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T. Rauscher

Publications and source records attributed to T. Rauscher.

At least 37 records · Page 2Linked to original sources
arXiv2018

The s-process nucleosynthesis in low mass stars: impact of the uncertainties in the nuclear physics determined by Monte Carlo variations

We investigated the impact of uncertainties in neutron-capture and weak reactions (on heavy elements) on the s-process nucleosynthesis in low-mass stars using a Monte-Carlo based approach. We performed extensive nuclear reaction network calculations that include newly evaluated temperature-dependent upper and lower limits for the individual reaction rates. Our sophisticated approach is able to evaluate the reactions that impact more significantly the final abundances. We found that beta-decay rate uncertainties affect typically nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are approximately 50%.

astro-ph.SR↗
arXiv2018

Formalism for inclusion of measured reaction cross sections in stellar rates including uncertainties and its application to neutron capture in the s-process

A general formalism to include experimental reaction cross sections into calculations of stellar rates is presented. It also allows to assess the maximally possible reduction of uncertainties in the stellar rates by experiments. As an example for the application of the procedure, stellar neutron capture reactivities from KADoNiS v0.3 are revised and the remaining uncertainties shown. Many of the uncertainties in the stellar rates are larger than those obtained experimentally. This has important consequences for s-process models and the interpretation of meteoritic data because it allows the rates of some reactions to vary within a larger range than previously assumed.

astro-ph.SR↗
arXiv2018

Women Scientists Who Made Nuclear Astrophysics

Female role models reduce the impact on women of stereotype threat, i.e., of being at risk of conforming to a negative stereotype about one's social, gender, or racial group. This can lead women scientists to underperform or to leave their scientific career because of negative stereotypes such as, not being as talented or as interested in science as men. Sadly, history rarely provides role models for women scientists; instead, it often renders these women invisible. In response to this situation, we present a selection of twelve outstanding women who helped to develop nuclear astrophysics.

physics.hist-ph↗
arXiv2018

Cross section measurements of $^{155,157}$Gd(n,$γ$) induced by thermal and epithermal neutrons

Neutron capture measurements on $^{155}$Gd and $^{157}$Gd were performed using the time-of-flight technique at the n\_TOF facility at CERN. Four samples in form of self-sustaining metallic discs isotopically enriched in $^{155}$Gd and $^{157}$Gd were used. The measurements were carried out at the experimental area (EAR1) at 185 m from the neutron source, with an array of 4 C$_6$D$_6$ liquid scintillation detectors. The capture cross sections of $^{155}$Gd and $^{157}$Gd at neutron kinetic energy of 0.0253 eV have been estimated to be 62.2(2.2) kb and 239.8(9.3) kb, respectively, thus up to 6\% different relative to the ones reported in the nuclear data libraries. A resonance shape analysis has been performed in the resolved resonance region up to 180 eV and 300 eV, respectively, in average resonance parameters have been found in good agreement with evaluations. Above these energies the observed resonance-like structures in the cross section have been tentatively characterised in terms of resonance energy and area up to 1 keV.

nucl-ex↗
arXiv2018

Uncertainties in s-process nucleosynthesis in low mass stars determined from Monte Carlo variations

The main s-process taking place in low mass stars produces about half of the elements heavier than iron. It is therefore very important to determine the importance and impact of nuclear physics uncertainties on this process. We have performed extensive nuclear reaction network calculations using individual and temperature-dependent uncertainties for reactions involving elements heavier than iron, within a Monte Carlo framework. Using this technique, we determined the uncertainty in the main s-process abundance predictions due to nuclear uncertainties link to weak interactions and neutron captures on elements heavier than iron. We also identified the key nuclear reactions dominating these uncertainties. We found that $β$-decay rate uncertainties affect only a few nuclides near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in neutron capture rates, either directly producing or destroying the nuclide of interest. Combined total nuclear uncertainties due to reactions on heavy elements are in general small (less than 50%). Three key reactions, nevertheless, stand out because they significantly affect the uncertainties of a large number of nuclides. These are $^{56}$Fe(n,$γ$), $^{64}$Ni(n,$γ$), and $^{138}$Ba(n,$γ$). We discuss the prospect of reducing uncertainties in the key reactions identified in this study with future experiments.

astro-ph.SR↗
arXiv2018

Photonuclear Reactions in Astrophysics

A brief overview of the importance of photodisintegration reactions in astrophysical environments is given and the relevance of photonuclear experiments for nucleosynthesis studies is discussed.

astro-ph.HE↗
arXiv2018

Mass Measurements of Neutron-Deficient Y, Zr, and Nb Isotopes and Their Impact on $rp$ and $νp$ Nucleosynthesis Processes

Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $α$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $νp$-process simulations.

