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— Partial decay scheme of the 186 Re isomer. Relative γ-ray intensities are indicated. Large number means the energy of the γ-ray and small number means the relative intensity . The isomer of 186 Re decays to the ground state through the internal transitions and subsequently the ground state disintegrates to 186 Os or 186 W.
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We study a new s-process path through an isomer of 186Re to improve a 187Re-187Os nucleo-cosmochronometer. The nucleus 187Re is produced by this new path of 185Re(n, γ)186Rem(n, γ)187Re. We measure a ratio of neutron-capture cross sections for the 185Re(n, γ)186Rem and 185Re(n, γ)186Reg reactions at thermal neutron energy because the ratio with the...
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Citations
... Despite the experimental and theoretical progress in terrestrial studies, much remains uncertain regarding the population of nuclear isomers in astrophysical en-vironments (Aprahamian & Sun 2005;Hayakawa et al. 2005Hayakawa et al. , 2009. The most well studied cases in nuclear astrophysics are in relatively light nuclei. ...
Certain nuclear isomers are well known to affect nucleosynthesis with important observable consequences (e.g., ²⁶ Al and ¹⁸⁰ Ta). We study the impact of nuclear isomers in the context of rapid neutron capture process ( r -process) nucleosynthesis. We demonstrate that nuclear isomers are dynamically populated in the r process and that some are populated far from thermal equilibrium; this makes them astrophysical isomers, or “astromers.” We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent β -decay feeding factors, which represent the fraction of material going to each of the isomer and ground state daughter species from the β -decay parent species. We simulate nucleosynthesis following the decay of a solar-like r -process composition and include as separate species nuclear excited states with measured terrestrial half-lives greater than 100 μ s. We introduce a new metric to identify those astromers most likely to be influential and summarize them in a table. Notable entries include many second peak nuclei (e.g., the Te isotopic chain) and previously overlooked isomers in stable nuclei (e.g., ¹¹⁹ Sn, ¹³¹ Xe, and ¹⁹⁵ Pt). Finally, we comment on the capacity of isomer production to alter radioactive heating in an r -process environment.
... Despite the experimental and theoretical progress in terrestrial studies, much remains uncertain regarding the population of nuclear isomers in astrophysical en-vironments (Aprahamian & Sun 2005;Hayakawa et al. 2005Hayakawa et al. , 2009. The most well studied cases in nuclear astrophysics are in relatively light nuclei. ...
We study the impact of astrophysically relevant nuclear isomers (astromers) in the context of the rapid neutron capture process (r-process) nucleosynthesis. We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent beta-decay feeding factors which represent the fraction of material going to each of the isomer and ground state daughter species from the beta-decay parent species. We simulate nucleosynthesis by including as separate species nuclear excited states with measured terrestrial half-lives greater than 100 microseconds. We find a variety of isomers throughout the chart of nuclides are populated, and we identify those most likely to be influential. We comment on the capacity of isomer production to alter radioactive heating in an r-process environment.
... Interest in the level structures above 186m Re is motivated by the fact that the isomer could contribute to the production of 187 Re in s-process nucleosynthesis. In this context, accurate cross sections for the production of 186m Re via slow-neutron capture on 185 Re are important for reducing the nuclear physics uncertainties in the 187 Re/ 187 Os cosmochronometer[5]. Previous measurements have suggested that 186m Re contributes negligibly to the chronometer uncertainty[5], but they were performed using the activation technique, which is sensitive to the imprecisely known half-life of the isomer. ...
... In this context, accurate cross sections for the production of 186m Re via slow-neutron capture on 185 Re are important for reducing the nuclear physics uncertainties in the 187 Re/ 187 Os cosmochronometer[5]. Previous measurements have suggested that 186m Re contributes negligibly to the chronometer uncertainty[5], but they were performed using the activation technique, which is sensitive to the imprecisely known half-life of the isomer. An alternative approach to determine the 185 Re(n,γ ) 186m Re cross section, which is independent of the isomer half-life, is to apply statistical modeling to the observed capture-γ cascades feeding the isomer. ...
Excited states in 186Re with spins up to J = 12h ̄ were investigated in two separate experiments using 186W(d,2n) reactions at beam energies of 12.5 and 14.5 MeV. Two- and threefold γ-ray coincidence data were collected using the CAESAR and CAGRA spectrometers, respectively, each composed of Compton-suppressed high-purity germanium detectors. Analysis of the data revealed rotational bands built on several two-quasiparticle intrinsic states, including a long-lived Kπ = (8+) isomer. Configuration assignments were supported by an analysis of in-band properties, such as |gK − gR | values. The excitation energies of the observed intrinsic states were compared with results from multi-quasiparticle blocking calculations, based on the Lipkin-Nogami pairing approach, that included contributions from the residual proton-neutron interactions.
