S-only Isotopes Research Articles

Shedding Light on the Origin of ^{204}Pb, the Heaviest s-Process-Only Isotope in the Solar System.

Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the r-process contribution by their stable isobars are defined as s-only nuclei. For a long time the abundance of ^{204}Pb, the heaviest s-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, ^{204}Tl (t_{1/2}=3.78 yr), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on ^{204}Tl, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9mg of ^{204}Tl produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the s-process temperatures of kT≈8 keV and kT≈30 keV, Maxwellian-averaged cross sections (MACS) of 580(168)mb and 260(90)mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new ^{204}Tl MACS, the uncertainty arising from the ^{204}Tl(n,γ) cross section on the s-process abundance of ^{204}Pb has been reduced from ∼30% down to +8%/-6%, and the s-process calculations are in agreement with the latest solar system abundance of ^{204}Pb reported by K. Lodders in 2021.

Neutron capture measurement at the n TOF facility of the 204Tl and 205Tl s-process branching points

Neutron capture cross sections are one of the fundamental nuclear data in the study of the s (slow) process of nucleosynthesis. More interestingly, the competition between the capture and the decay rates in some unstable nuclei determines the local isotopic abundance pattern. Since decay rates are often sensible to temperature and electron density, the study of the nuclear properties of these nuclei can provide valuable constraints to the physical magnitudes of the nucleosynthesis stellar environment. Here we report on the capture cross section measurement of two thallium isotopes, 204Tl and 205Tl performed by the time-of-flight technique at the n TOF facility at CERN. At some particular stellar s-process environments, the decay of both nuclei is strongly enhanced, and determines decisively the abundance of two s-only isotopes of lead, 204Pb and 205Pb. The latter, as a long-lived radioactive nucleus, has potential use as a chronometer of the last s-process events that contributed to final solar isotopic abundances.

The s-process in the Nd-Pm-Sm region: Neutron activation of 147Pm

The Nd-Pm-Sm branching is of interest for the study of the s-process, related to the production of heavy elements in stars. As 148Sm and 150Sm are s-only isotopes, the understanding of the branching allows constraining the s-process neutron density. In this context the key physics input needed is the cross section of the three unstable nuclides in the region: 147Nd (10.98 d half-life), 147Pm (2.62 yr) and 148Pm (5.37 d). This paper reports on the activation measurement of 147Pm, the longest-lived of the three nuclides. The cross section measurement has been carried out by activation at the SARAF LiLiT facility using a 56(2) μg target. Compared to the single previous measurement of 147Pm, the measurement presented herein benefits from a target 2000 times more massive. The resulting Maxwellian Averaged Cross Section (MACS) to the ground and metastable states in 148Pm are 469(50) mb and 357(27) mb. These values are 41% higher (to the ground state) and 15% lower (to the metastable state) than the values reported so far, leading however to a total cross section of 826(107) mb consistent within uncertainties with the previous result and hence leaving unchanged the previous calculation of the s-process neutron density.

The branchings of the main s-process: their sensitivity to α-induced reactions on 13C and 22Ne and to the uncertainties of the nuclear network

This paper provides a detailed analysis of the main component of the slow neutron capture process (the s-process), which accounts for the solar abundances of half of the nuclei with 90 <~ A <~ 208. We examine the impact of the uncertainties of the two neutron sources operating in low-mass asymptotic giant branch (AGB) stars: the 13C(alpha, n)16O reaction, which releases neutrons radiatively during interpulse periods (kT ~ 8 keV), and the 22Ne(alpha, n)25Mg reaction, partially activated during the convective thermal pulses (TPs). We focus our attention on the branching points that mainly influence the abundance of s-only isotopes. In our AGB models, the 13C is fully consumed radiatively during interpulse. In this case, we find that the present uncertainty associated to the 13C(alpha, n)16O reaction has marginal effects on s-only nuclei. On the other hand, a reduction of this rate may increase the amount of residual (or unburned) 13C at the end of the interpulse: in this condition, the residual 13C is burned at higher temperature in the convective zone powered by the following TP. The neutron burst produced by the 22Ne(alpha, n)25Mg reaction has major effects on the branches along the s path. The contributions of s-only isotopes with 90 <~ A <= 204 are reproduced within solar and nuclear uncertainties, even if the 22Ne(alpha, n)25Mg rate is varied by a factor of two. Improved beta-decay and neutron capture rates of a few key radioactive nuclides would help to attain a comprehensive understanding of the solar main component.