nucl-ex↗
arXiv2018

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

The aim of the present work is to measure the $^{121}$Sb($α,γ$)$^{125}$I, $^{121}$Sb($α$,n)$^{124}$I, and $^{123}$Sb($α$,n)$^{126}$I reaction cross sections. The $α$-induced reactions on natural and enriched antimony targets were investigated using the activation technique. The ($α$,$γ$) cross sections of $^{121}$Sb were measured and are reported for first time. To determine the cross section of the $^{121}$Sb($α$,$γ$)$^{125}$I, $^{121}$Sb($α$,n)$^{124}$I, and $^{123}$Sb($α$,n)$^{126}$I 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 LEPS (Low Energy Photon Spectrometer) detector. The cross section of the $^{121}$Sb($α$,$γ$)$^{125}$I, $^{121}$Sb($α$,n)$^{124}$I and $^{123}$Sb($α$,n)$^{126}$I reactions were measured with high precision in an energy range between 9.74 MeV to 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 due to the applied neutron- and $γ$ widths. 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.

nucl-ex↗
arXiv2018

Sensitivity to neutron captures and beta-decays of the enhanced s-process in rotating massive stars at low metallicities

The s-process in massive stars, producing nuclei up to $A\approx 90$, has a different behaviour at low metallicity if stellar rotation is significant. This enhanced s-process is distinct from the s-process in massive stars around solar metallicity, and details of the nucleosynthesis are poorly known. We investigated nuclear physics uncertainties in the enhanced s-process in metal-poor stars within a Monte-Carlo framework. We applied temperature-dependent uncertainties of reaction rates, distinguishing contributions from the ground state and from excited states. We found that the final abundance of several isotopes shows uncertainties larger than a factor of 2, mostly due to the neutron capture uncertainties. A few nuclei around branching points are affected by uncertainties in the $β$-decay.

astro-ph.SR↗
arXiv2018

Impacts of nuclear-physics uncertainties in the s-process determined by Monte-Carlo variations

The s-process, a production mechanism based on slow-neutron capture during stellar evolution, is the origin of about half the elements heavier than iron. Abundance predictions for s-process nucleosynthesis depend strongly on the relevant neutron-capture and $β$-decay rates, as well as on the details of the stellar model being considered. Here, we have used a Monte-Carlo approach to evaluate the nuclear uncertainty in s-process nucleosynthesis. We considered the helium burning of massive stars for the weak s-process and low-mass asymptotic-giant-branch stars for the main s-process. Our calculations include a realistic and general prescription for the temperature dependent uncertainty for the reaction cross sections. We find that the adopted uncertainty for (${\rm n},γ$) rates, tens of per cent on average, effects the production of s-process nuclei along the line of $β$-stability, and that the uncertainties in $β$-decay from excited state contributions, has the strongest impact on branching points.

astro-ph.SR↗
arXiv2018

Role of core-collapse supernovae in explaining Solar System abundances of p nuclides

The production of the heavy stable proton-rich isotopes between 74Se and 196Hg -- the p nuclides -- is due to the contribution from different nucleosynthesis processes, activated in different types of stars. Whereas these processes have been subject to various studies, their relative contributions to Galactic Chemical Evolution are still a matter of debate. We investigate for the first time the nucleosynthesis of p nuclides in GCE by including metallicity and progenitor mass-dependent yields of core-collapse supernovae (ccSNe) into a chemical evolution model. We used a grid of metallicities and progenitor masses from two different sets of stellar yields and followed the contribution of ccSNe to the Galactic abundances as function of time. In combination with previous studies on p-nucleus production in thermonuclear supernovae (SNIa), and using the same GCE description, this allows us to compare the respective roles of SNeIa and ccSNe in the production of p-nuclei in the Galaxy. The gamma process in ccSN is very efficient for a wide range of progenitor masses (13M_sun_- 25M_sun) at solar metallicity. Since it is a secondary process with its efficiency depending on the initial abundance of heavy elements, its contribution is strongly reduced below solar metallicity. This makes it challenging to explain the inventory of the p nuclides in the Solar System by the contribution from ccSNe alone. In particular, we find that ccSNe contribute less than 10% of the solar p nuclide abundances, with only a few exceptions. Due to the uncertain contribution from other nucleosynthesis sites in ccSNe, such as neutrino winds or $α$-rich freeze out, we conclude that the light p-nuclides 74$Se,78$Kr, 84Sr, and 92Mo may either still be completely or only partially produced in ccSNe.

astro-ph.SR↗
arXiv2017

Uncertainties in the production of p nuclides in thermonuclear supernovae determined by Monte Carlo variations