... Still little is known about the structure of the 186 Re isomer and the excited states above it, which are needed to improve predictions of the various production and destruction cross sections of the isomer. While recent activation studies [375] indicated that the isomer may be of lesser importance to the 187 Re-187 Os clock, the results depend sensitively on the half-life of the isomer, which is not yet well established. ...
The structure of nuclear isomeric states is reviewed in the context of their role in contemporary nuclear physics research. Emphasis is given to high-spin isomers in heavy nuclei, with [Formula: see text]. The possibility to exploit isomers to study some of the most exotic nuclei is a recurring theme. In spherical nuclei, the role of octupole collectivity is discussed in detail, while in deformed nuclei the limitations of the K quantum number are addressed. Isomer targets and isomer beams are considered, along with applications related to energy storage, astrophysics, medicine, and experimental advances.
... Calculations that include this level result in a cross section for populating the isomer equal to 0.071(24) b. Two measurements have been made of the ratio of thermal-neutron capture cross sections for the 185 Re(n,γ ) 186 Re m and 185 Re(n,γ ) 186 Re g reactions using activation techniques: 0.3% [57] and 0.54(11)% [62]. When combined with the adopted value of σ 0 = 112(2) b for populating the 186 Re ground state [35], these ratios yield isomer cross sections of 0.34(10) b and 0.60(12) b, respectively. ...
... PGAA 111(6) Mughabghab [35] Evaluation 112(2) Seren et al. [64] Activation 101(20) Pomerance [65] Pile oscillator 100(8) Lyon [66] Activation 127.0(127) Karam et al. [67] Activation 96.5(100) Friesenhahn et al. [68] Activation 105(10) Heft [69] Activation 116(5) De Corte et al. [70] Activation 112(18) a Hayakawa et al. [62] Activation 132(26) Farina-Arbocco et al. [71] Activation 111.6 (11) ...
Partial γ-ray production cross sections and the total radiative thermal-neutron capture cross section for the 185Re(n,γ)186Re reaction were measured using the Prompt Gamma Activation Analysis facility at the Budapest Research Reactor with an enriched 185Re target. The 186Re cross sections were standardized using well-known 35Cl(n,γ)36Cl cross sections from irradiation of a stoichiometric natReCl3 target. The resulting cross sections for transitions feeding the 186Re ground state from low-lying levels below a cutoff energy of E_c = 746 keV were combined with a modeled probability of ground-state feeding from levels above E_c to arrive at a total cross section of σ_0 = 111(6) b for radiative thermal-neutron capture on 185Re. A comparison of modeled discrete-level populations with measured transition intensities led to proposed revisions for seven tentative spin-parity assignments in the adopted level scheme for 186Re. Additionally, 102 primary γ rays were measured, including 50 previously unknown. A neutron-separation energy of S_n = 6179.59(5) keV was determined from a global least-squares fit of the measured γ-ray energies to the known 186Re decay scheme. The total capture cross section and separation energy results are comparable to earlier measurements of these values.
... This indicates that the 190 Os/ 188 Os ratio in the projected s-process component is in good agreement with the theoretical value, whereas the s-process 186 Os/ 188 Os ratio obtained from chondrite data is lower than the theoretical value. Yokoyama et al. (2007) concluded that the mismatch could be explained either by another s-process path to produce 187 Re through the relatively long-lived isomer of 186 Re (T 1/2 = 2.0 × 10 5 yr) (Hayakawa et al. 2005;Meyer and Wang 2007), or by errors in the input parameters to the nucleosynthetic calculations. For example, the Maxwellian-averaged neutron capture cross sections (MACS; s n ) of 186 Os, 187 Os, and 188 Os from Mosconi et al. (2006) yield lower s-process 186 Os/ 188 Os ratios than the earlier parameters used in Arlandini (1999) or Bao et al. (2000). ...
Numerous investigations have been devoted to understanding how the materials that contributed to the Solar System formed, were incorporated into the precursor molecular cloud and the protoplanetary disk, and ultimately evolved into the building blocks of planetesimals and planets. Chemical and isotopic analyses of extraterrestrial materials have played a central role in decoding the signatures of individual processes that led to their formation. Among the elements studied, the siderophile and chalcophile elements are crucial for considering a range of formational and evolutionary processes. Consequently, over the past 60 years, considerable effort has been focused on the development of abundance and isotopic analyses of these elements in terrestrial and extraterrestrial materials (e.g., Shirey and Walker 1995; Birck et al. 1997; Reisberg and Meisel 2002; Meisel and Horan 2016, this volume).