ON THE NEED FOR THE LIGHT ELEMENTS PRIMARY PROCESS (LEPP)

Extant chemical evolution models underestimate the Galactic production of Sr, Y and Zr as well as the Solar System abundances of s-only isotopes with 90<A<130. To solve this problem, an additional (unknown) process has been invoked, the so-called LEPP (Light Element Primary Process). In this paper we investigate possible alternative solutions. Basing on Full Network Stellar evolutionary calculations, we investigate the effects on the Solar System s-only distribution induced by the inclusion of some commonly ignored physical processes (e.g. rotation) or by the variation of the treatment of convective overshoot, mass-loss and the efficiency of nuclear processes. Our main findings are: 1) at the epoch of the formation of the Solar System, our reference model produces super-solar abundances for the whole s-only distribution, even in the range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN) as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the inclusion of rotation implies a downward shift of the whole distribution with an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only distribution; 5) different prescriptions on convection or mass-loss produce nearly rigid shifts of the whole distribution. In summary, a variation of the standard paradigm of AGB nucleosynthesis would allow to reconcile models predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot be definitely ruled out, because of the uncertainties still affecting stellar and Galactic chemical evolution models.

THE IMPACT OF HELIUM-BURNING REACTION RATES ON MASSIVE STAR EVOLUTION AND NUCLEOSYNTHESIS

We study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the helium-burning reaction rates within the range of their uncertainties. We use the current solar abundances from Lodders (2009) for the initial stellar composition. We compute a grid of 12 initial stellar masses and 176 models per stellar mass to explore the effects of independently varying the 12^C(a,g)16^O and 3a reaction rates, denoted R_a12 and R_3a, respectively. The production factors of both the intermediate-mass elements (A=16-40) and the s-only isotopes along the weak s-process path (70Ge, 76Se, 80Kr, 82Kr, 86Sr, and 87Sr) were found to be in reasonable agreement with predictions for variations of R_3a and R_a12 of +/-25%; the s-only isotopes, however, tend to favor higher values of R_3a than the intermediate-mass isotopes. The experimental uncertainty (one standard deviation) in R_3a(R_a12) is approximately +/-10%(+/-25%). The results show that a more accurate measurement of one of these rates would decrease the uncertainty in the other as inferred from the present calculations. We also observe sharp changes in production factors and standard deviations for small changes in the reaction rates, due to differences in the convection structure of the star. The compactness parameter was used to assess which models would likely explode as successful supernovae, and hence contribute explosive nucleosynthesis yields. We also provide the approximate remnant masses for each model and the carbon mass fractions at the end of core-helium burning as a key parameter for later evolution stage

The long-standing problem of 176Lu/176Hf branching: A new approach with full stellar evolutionary models

We present an analysis of the s-process branching at A=176, which is determined by the two s-only isotopes 176Lu and 176Hf. This work is motivated by the discrepancy between the current theoretical expectations and solar-system abundances of the two isotopes involved in the branching. Asymptotic Giant Branch models have been calculated with the FRANEC evolutionary code, coupling a full nuclear reaction network with the equation describing the physical evolution of the stellar structure. With respect to previous studies, we can better follow the complex interplay between mixing and burning, which both strongly influence this branching. We show that a substantial reduction of the mixing velocity within the convective zone generated by a He-shell flash (or thermal pulse) may alleviate the discrepancy between the theoretical expectations and the observed solar abundances. Nonetheless, we reckon that a definitive solution of this problem has to be searched in the nuclear coupling scheme between the thermally populated levels in 176Lu.

The effect of 12C + 12C rate uncertainties on s-process yields

The slow neutron capture process in massive stars (the weak s-process) produces most of the s-only isotopes in the mass region 60 < A < 90. The nuclear reaction rates used in simulations of this process have a profound effect on the final s-process yields. We generated 1D stellar models of a 25M⊙ star varying the 12C + 12C rate by a factor of 10 and calculated full nucleosynthesis using the post-processing code PPN. Increasing or decreasing the rate by a factor of 10 affects the convective history and nucleosynthesis, and consequently the final yields.