Thermonuclear supernovae originating from the explosion of a white dwarf accreting mass from a companion star have been suggested as a site for the production of $p$ nuclides. Such nuclei are produced during the explosion, in layers enriched with seed nuclei coming from prior strong $s$ processing. These seeds are transformed to proton-richer isotopes mainly by photodisintegration reactions. Several thousand trajectories from a 2D explosion model were used in a Monte Carlo approach. Temperature-dependent uncertainties were assigned individually to thousands of rates varied simultaneously in post-processing in an extended nuclear reaction network. The uncertainties in the final nuclear abundances originating from uncertainties in the astrophysical reaction rates were determined. In addition to the 35 classical $p$ nuclides, abundance uncertainties were also determined for the radioactive nuclides $^{92}$Nb, $^{97,98}$Tc, $^{146}$Sm, and for the abundance ratios $Y$(${}^{92}$Mo)/$Y$(${}^{94}$Mo), $Y$(${}^{92}$Nb)/$Y$(${}^{92}$Mo), $Y$(${}^{97}$Tc)/$Y$(${}^{98}$Ru), $Y$(${}^{98}$Tc)/$Y$(${}^{98}$Ru), and $Y$(${}^{146}$Sm)/$Y$(${}^{144}$Sm), important for Galactic Chemical Evolution studies. Uncertainties found were generally lower than a factor of two, although most nucleosynthesis flows mainly involve predicted rates with larger uncertainties. The main contribution to the total uncertainties comes from a group of trajectories with high peak density originating from the interior of the exploding white dwarf. The distinction between low-density and high-density trajectories allows more general conclusions to be drawn, also applicable to other simulations of white dwarf explosions.

astro-ph.SR↗
arXiv2017

Cross section of $α$-induced reactions on iridium isotopes obtained from thick target yield measurement for the astrophysical $γ$ process

The stellar reaction rates of radiative $α$-capture reactions on heavy isotopes are of crucial importance for the $γ$ process network calculations. These rates are usually derived from statistical model calculations, which need to be validated, but the experimental database is very scarce. This paper presents the results of $α$-induced reaction cross section measurements on iridium isotopes carried out at first close to the astrophysically relevant energy region. Thick target yields of $^{191}$Ir($α$,$γ$)$^{195}$Au, $^{191}$Ir($α$,n)$^{194}$Au, $^{193}$Ir($α$,n)$^{196m}$Au, $^{193}$Ir($α$,n)$^{196}$Au reactions have been measured with the activation technique between E$_α= 13.4$ MeV and 17 MeV. For the first time the thick target yield was determined with X-ray counting. This led to a previously unprecedented sensitivity. From the measured thick target yields, reaction cross sections are derived and compared with statistical model calculations. The recently suggested energy-dependent modification of the $α$+nucleus optical potential gives a good description of the experimental data.

nucl-ex↗
arXiv2017

Abundance Uncertainties Obtained With the PizBuin Framework For Monte Carlo Reaction Rate Variations

Uncertainties in nucleosynthesis models originating from uncertainties in astrophysical reaction rates were estimated in a Monte Carlo variation procedure. Thousands of rates were simultaneously varied within individual, temperature-dependent errors to calculate their combined effect on final abundances. After a presentation of the method, results from application to three different nucleosynthesis processes are shown: the $γ$-process and the s-process in massive stars, and the main s-process in AGB stars (preliminary results). Thermal excitation of nuclei in the stellar plasma and the combined action of several reactions increase the final uncertainties above the level of the experimental errors. The total uncertainty, on the other hand, remains within a factor of two even in processes involving a large number of unmeasured rates, with some notable exceptions for nuclides whose production is spread over several stellar layers and for s-process branchings.

astro-ph.SR↗
arXiv2017

Experimental setup and procedure for the measurement of the 7Be(n,p)7Li reaction at n_TOF

Following the completion of the second neutron beam line and the related experimental area (EAR2) at the n_TOF spallation neutron source at CERN, several experiments were planned and performed. The high instantaneous neutron flux available in EAR2 allows to investigate neutron indiced reactions with charged particles in the exit channel even employing targets made out of small amounts of short-lived radioactive isotopes. After the successful measurement of the 7Be(n,α)α cross section, the 7Be(n,p)7Li reaction was studied in order to provide still missing cross section data of relevance for Big Bang Nucleosynthesis (BBN), in an attempt to find a solution to the cosmological Lithium abundance problem. This paper describes the experimental setup employed in such a measurement and its characterization.

physics.ins-det↗
arXiv2017

The s-process nucleosynthesis: impact of the uncertainties in the nuclear physics determined by Monte Carlo variations

We investigated the impact of uncertainties in neutron-capture and weak reactions (on heavy elements) on the s-process nucleosynthesis in low-mass stars and massive stars using a Monte-Carlo based approach. We performed extensive nuclear reaction network calculations that include newly evaluated temperature-dependent upper and lower limits for the individual reaction rates. We found beta-decay rate uncertainties affect only a few nuclei near s-process branchings, whereas most of the uncertainty in the final abundances is caused by uncertainties in the neutron capture rates. We suggest a list of uncertain rates as candidates for improved measurement by future experiments.

astro-ph.SR↗
arXiv2017

Impacts of nuclear-physics uncertainty in stellar temperatures on the s-process nucleosynthesis

We evaluated the uncertainty relevant to s-process nucleosynthesis using a Monte-Carlo centred approach. We are based on a realistic and general prescription of temperature dependent uncertainty for the reactions. We considered massive stars for the weak s-process and AGB stars for the main s-process. We found that the adopted uncertainty for (n,$γ$) rates, tens of per cent on average, affect the production of s-process nuclei along the $β$-stability line, while for $β$-decay, for which contributions from excited states enhances the uncertainty, has the strongest impact on branching points.

astro-ph.SR↗