Stellar Cross Sections Research Articles

Measurement of the Tb159(n,γ) cross section at the CSNS Back-n facility

The stellar ($n,\ensuremath{\gamma}$) cross section data for the mass numbers around $A\ensuremath{\approx}160$ are of key importance to nucleosynthesis in the main component of the slow neutron capture process, which occurs in the thermally pulsing asymptotic giant branch (TP-AGB). The new measurement of ($n,\ensuremath{\gamma}$) cross sections for $^{159}\mathrm{Tb}$ was performed using the ${\mathrm{C}}_{6}{\mathrm{D}}_{6}$ detector system at the back streaming white neutron beam line (Back-n) of the China spallation neutron source with neutron energies ranging from 1 eV to 1 MeV. Experimental resonance capture kernels are reported up to 1.2 keV neutron energy with this capture measurement. Maxwellian-averaged cross sections (MACS) are derived from the measured $^{159}\mathrm{Tb}(n,\ensuremath{\gamma})$ cross sections at $kT=5$--100 keV and are in good agreement with the recommended data of KADoNiS-v0.3 and JEFF-3.3, while KADoNiS-v1.0 and ENDF-VIII.0 significantly overestimate the present MACS up to $40%$ and $20%$, respectively. A sensitive test of the $s$-process nucleosynthesis is also performed with the stellar evolution code mesa. Significant changes in abundances around $A\ensuremath{\approx}160$ are observed between the ENDF/B-VIII.0 and present measured rate of $^{159}\mathrm{Tb}(n,\ensuremath{\gamma})^{160}\mathrm{Tb}$ in the mesa simulation.

Ta179(n,γ) cross-section measurement and the astrophysical origin of the Ta180 isotope

Tantalum-180m is nature's rarest (quasi) stable isotope and its astrophysical origin is an open question. A possible production site of this isotope is the slow neutron capture process in Asymptotic Giant Branch stars, where it can be produced via neutron capture reactions on unstable $^{179}$Ta. We report a new measurement of the $^{179}$Ta($n,\gamma$)$^{180}$Ta cross section at thermal neutron energies via the activation technique. Our results for the thermal and resonance-integral cross-sections are 952 $\pm$ 57 b and 2013 $\pm$ 148 b, respectively. The thermal cross section is in good agreement with the only previous measurement (Phys. Rev C {\bf 60} 025802, 1999), while the resonance integral is different by a factor of $\approx$1.7. While neutron energies in this work are smaller than the energies in a stellar environment, our results may lead to improvements in theoretical predictions of the stellar cross section.

^{74}Ge(n,gamma ) cross section below 70 keV measured at n_TOF CERN

Neutron capture reaction cross sections on ^{74}Ge are of importance to determine ^{74}Ge production during the astrophysical slow neutron capture process. We present new resonance data on ^{74}Ge(n,gamma ) reactions below 70 keV neutron energy. We calculate Maxwellian averaged cross sections, combining our data below 70 keV with evaluated cross sections at higher neutron energies. Our stellar cross sections are in agreement with a previous activation measurement performed at Forschungszentrum Karlsruhe by Marganiec et al., once their data has been re-normalised to account for an update in the reference cross section used in that experiment.

Two absolute integral measurements of the 197Au(n,γ) stellar cross-section and solution of the historic discrepancies

The Maxwellian averaged cross section (MACS) for 197Au(n,γ) is used in neutron capture cross section measurements as a reference for reactions important for astrophysics, reactor and dosimetry applications. The traditionally adopted value for this reference cross section, in the energy range relevant for astrophysical (3 < E n < 200 keV), was obtained by Ratynski and Käppeler in 1988. However, the MACS calculated using the 2006 standards evaluation is approximately 6 % above the Ratynski and Käppeler (R&K) evaluation. Because of this discrepancy new experiments and reanalyses were done in an attempt to resolve the problem. In 2011 we started as well a series of integral experiments (activation) for determining the MACS-30 (kT=30 keV) of Au with two different Maxwellian neutron spectra: i) QMNS-25 (as R&K) and ii) MNS-30 (new method). Our results agree with those obtained with the standard evaluation. At present (2018), the updated MACS-30 has been included as standard. Here we present the results of our measurements and the reasons for the lower value of the R&K measurement.