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

Photon scattering experiments on the quasistable, odd-odd mass nucleusLu176

The quasistable odd-odd-mass nucleus $^{176}\mathrm{Lu}$ is of special interest in nuclear structure physics and, above all, in nuclear astrophysics. Systematic photon scattering experiments have been performed at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator with bremsstrahlung end-point energies of 2.3 and 3.1 MeV to determine the low-energy dipole strength distribution in the s-only isotope $^{176}\mathrm{Lu}$. The main goal was to pin down possible intermediate states (IS) for the photoactivation of the short-lived 123-keV isomer, which is the key process determining the effective lifetime of $^{176}\mathrm{Lu}$ in a stellar photon bath and hence for the use of this isotope as a stellar chronometer. Using an enriched sample, 29 transitions ascribed to $^{176}\mathrm{Lu}$ were detected below 2.9-MeV excitation energy. The corresponding excitation strengths were determined. For the previously proposed lowest IS at 839 keV, an upper limit for the excitation strength corresponding to a lifetime of $\ensuremath{\tau}\ensuremath{\geqslant}$ 1.5 ps can be given. Astrophysical consequences, also in view of new Stuttgart photoactivation experiments, are discussed. The fragmentation of the dipole strength is compared to those in neighboring even-even and odd-even nuclei.

128Xe and130Xe: Testing He‐Shell Burning in Asymptotic Giant Branch Stars

The s-process branching at 128I has been investigated on the basis of new, precise, experimental (n, γ) cross sections for the s-only isotopes 128Xe and 130Xe. This branching is unique, since it is essentially determined by the temperature- and density-sensitive stellar decay rates of 128I and only marginally affected by the specific stellar neutron flux. For this reason it represents an important test for He-shell burning in asymptotic giant branch stars. The description of the branching by means of the complex stellar scenario reveals a significant sensitivity to the timescales for convection during He shell flashes, thus providing constraints for this phenomenon. The s-process ratio 128Xe/130Xe deduced from stellar models allows for a 9% ± 3% p-process contribution to solar 128Xe, in agreement with the Xe-S component found in meteoritic presolar SiC grains.

Galactic Evolution of Sr, Y, and Zr: A Multiplicity of Nucleosynthetic Processes

We follow the Galactic enrichment of three easily observed light n-capture elements Sr,Y,and Zr.Input stellar yields have been first separated into their respective main and weak s-process,and r-process components.The s-process yields from AGB stars are computed,exploring a wide range of efficiencies of the major neutron source,13C,and covering both disk and halo metallicities.AGBs have been shown to reproduce the main s-component in the solar system.The concurrent weak s-process,which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22Ne n-source,is discussed in detail.Neither the main s-,nor the weak s-components are shown to contribute significantly to the n-capture element abundances observed in unevolved halo stars.We present a detailed analysis of a large database of spectroscopic observations of Sr,Y,Zr, Ba,and Eu for Galactic stars at various metallicities.Spectroscopic observations of Sr,Y,and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of n-capture processes forming Sr,Y and Zr.The observed [Sr,Y,Zr/Ba,Eu] ratio is clearly not flat at low metallicities,as we would expect if Ba,Eu and Sr,Y,Zr all had the same r-process origin.We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr.We find hints for a primary process in low-metallicity massive stars, different from the 'classical s-process' and from the 'classical r-process',that we tentatively define LEPP (Lighter Element Primary Process).This allows us to revise the estimates of the r-process contributions to the solar Sr,Y and Zr abundances,as well as of the contribution to the s-only isotopes 86Sr,87Sr,96Mo.

