section and for the partial cross section 68Zn(n,g)69Zn-m feeding the isomeric state in 69Zn. For the partial cross sections 70Zn(n,g)71Zn-m and 70Zn(n,g)71Zn-g, which had not been measured so far, uncertainties of only 16% and 6% could be reached because of limited counting statistics and decay intensities. Compared to previous measurements on 64,68Zn, the uncertainties could be significantly improved, while the 70Zn cross section was found to be two times smaller than existing model calculations. From these results Maxwellian average cross sections were determined between 5 and 100 keV. Additionally, the beta-decay half-life of 71Zn-m could be determined with significantly improved accuracy. The consequences of these data have been studied by network calculations for convective core He burning and convective shell C burning in massive stars.

astro-ph.IM↗

arXiv2013

PINO - a tool for simulating neutron spectra resulting from the 7Li(p,n) reaction

The 7Li(p,n) reaction in combination with a 3.7 MV Van de Graaff accelerator was routinely used at FZK to perform activation as well as time-of-flight measurements with neutrons in the keV-region. Planned new setups with much higher proton currents like SARAF and FRANZ and the availability of liquid-lithium target technology will trigger a renaissance of this method. A detailed understanding of the neutron spectrum is not only important during the planning phase of an experiment, but also during for the analysis of activation experiments. Therefore, the Monte-Carlo based program PINO (Protons In Neutrons Out) was developed, which allows the simulation of neutron spectra considering the geometry of the setup and the proton-energy distribution.

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arXiv2013

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis

The $^{63}$Ni($n, γ$) cross section has been measured for the first time at the neutron time-of-flight facility n\_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian Averaged Cross Sections were calculated for thermal energies from kT = 5 keV to 100 keV with uncertainties around 20%. Stellar model calculations for a 25 M$_\odot$ star show that the new data have a significant effect on the $s$-process production of $^{63}$Cu, $^{64}$Ni, and $^{64}$Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

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arXiv2011

Cross sections for proton-induced reactions on Pd isotopes at energies relevant for the gamma process

Proton-activation reactions on natural and enriched palladium samples were investigated via the activation technique in the energy range of E_p=2.75 MeV to 9 MeV, close to the upper end of the respective Gamow window of the gamma process. We have determined cross sections for 102Pd(p,gamma)103Ag, 104Pd(p,gamma)105Ag, and 105Pd(p,n)105Ag, as well as partial cross sections of 104Pd(p,n)104Ag^g, 105Pd(p,gamma)106Ag^m, 106Pd(p,n)106Ag^m, and 110Pd(p,n)110Ag^m with uncertainties between 3% and 15% for constraining theoretical Hauser-Feshbach rates and for direct use in gamma-process calculations.

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