arXiv subjects

T. Rauscher

Publications and source records attributed to T. Rauscher.

At least 73 records · Page 4Linked to original sources

arXiv2014

Testing the role of SNe Ia for Galactic chemical evolution of p-nuclei with 2D models and with s-process seeds at different metallicities

The bulk of p isotopes is created in the 'gamma processes' mainly by sequences of photodisintegrations and beta decays in explosive conditions in Type Ia supernovae (SNIa) or in core collapse supernovae (ccSN). The contribution of different stellar sources to the observed distribution of p-nuclei in the Solar System is still under debate. We explore single degenerate Type Ia supernovae in the framework of two-dimensional SNIa delayed-detonation explosion models. Travaglio et al. (2011, TRV11) discussed the sensitivity of p-nuclei production to different SNIa models, i.e. delayed detonations of different strength, deflagrations, and the dependence on selected s-process seed distributions. Here we present a detailed study of p-process nucleosynthesis occuring in SNIa with s-process seeds at different metallicities. Based on the delayed-detonation model DDT-a of TRV11, we analyze the dependence of p-nucleosynthesis on the s-seed distribution obtained from different strengths of the 13C-pocket. Using a Galactic chemical evolution code (see Travaglio et al. 2004) we study the contribution of SNIa to the solar stable p-nuclei. We find that explosions of Chandrasekhar-mass single degenerate systems produce a large amount of p-nuclei in our Galaxy, both in the range of light (A < 120) and heavy p-nuclei, at almost flat average production factors (within a factor of about 3). We discussed in details p-isotopes such as 94Mo with a behavior diverging from the average, which we attribute to uncertainties in the nuclear data or in SNIa modelling.

astro-ph.SR↗

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.

nucl-ex↗

arXiv2014

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

A measurement of total cross-section values of the $^{130}$Ba(p,$γ$)$^{131}$La 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~$\approx$~130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials. 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 $\leq E_p \leq$ 5.0 MeV, thus, inside the astrophysically relevant energy region. 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 semi-microscopic 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 semi-microscopic Bauge proton+nucleus optical model potential using a constant renormalization factor.

nucl-ex↗

arXiv2014

Radiogenic p-isotopes from SNIa, nuclear physics uncertainties and Galactic chemical evolution compared with values in primitive meteorites

The nucleosynthesis of proton-rich isotopes is calculated for multi-dimensional Chandrasekhar-mass models of Type Ia supernovae with different metallicities. The predicted abundances of the short-lived radioactive isotopes 92Nb, 97Tc, 98Tc and 146Sm are given in this framework. The abundance seeds are obtained by calculating s-process nucleosynthesis in the material accreted onto a carbon-oxygen white dwarf from a binary companion. A fine grid of s-seeds at different metallicities and 13C-pocket efficiencies is considered. A galactic chemical evolution model is used to predict the contribution of SNIa to the solar system p-nuclei composition measured in meteorites. Nuclear physics uncertainties are critical to determine the role of SNeIa in the production of 92Nb and 146Sm. We find that, if standard Chandrasekhar-mass SNeIa are at least 50% of all SNIa, they are strong candidates for reproducing the radiogenic p-process signature observed in meteorites.

astro-ph.SR↗

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.

nucl-ex↗

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.

physics.ins-det↗

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.

nucl-ex↗

arXiv2014

Measurement of the $^{58}$Ni($α$,$γ$)$^{62}$Zn reaction and its astrophysical impact

Cross section measurements of the $^{58}$Ni($α$,$γ$)$^{62}$Zn reaction were performed in the energy range $E_α=5.5-9.5$ MeV at the Nuclear Science Laboratory of the University of Notre Dame, using the NSCL Summing NaI(Tl) detector and the $γ$-summing technique. The measurements are compared to predictions in the statistical Hauser-Feshbach model of nuclear reactions using the SMARAGD code. It is found that the energy dependence of the cross section is reproduced well but the absolute value is overestimated by the prediction. This can be remedied by rescaling the $α$ width by a factor of 0.45. Stellar reactivities were calculated with the rescaled $α$ width and their impact on nucleosynthesis in type Ia supernovae has been studied. It is found that the resulting abundances change by up to 5\% when using the new reactivities.

nucl-ex↗

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.

nucl-ex↗

arXiv2014

Thermonuclear 42Ti(p,gamma)43V rate in type I X-ray bursts

The thermonuclear rate of the $^{42}$Ti($p$,$γ$)$^{43}$V reaction has been reevaluated based on a recent precise proton separation energy measurement of $S_p$($^{43}$V)=83$\pm$43 keV. The astrophysical impact of our new rates has been investigated through one-zone postprocessing type I x-ray burst calculations. It shows that the new experimental value of $S_p$ significantly affects the yields of species between A$\approx$40--45. As well, the precision of the recent experimental $S_p$ value constrains these yields to better than a factor of three.

nucl-th↗

arXiv2014

Challenges in nucleosynthesis of trans-iron elements

Nucleosynthesis beyond Fe poses additional challenges not encountered when studying astrophysical processes involving light nuclei. Astrophysical sites and conditions are not well known for some of the processes involved. On the nuclear physics side, different approaches are required, both in theory and experiment. The main differences and most important considerations are presented for a selection of nucleosynthesis processes and reactions, specifically the $s$-, $r$-, $γ$-, and $νp$-processes. Among the discussed issues are uncertainties in sites and production conditions, the difference between laboratory and stellar rates, reaction mechanisms, important transitions, thermal population of excited states, and uncertainty estimates for stellar rates. The utility and limitations of indirect experimental approaches are also addressed. The presentation should not be viewed as confining the discussed problems to the specific processes. The intention is to generally introduce the concepts and possible pitfalls along with some examples. Similar problems may apply to further astrophysical processes involving nuclei from the Fe region upward and/or at high plasma temperatures. The framework and strategies presented here are intended to aid the conception of future experimental and theoretical approaches.