S-process branchings at151Sm,154Eu,and163Dy

The $(n,\ensuremath{\gamma})$ cross sections of the stable europium isotopes ${}^{151}\mathrm{Eu}$ and ${}^{153}\mathrm{Eu}$ have been measured by irradiating oxide samples in a quasistellar neutron spectrum. From the induced activities, the stellar cross sections of ${}^{151}\mathrm{Eu}$ and ${}^{153}\mathrm{Eu}$ at a thermal energy of $kT=30 \mathrm{keV}$ were found to be $〈\ensuremath{\sigma}v〉{/v}_{T}=3821\ifmmode\pm\else\textpm\fi{}152 \mathrm{mb}$ and $2733\ifmmode\pm\else\textpm\fi{}110 \mathrm{mb},$ respectively. These results allowed us to resolve severe discrepancies among previous data. Similar activation measurements were also performed on ${}^{152,154}\mathrm{Sm}$ and ${}^{164,170}\mathrm{Er}.$ Among these results, the stellar cross section of the s-only isotope ${}^{164}\mathrm{Er}$ $〈\ensuremath{\sigma}v〉{/v}_{T}=1084\ifmmode\pm\else\textpm\fi{}51 \mathrm{mb}$ at $kT=30 \mathrm{keV}$ is particularly important. Statistical model calculations were performed with emphasis on the effect of excited states as well as on the unstable isotopes ${}^{151}\mathrm{Sm},$ ${}^{152}\mathrm{Eu},$ and ${}^{154}\mathrm{Eu}.$ The combined set of cross sections was used for an updated analysis of the branchings at $A=151,$ 152, 154, and 163. The temperature and density estimates derived via the classical approach are discussed and compared to stellar models for helium shell burning in low mass stars.

Stellar neutron capture cross sections of the Nd isotopes

The neutron capture cross sections of {sup 142}Nd, {sup 143}Nd, {sup 144}Nd, {sup 145}Nd, {sup 146}Nd, and {sup 148}Nd have been measured in the energy range from 3 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the {sup 7}Li(p,n){sup 7}Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4{pi} Barium Fluoride Detector. The cross sections were determined relative to the gold standard. The experiment was difficult due to the small cross sections of the even isotopes at or near the magic neutron number N=82, and also since the isotopic enrichment of some samples was comparably low. The necessary corrections for capture of scattered neutrons and for isotopic impurities could be determined reliably thanks to the high efficiency and the spectroscopic quality of the BaF{sub 2} detector, resulting in a consistent set of (n,{gamma}) cross sections for the six stable neodymium isotopes involved in the s process with typical uncertainties of 1.5{endash}2{percent}. From these data, Maxwellian averaged cross sections were calculated between kT=10 and 100 keV. The astrophysical implications of these results were investigated in an s-process analysis, which deals with the rolemore » of the s-only isotope {sup 142}Nd for the N{sub s}{l_angle}{sigma}{r_angle} systematics near the magic neutron number N=82, the decomposition of the Nd abundances into the respective r-, s-, and p-process components, and the interpretation of isotopic anomalies in meteoritic material. {copyright} {ital 1998} {ital The American Physical Society}« less

Measurements of 142,144Nd(n,γ) cross sections at ORELA for astrophysical s-process studies

We have completed measurements of the 142,144Nd(n,γ) cross sections from approximately 20 eV to 200 keV at the Oak Ridge Electron Linear Accelerator (ORELA) using a recently improved C 6D 6 detector apparatus. 142Nd is an s-only isotope, i.e. it is only formed during the s-process. It has a closed neutron shell and therefore defines a step in the 〈 σ 〉N s curve from which the mean s-process neutron exposure can be calculated. In addition, 144Nd is the normalization point for the neodymium abundances. Also accurate (n,γ) cross sections would help to determine the r- and p-process residuals of these isotopes and will impact the interpretation of the recently discovered isotopic anomalies in silicon carbide grains from the Murchison meteorite. Our new (n,γ) cross sections also show that reaction rate extrapolations for nuclei near closed neutron shells from measured values at 30 keV down to 8 keV can be inaccurate.

The stellar capture rate of 136Ba

High resolution 136Ba(n,γ) measurements were carried out at GELINA and the capture areas of 184 resonances in the range 0.4–60 keV were derived. From these values the Maxwellian - averaged neutron capture cross section was calculated vs temperature kT from 5 to 50 keV. The results for this unbranched s-only isotope are discussed in the light of the recent models of s-process nucleosynthesis.

Neutron capture in neodymium isotopes: implications for the s-process

The neutron capture cross sections of six neodymium isotopes were measured in the energy range from 3 to 225 keV using the Karlsruhe 4π Barium Fluoride Detector. The obtained uncertainty of 1–2% is an improvement by factors of five compared to previous measurements. Stellar capture cross sections were calculated for thermal energies between kT = 10 and 100 keV. These new data allow a detailed analysis of the s-process branchings at 141Ce and 142Pr which are determining the abundance of the s-only isotope 142Nd. The s-process abundance pattern of the six isotopes is in perfect agreement with isotopic anomalies found in the Murchison meteorite.

Stellar neutron capture cross sections of the Ba isotopes.