In this review, we consider nucleosynthetic isotopic variability of siderophile and chalcophile elements in meteorites. Chapter 4 provides a review for siderophile and chalcophile elements in planetary materials in general (Day et al. 2016, this volume). In many cases, such variability is denoted as an “isotopic anomaly”; however, the term can be ambiguous because several pre- and post- Solar System formation processes can lead to variability of isotopic compositions as recorded in meteorites. Here we strictly define the term “isotopic anomaly” as referring to an isotopic deviation from the terrestrial composition resulting from the incorporation of varying proportions of elements with diverse nucleosynthetic origins into a meteorite component or parent body. The term will not be used here to refer to isotopic variations that result from mass-dependent isotopic fractionation, radioactive decay in the Solar System, or spallation effects.
Based on astronomical observations and physical modelling, the formation of the Solar System has generally been thought to have initiated by the collapse of a dense molecular cloud …
The neutron capture measurement of natural rhenium was performed with the time-of-flight technique at the Back-n facility of the China Spallation Neutron Source in the energy range from 1 to 500 eV of the resolved resonance region (RRR) and from 5 to 400 keV in the unresolved resonance region (URR). Prompt γ-rays originating from neutron-induced capture events were detected by four C6D6 detectors. The pulse height weighting technique and the double-bunch unfolding method based on the Bayesian theory were used in the data analysis. To obtain reliable measurement results, background subtraction, normalization, and corrections were carefully considered. The multilevel R-matrix Bayesian code SAMMY was used to extract the resonance parameters in the RRR. An absence of resonance near 392 eV is observed in our measurement, which has been observed in previous works. The average cross sections in the URR of natural rhenium relative to Au197 were obtained in logarithmical equidistant energy bins with 20 bins per energy decade. The talys code was used to describe the average cross sections in the URR; the Maxwellian-averaged cross sections (MACSs) of Re185 and Re187 are given from kT=5 to 100 keV. At a thermal energy of kT=30 keV, the MACS value for Re185 (1469±127 mb) is in good agreement with the Karlsruhe Astrophysical Database of Nucleosynthesis in Stars (KADoNIS) recommended value (1535±62 mb) within the error bars. By contrast, the value of 1361±118 mb for Re187 shows a discrepancy with the KADoNiS recommended value (1160±57 mb).
The isomeric ratio of 184m, g Re and the half-life of 184g Re were measured in the ¹⁸⁵ Re (n, 2n) ¹⁸⁴ Re reaction at 14.8 MeV, as well as the uncertainty was discussed in detail. The measurements were performed by the activation method implemented for the rhenium sample using the K-400 neutron generator at the Chinese Academy of Engineering Physics (CAEP). Isomeric state and ground state nuclei of ¹⁸⁴ Re were identified by their γ-ray spectra. In order to eliminate the effect of the γ-ray emitted from the isomer on the counting of the ground state characteristic peak, the isomeric ratio of 184m, g Re was calculated to be 0.29 ± 0.11 according to the neutron activation cross-section formula. This result is consistent with previous data within the uncertainty, whereby it can be used to determine parameters that characterize the dependence of the level density on the excitation energy and angular momentum. Through exponential function fitting and a detailed discussion of the uncertainty evaluation, the new recommended half-life of 184g Re was determined as 35.43 ± 0.16 d, which was consistent with the currently recommended value, however, the uncertainty assessment of the latter was barely documented. In addition, this work indicates that the half-life of the ground state can be obtained by eliminating the contamination of the γ-rays emitted from the isomer, which provides the possibility to determine the half-lives of nuclides containing isomers.
The neutron capture cross section of 185Re was measured in the astrophysically important energy region. Measurements were made using a neutron beam from a 7Li(p,n)7Be neutron source with energies ranging from 3 to 90 keV. Two different experimental techniques, time-of-flight (TOF) and activation methods, were employed. In the TOF experiments, the total neutron capture cross section of 185Re was determined by the pulse-height weighting technique. In the activation method, the partial capture cross section leading to the ground state of 186Re was measured by detecting decay γ-rays from neutron activated samples. The present cross section values were compared with evaluated cross section data and previous measurements. The difference between the TOF and activation results was smaller than experimental uncertainties. This suggests that the production cross section of isomer states of 186Re is very small.