Measurement of the Ge70(n,γ) cross section up to 300 keV at the CERN n_TOF facility

Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The ($n,\gamma$) cross section on $^{70}$Ge, which is mainly produced in the $s$~process, was measured at the neutron time-of-flight facility n_TOF at CERN. Resonance capture kernels were determined up to 40~keV neutron energy, and average cross sections up to 300~keV. Stellar cross sections were calculated from $kT=5$~keV to $kT=100$ keV and are in very good agreement with a previous measurement by Walter and Beer (1985), and recent evaluations. Average cross sections are in agreement with Walter and Beer (1985) over most of the neutron energy range covered, while being systematically smaller for neutron energies above 150~keV. We have calculated isotopic abundances produced in $s$-process environments in a 25 solar mass star for two initial metallicities (below solar, and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances the close to solar model shows a good global match to solar system abundances between mass numbers A=60-80.

Measurement of 73Ge(n,γ) cross sections and implications for stellar nucleosynthesis

73Ge(n,γ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT=30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73Ge produced in stars, which would explain the low isotopic abundance of 73Ge in the solar system.

Ni62(n,γ) andNi63(n,γ) cross sections measured at the n_TOF facility at CERN

The cross section of the $^{62}$Ni($n,\gamma$) 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($n,\gamma$) 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.

Current quests in nucleosynthesis: present and future neutron-induced reaction measurements

We present some open questions in nucleosynthesis focused on the measure- ment of relevant neutron capture cross-sections and on new experimental methods. We review the recent 63 Ni(n, ) experiment carried out at the n_TOF facility at CERN and its astrophysical implications as well as future experiments and opportunities at n_TOF. We argue some improvements in the measurement of cross-sections by activation arising from a new method for the generation of stellar neutron spectra. We show preliminary results of the experimental validation of the method. We discuss the astrophysical im- plications of the 181 Ta(n, ) stellar cross-section measured with this method. Finally, we describe challenging experiments consisting of in situ radioactive ion beams and stellar neutron beams.

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

We have measured the neutron capture cross sections of the stable magnesium isotopes 24,25,26Mg in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in 25Mg and 26Mg were recorded using the total energy method based on C62H6 detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for 24Mg, 25Mg, and 26Mg are 3.8±0.2 mb, 4.1±0.6 mb, and 0.14±0.01 mb, respectively.5 MoreReceived 9 January 2012DOI:https://doi.org/10.1103/PhysRevC.85.044615©2012 American Physical Society

Stellar neutron capture rates and thesprocess

Neutron reactions are responsible for the formation of the elements heavier than iron. The corresponding scenarios relate to helium burning in Red Giant stars (s process) and to supernova explosions (r and p processes). The s process, which operates in or near the valley of β -stability, has produced about half of the elemental abundances between Fe and Bi. Accurate (n, γ ) cross sections are the essential input for s process studies, because they determine the abundances produced by that process. Following a brief summary of the neutron capture processes, the focus will be set on the s process in massive stars, where the role of reliable cross section information is particularly important. Eventually, the intriguing aspects of the origin of 60 Fe will be addressed. Attempts to determine the stellar cross section of that isotope are pushing experimental possibilities to their limits and present a pertinent challenge for future facilities.

Stellar (n,γ) cross sections ofp-process isotopes. II.Yb168,W180,Os184,Pt190, andHg196

The neutron-capture cross sections of {sup 168}Yb, {sup 180}W, {sup 184}Os, {sup 190}Pt, and {sup 196}Hg have been measured by means of the activation technique. The samples were irradiated in a quasistellar neutron spectrum of kT=25 keV, which was produced at the Karlsruhe 3.7-MV Van de Graaff accelerator via the {sup 7}Li(p,n){sup 7}Be reaction. Systematic uncertainties were investigated in repeated activations with different samples and by variation of the experimental parameters, that is, irradiation times, neutron fluxes, and {gamma}-ray counting conditions. The measured data were converted into Maxwellian-averaged cross sections at kT=30 keV, yielding 1214{+-}61, 624{+-}54, 590{+-}43, 511{+-}46, and 201{+-}11 mb for {sup 168}Yb, {sup 180}W, {sup 184}Os, {sup 190}Pt, and {sup 196}Hg, respectively. The present results either represent first experimental data ({sup 168}Yb, {sup 184}Os, and {sup 196}Hg) or could be determined with significantly reduced uncertainties ({sup 180}W and {sup 190}Pt). These measurements are part of a systematic study of stellar (n,{gamma}) cross sections of the stable p isotopes.