The neutron capture cross sections of $^{134}\mathrm{Ba}$, $^{135}\mathrm{Ba}$, $^{136}\mathrm{Ba}$, and $^{137}\mathrm{Ba}$ were measured in the energy range from 5 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$ (p,n${)}^{7}$ Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4\ensuremath{\pi} barium fluoride detector. The cross section ratios were determined with an overall uncertainty of \ensuremath{\sim}3%, an improvement by factors of 5 to 8 compared to existing data. Severe discrepancies were found with respect to previous results. As a new possibility in time of flight experiments, isomeric cross section ratios could be determined for $^{135}\mathrm{Ba}$, $^{136}\mathrm{Ba}$, and $^{137}\mathrm{Ba}$. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT=10 keV and 100 keV. These stellar cross sections were used in an s-process analysis. For the s-only isotopes $^{134}\mathrm{Ba}$ and $^{136}\mathrm{Ba}$ the ${\mathit{N}}_{\mathit{s}}$〈\ensuremath{\sigma}〉 ratio was determined to 0.875\ifmmode\pm\else\textpm\fi{}0.025. Hence, a significant branching of the s-process path at $^{134}\mathrm{Cs}$ can be claimed for the first time, in contrast to predictions from the classical approach. This branching yields information on the s-process temperature, indicating values around ${\mathit{T}}_{8}$=2. The new cross sections are also important for the interpretation of barium isotopic anomalies, which were recently discovered in SiC grains of carbonaceous chondrite meteorites. Together with the results from previous experiments on tellurium and samarium, a general improvement of the ${\mathit{N}}_{\mathit{s}}$〈\ensuremath{\sigma}〉 systematics in the mass range A=120\char21{}150 is achieved. This yields a more reliable separation of s- and r-process contributions for comparison with stellar observations, but reveals a 20% discrepancy with respect to the solar barium abundance.

S-Process studies with improved cross sections

New experimental techniques for the determination of neutron capture cross sections, e.g. the Karlsruhe 4 pi Barium Fluoride Detector, have reduced the respective uncertainties by a factor five compared to previous methods. With these improved data, detailed analyses of branchings in the s-process path allow the authors to derive constraints on the physical parameters during the s-process. As first examples, the neutron capture cross sections of the s-only isotopes of tellurium and samarium have been determined. The authors discuss the results in the framework of the classical s-process approach and with a stellar model describing the s-process during helium shell burning in low mass stars. The results for tellurium confirm the prediction of the classical approach of a 'local approximation' within the experimental uncertainty of 1%.

Neutron capture in 148,150Sm: A sensitive probe of the s-process neutron density.

The neutron capture cross sections of $^{147,148,149,150,152}\mathrm{Sm}$ were measured in the energy range from 3 to 225 keV at the Karlsruhe Van de Graaff accelerator using gold as a standard. Neutrons were produced via the $^{7}\mathrm{Li}$(p,n${)}^{7}$Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4\ensuremath{\pi} barium fluoride detector. Several runs were performed under different experimental conditions to study the systematic uncertainties in detail. For the first time, data were recorded with an ADC system that allows one to register gamma-ray energy and time of flight of the individual detector modules. The cross-section ratios were determined with an overall uncertainty of \ensuremath{\sim}1%. This is an improvement by about a factor of 5 compared to the existing data. Severe discrepancies were found with the results of previous measurements. Maxwellian-averaged neutron capture cross sections were calculated for thermal energies between kT=10 and 100 keV by normalizing the cross-section shape up to 700 keV neutron energy reported in literature to the present data. These stellar cross sections were used in an s-process analysis. The ratio of the values of the s-process current 〈\ensuremath{\sigma}〉${\mathit{N}}_{\mathit{s}}$ (Maxwellian-averaged neutron capture cross section times s-process abundance) for the s-only isotopes $^{148,150}\mathrm{Sm}$ is 0.870\ifmmode\pm\else\textpm\fi{}0.009 rather than unity as expected by the local approximation. The corresponding branching in the s-process path is analyzed in the framework of the classical approach. The resulting mean neutron density, ${\mathit{n}}_{\mathit{n}}$=(3.8\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, is the most stringent value obtained so far. Finally the new cross sections are used to derive constraints for a stellar model and to check recently discovered isotopic anomalies in meteoritic samarium.