150 research outputs found
The Ni(n,) cross section measured with DANCE
The neutron capture cross section of the s-process branch nucleus Ni
affects the abundances of other nuclei in its region, especially Cu and
Zn. In order to determine the energy dependent neutron capture cross
section in the astrophysical energy region, an experiment at the Los Alamos
National Laboratory has been performed using the calorimetric 4 BaF
array DANCE. The (n,) cross section of Ni has been determined
relative to the well known Au standard with uncertainties below 15%.
Various Ni resonances have been identified based on the Q-value.
Furthermore, the s-process sensitivity of the new values was analyzed with the
new network calculation tool NETZ.Comment: 11 pages, 13 page
Nuclear astrophysics with radioactive ions at FAIR
The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process, β-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes
Neutron activation of Ga and Ga at kBT≈25 keV
Background: About 50% of heavy elements are produced by the slow neutron capture process (s process) in stars. The element gallium is mostly produced during the weak s process in massive stars.
Purpose: Our activation at kT≈25 keV is the first experiment in a series of activation and time-of-flight measurements on Ga and Ga relevant for astrophysics.
Methods: We activated Ga and Ga with a neutron distribution that corresponds to a quasistellar distribution with kT=25 keV at the Joint Research Centre (JRC), Geel, Belgium. Protons were provided by an electrostatic Van de Graaff accelerator to produce neutrons via the reaction Li(p,n). The produced activity was measured via the γ emission by the decaying product nuclei by high-purity germanium detectors.
Results: We provide spectrum-averaged cross sections (SACS) and ratios of the cross sections σ/σ for the neutron spectrum of the activation. We obtain values of σ=(186±12) mb and σ = (112±7) mb, and cross section ratios of σ/σ=0.29±0.02 and σ/σ = 0.17±0.01.
Conclusions: Our data disagree with the available evaluated data provided by KADoNiS v0.3, our cross-section ratio is about 20% higher for Ga and about 20% lower for Ga
Thermal neutron capture cross section of the radioactive isotope Fe 60
Background: Fifty percent of the heavy element abundances are produced via slow neutron capture reactions in different stellar scenarios. The underlying nucleosynthesis models need the input of neutron capture cross sections. Purpose: One of the fundamental signatures for active nucleosynthesis in our galaxy is the observation of long-lived radioactive isotopes, such as Fe60 with a half-life of 2.60×106 yr. To reproduce this γ activity in the universe, the nucleosynthesis of Fe60 has to be understood reliably. Methods: An Fe60 sample produced at the Paul Scherrer Institut (Villigen, Switzerland) was activated with thermal and epithermal neutrons at the research reactor at the Johannes Gutenberg-Universität Mainz (Mainz, Germany). Results: The thermal neutron capture cross section has been measured for the first time to σth=0.226(-0.049+0.044)b. An upper limit of σRI<0.50b could be determined for the resonance integral. Conclusions: An extrapolation towards the astrophysically interesting energy regime between kT=10 and 100 keV illustrates that the s-wave part of the direct capture component can be neglected
Thermal (n, γ) cross section and resonance integral of 171Tm
Background: About 50% of the heavy elements are produced in stars during the slow neutron capture process.
The analysis of branching points allows us to set constraints on the temperature and the neutron density in the
interior of stars.
Purpose: The temperature dependence of the branch point 171Tm is weak. Hence, the 171Tm neutron capture
cross section can be used to constrain the neutron density during the main component of the s process in thermally
pulsing asymptotic giant branch (TP-AGB) stars.
Methods: A 171Tm sample produced at the ILL was activated with thermal and epithermal neutrons at the
TRIGA research reactor at the Johannes Gutenberg-Universität Mainz.
Results: The thermal neutron capture cross section and the resonance integral have been measured for the first
time to be σth = 9.9 ± 0.9 b and σRI = 193 ± 14 b.
Conclusions: Based on our results, new estimations of the direct capture components’ impact on the
Maxwellian-nAveraged cross sections (MACS) are possible.European Unions’s Seventh Framework Programme (FP/2007-2013
Ta(n, γ) cross-section measurement and the astrophysical origin of the Ta isotope
Ta is nature\u27s 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 Ta. We report a new measurement of the Ta(n,γ) Ta cross section at thermal-neutron energies via the activation technique. Our results for the thermal and resonance-integral cross sections are 952±57 and 2013±148 b, respectively. The thermal cross section is in good agreement with the only previous measurement [Phys. Rev. C 60, 025802 (1999)], while the resonance integral is different by a factor of ≈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
The unusual spectral energy distribution of LBQS 0102-2713
We have studied the SED of the quasar LBQS 0102-2713. The available
multiwavelength data are one optical spectrum between 3200 and 7400 A, 7 HST
FOS spectra between 1700 and 2300 A, one GALEX NUV flux density and a K_S
magnitude obtained from NED, and 3 public ROSAT PSPC pointed observations in
the 0.12.4 keV energy band. The alpha_ox values obtained are -2.3 and -2.2,
respectively, comparable to BAL quasars. The ROSAT photon index is 6.0+-1.3.
The 2500 A luminosity density is about a factor of 10 higher compared to the
mean of the most luminous SDSS quasars. We argue that the object might be
indicative for a new class of quasars with an unusual combination in their UV-,
X-ray, and N_H properties.Comment: 16 pages, 8 figures, accepted by Ap
63Cu(n,γ) cross section measured via 25 keV activation and time of flight
In the nuclear mass range A ≈ 60 to 90 of the solar abundance distribution the weak s-process component is the dominant contributor. In this scenario, which is related to massive stars, the overall neutron exposure is not sufficient for the s process to reach mass flow equilibrium. Hence, abundances and isotopic ratios are very sensitive to the neutron capture cross sections of single isotopes, and nucleosynthesis models need accurate experimental data. In this work we report on a new measurement of the 63Cu(n,γ ) cross section for which the existing experimental data show large discrepancies. The 63Cu(n,γ ) cross section at kBT = 25 keV was determined via activation with a quasistellar neutron spectrum at the Joint Research Centre (JRC) in Geel, and the energy dependence was determined with the time-of-flight technique and the calorimetric 4π BaF2 detector array DANCE at the Los Alamos National Laboratory.We provide new cross section data for the whole astrophysically relevant energy range.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard
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