Neutron physics of the Re/Os clock. III. Resonance analyses and stellar (n,γ) cross sections ofOs186,187,188

Neutron resonance analyses have been performed for the capture cross sections of Os-186, Os-187, and Os-188 measured at the n_TOF facility at CERN. Resonance parameters have been extracted up to 5, 3, and 8 keV, respectively, using the SAMMY code for a full R-matrix fit of the capture yields. From these results average resonance parameters were derived by a statistical analysis to provide a comprehensive experimental basis for modeling of the stellar neutron capture rates of these isotopes in terms of the Hauser-Feshbach statistical model. Consistent calculations for the capture and inelastic reaction channels are crucial for the evaluation of stellar enhancement factors to correct the Maxwellian averaged cross sections obtained from experimental data for the effect of thermally populated excited states. These factors have been calculated for the full temperature range of current scenarios of s-process nucleosynthesis using the combined information of the experimental data in the region of resolved resonances and in the continuum. The consequences of this analysis for the s-process component of the Os-187 abundance and the related impact on the evaluation of the time duration of galactic nucleosynthesis via the Re/Os cosmochronometer are discussed.

Stellar(n,γ)cross sections ofp-process isotopes Part I:Pd102,Te120,Ba130,132, andDy156

We have investigated the (n,gamma) cross sections of p-process isotopes with the activation technique. The measurements were carried out at the Karlsruhe Van de Graaff accelerator using the 7Li(p,n)7Be source for simulating a Maxwellian neutron distribution of kT = 25 keV. Stellar cross section measurements are reported for the light p-process isotopes 102Pd, 120Te, 130,132Ba, and 156Dy. In a following paper the cross sections of 168Yb, 180W, 184Os, 190Pt, and 196Hg will be discussed. The data are extrapolated to p-process energies by including information from evaluated nuclear data libraries. The results are compared to standard Hauser-Feshbach models frequently used in astrophysics.

Numerical investigation of corner angle and wing number effects on fluid flow characteristics of a turbulent stellar jet

In this research the fluid dynamics characteristics of a stellar turbulent jet flow is studied numerically and the results of three dimensional jet issued from a stellar nozzle are presented. A numerical method based on control volume approach with collocated grid arrangement is employed. The turbulent stresses are approximated using k–e and k–ω models with four different inlet conditions. The velocity field is presented and the rate of decay at jet centerline is noted. Special attention is drawn on the influence of corner angle and number of wings on mixing in stellar cross section jets. Stellar jets with three; four and five wings and 15–65° corner angles are studied. Also the effect of Reynolds number (based on hydraulic diameter) as well as the inflow conditions on the evolution of the stellar jet is studied. The Numerical results show that the jet entrains more with corner angle 65° and five wings number. The jet is close to a converged state for high Reynolds numbers. Also the influence of the inflow conditions on the jet characteristics is so strong.

Neutron capture cross sections ofGe74,Ge76, andAs75at 25 keV

The neutron capture cross sections of $^{74}\mathrm{Ge}$, $^{76}\mathrm{Ge}$, and $^{75}\mathrm{As}$ have been measured at the Karlsruhe Van de Graaff accelerator by means of the activation technique. The neutron source reaction $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ was used for simulating a thermal energy distribution for $\mathit{kT}=25$ keV to determine the astrophysically relevant stellar cross sections directly. Based on an extended series of activations, consistent results could be obtained, corresponding to Maxwellian average cross sections at $\mathit{kT}=30$ keV of $37.6\ifmmode\pm\else\textpm\fi{}3.9$ mb for $^{74}\mathrm{Ge}$, $21.5\ifmmode\pm\else\textpm\fi{}1.8$ mb for $^{76}\mathrm{Ge}$, and $362\ifmmode\pm\else\textpm\fi{}19$ mb for $^{75}\mathrm{As}$. These results are more accurate than previous data and are, therefore, important for resolving previous discrepancies. By extrapolation Maxwellian averaged cross sections were derived for thermal energies from 5 to 100 keV. The implications of these results for the $s$-process abundances in massive stars and for the background conditions in double $\ensuremath{\beta}$ decay experiments are discussed.