astro-ph.SR↗

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.

nucl-ex↗

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.

nucl-ex↗

arXiv2013

Measurement of the $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb reactions for the nucleosynthesis of elements around A=90

Cross section measurements of the reactions $^{90, 92}$Zr(p,$γ$)$^{91,93}$Nb were performed using the NSCL SuN detector at the University of Notre Dame. These reactions are part of the nuclear reaction flow for the synthesis of the light p nuclei. For the $^{90}$Zr(p,$γ$)$^{91}$Nb reaction the new measurement resolves the disagreement between previous results. For the $^{92}$Zr(p,$γ$)$^{93}$Nb reaction the present work reports the first measurement of this reaction cross section. Both reaction cross sections are compared to theoretical calculations and a very good agreement with the standard NON-SMOKER model is observed.

nucl-ex↗

arXiv2013

Revision of the derivation of stellar rates from experiment and impact on Eu s-process contributions

A new, general formalism to include experimental data in revised stellar rates is discussed, containing revised uncertainties. Application to the s-process shows that the actual uncertainties in the neutron capture rates can be larger than would be expected from the experimental errors alone. As a specific example for how astrophysical conclusions can depend on the approach selected to derive stellar rates, the 151Eu/(151Eu+153Eu) abundance ratio from AGB star models is presented. Finally, a recommended workflow for the derivation of stellar rates from experiment is laid out.

astro-ph.SR↗

arXiv2013

Suppression of Excited-State Contributions to Stellar Reaction Rates

It has been shown in previous work [Phys. Rev. Lett. 101, 191101 (2008); Phys. Rev. C 80, 035801 (2009)] that a suppression of the stellar enhancement factor (SEF) occurs in some endothermic reactions at and far from stability. This effect is re-evaluated using the ground-state contributions to the stellar reaction rates, which were shown to be better suited to judge the importance of excited state contributions than the previously applied SEFs. An update of the tables shown in Phys. Rev. C 80, 035801 (2009) is given. The new evalution finds 2350 cases (out of a full set of 57513 reactions) for which the ground-state contribution is larger in the reaction direction with negative reaction Q-value than in the exothermic direction, thus providing exceptions to the commonly applied Q-value rule. The results confirm the Coulomb suppression effect but lead to a larger number of exceptions than previously found. This is due to the fact that often a large variation in the g.s. contribution does not lead to a sizeable change in the SEF. On the other hand, several previously identified cases do not appear anymore because it is found that their g.s. contribution is smaller than inferred from the SEF.

astro-ph.SR↗

arXiv2013

Solution of the alpha-potential mystery in the gamma-process and its impact on the Nd/Sm ratio in meteorites

The 146Sm/144Sm ratio in the early solar system has been constrained by Nd/Sm isotope ratios in meteoritic material. Predictions of 146Sm and 144Sm production in the gamma-process in massive stars are at odds with these constraints and this is partly due to deficiences in the prediction of the reaction rates involved. The production ratio depends almost exclusively on the (gamma,n)/(gamma,alpha) branching at 148Gd. A measurement of 144Sm(alpha,gamma)148Gd at low energy had discovered considerable discrepancies between cross section predictions and the data. Although this reaction cross section mainly depends on the optical alpha+nucleus potential, no global optical potential has yet been found which can consistently describe the results of this and similar alpha-induced reactions at the low energies encountered in astrophysical environments. The untypically large deviation in 144Sm(alpha,gamma) and the unusual energy dependence can be explained, however, by low-energy Coulomb excitation which is competing with compound nucleus formation at very low energies. Considering this additional reaction channel, the cross sections can be described with the usual optical potential variations, compatible with findings for (n,alpha) reactions in this mass range. Low-energy (alpha,gamma) and (alpha,n) data on other nuclei can also be consistently explained in this way. Since Coulomb excitation does not affect alpha-emission, the 148Gd(gamma,alpha) rate is much higher than previously assumed. This leads to small 146Sm/144Sm stellar production ratios, in even more pronounced conflict with the meteorite data.

astro-ph.SR↗

arXiv2013

Silicon carbide grains of type C provide evidence for the production of the unstable isotope $^{32}$Si in supernovae

Carbon-rich grains are observed to condense in the ejecta of recent core-collapse supernovae, within a year after the explosion. Silicon carbide grains of type X are C-rich grains with isotpic signatures of explosive supernova nucleosynthesis have been found in primitive meteorites. Much rarer silicon carbide grains of type C are a special sub-group of SiC grains from supernovae. They show peculiar abundance signatures for Si and S, isotopically heavy Si and isotopically light S, which appear to to be in disagreement with model predictions. We propose that C grains are formed mostly from C-rich stellar material exposed to lower SN shock temperatures than the more common type X grains. In this scenario, extreme $^{32}$S enrichments observed in C grains may be explained by the presence of short-lived $^{32}$Si ($τ$$_{1/2}$ = 153 years) in the ejecta, produced by neutron capture processes starting from the stable Si isotopes. No mixing from deeper Si-rich material and/or fractionation of Si from S due to molecular chemistry is needed to explain the $^{32}$S enrichments. The abundance of $^{32}$Si in the grains can provide constraints on the neutron density reached during the supernova explosion in the C-rich He shell material. The impact of the large uncertainty of the neutron capture cross sections in the $^{32}$Si region is discussed.

astro-ph.SR↗