Time-Scales of the s Process: from Minutes to Ages

AbstractThe time scales in the s process appears to be an approriate aspect to discuss at the occasion of Roberto's 70th anniversary, the more as this subject has been repeatedly addressed during the 20 years of collaboration between Torino and Karlsruhe. The two chronometers presented in this text were selected to illustrate the intense mutual stimulation of both groups. Based on a reliable set of accurate stellar (n, γ) cross sections determined mostly at FZK, the Torino group succeeded to develop a comprehensive picture of the various s-process scenarios, which are most valuable for understanding the composition of the solar system as well as for the interpretation of an increasing number of astronomical observations.

Stellar (n, γ) cross sections for Br and Rb: Matching the weak and mains-process components

The stellar ($n, \ensuremath{\gamma}$) cross sections of $^{79}\mathrm{Br}$, $^{81}\mathrm{Br}$, $^{85}\mathrm{Rb}$, and $^{87}\mathrm{Rb}$ have been determined by a series of ten activation measurements in a quasistellar neutron spectrum corresponding to a thermal energy of $\mathit{kT} = 25$ keV. The final uncertainties between 3 and 10% were dominated by the \ensuremath{\gamma}-ray intensities in the decay of the respective reaction products. All other uncertainties were significantly reduced through variation of the experimental parameters. The consequences of these results for $s$-process nucleosynthesis of Br and Rb, and in particular of the local $s$-only isotopes $^{80, 82}\mathrm{Kr}$ and $^{86, 87}\mathrm{Sr}$, are discussed with respect to the branching points at $^{79}\mathrm{Se}$ and $^{85}\mathrm{Kr}$. The contributions of both the main $s$ component from thermally pulsing low mass asymptotic giant branch stars and the weak $s$ component from massive stars are considered in this analysis.

176Lu/176Hf: A Sensitive Test ofs‐Process Temperature and Neutron Density in AGB Stars

The s-process branching at A ¼ 176 has been analyzed on the basis of significantly improved experimental cross sections.Thisworkreportsonactivationmeasurementsofthepartial(n ;� )crosssectionof 176Lufeedingtheisomeric state in 176 Lu. In total, six irradiations were performed at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured with HPGe clover detectors. In combination with previous data, partial cross sections of 3185 � 156 and 1153 � 30 mbarn were deduced at kT ¼ 5:1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n ;� ) cross section, the isomeric ratio was found to be constant in the relevant thermal energy range of the main s-process component. Based on these new data, a comprehensive analysis of the branching at 176 Lu was carried out for testing the temperature and neutron density conditions duringHeshellflashesinthermallypulsinglow-massasymptoticgiantbranchstars.Itwasfoundthatthelong-standing problem of the mother/daughter ratio of the two s-only isotopes 176 Lu and 176 Hf could be solved, if the temperaturedependent � -decay half-life of 176 Lu was considered with sufficient resolution over the temperature profile of the

P-Process simulations with a modified reaction library

We have performed p-process simulations with the most recent stellar (n, γ) cross sections from the Karlsruhe Astrophysical Database of Nucleosynthesis in Stars project (version v0.2, http://nuclear-astrophysics.fzk.de/kadonis). The simulations were carried out with a parametrized supernova type II shock front model (‘γ-process’) of a 25 solar mass star and compared to recently published results. A decrease in the normalized overproduction factor could be attributed to lower cross sections of a significant fraction of seed nuclei located in Bi and Pb regions around the N = 126 shell closure.

Neutron capture cross section of 76Ge

We have measured the neutron capture cross section of 76Ge by means of the activation technique. Neutrons were produced via the 7Li(p, n)7Be reaction, which offered the possibility of simulating a quasi-stellar neutron spectrum for kT = 25 keV, thus allowing us to measure the Maxwellian averaged cross section directly. The stellar cross section extrapolated to kT = 30 keV was found to be 12.4 ± 0.7 mbarn. This result carries a considerably smaller uncertainty than previous data.