819 research outputs found
Neutron capture on ^{205}Tl: depicting the abundance pattern of lead isotopes in s-process nucleosynthesis
Proposal: Neutron capture on 205Tl: depicting the abundance pattern of lead isotopes in s-process nucleosynthesisWe propose to use the TOF technique to measure the neutron capture cross section of 205Tl(n,gamma) over the full energy range of stellar interest. An accurate measurement of this cross section is needed for a complete and consistent understanding of the s-process nucleosynthesis of the heaviest nuclei which are produced in low-mass and low metallicity AGB-stars. The only previous TOF measurement has yield only a partial information, insufficient for a reliable analysis of the complex branching pattern around 205Pb and 205Tl. Furthermore, there is also a discrepancy of 40% between the two previous activation measurements made at kT=24 keV. The cross section of 205Tl(n,gamma) is particularly relevant because it affects the equilibrium that is established in some stellar conditions between the 205Tl -> 205Pb bound-state Beta-decay and the 205Pb -> 205Tl E.C. decay. This effect induces a complex interplay which influences the final s-process abundance of both nuclei. We propose to measure accurately and with high resolution the 205Tl(n,gamma) cross section by using a set of four C6D6 detectors in combination with the pulsed neutron-source of CERN n_TOF.Preprin
Forthcoming (n,Îł) measurements on the Fe and Ni isotopes at CERN n_TOF
An overview of the past, present and future research activities at the CERN neutron time of flight facility n TOF is given, with special focus on the astrophysical aspects. During the first campaign (Phase I), neutron capture cross sections of relevance for several aspects of the s-process nucleosynthesis have been measured. A second campaign has recently started (Phase II), aiming at the study of the weak s-process component via accurate (n, Îł) measurements of the Fe and Ni isotopes. Some changes in the facility will allow us to perform these measurements with improved experimental conditions
Approaching the precursor nuclei of the third r-process peak with RIBs
The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the
main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary 238U beam at GSI we were able to measure such properties for several neutron-rich nuclei from 208Hg to 218Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical.
The impact of the uncertainties connected with the eta-decay rates and with beta-delayed neutron emission is illustrated on the basis of r-process network calculations. In order to
obtain a reasonable reproduction of the third r-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA,
commonly used in r-process model calculations. Further measurements around N 126 are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic r-process abundance calculations.Postprint (published version
New Stellar Cross Sections and The "Karlsruhe Astrophysical Database of Nucleosynthesis in Stars"
Since April 2005 a regularly updated stellar neutron cross section
compilation is available online at http://nuclear-astrophysics.fzk.de/kadonis.
This online-database is called the "Karlsruhe Astrophysical Database of
Nucleosynthesis in Stars" project and is based on the previous Bao et al.
compilation from the year 2000. The present version \textsc{KADoNiS} v0.2
(January 2007) includes recommended cross sections for 280 isotopes between
H and Po and 75 semi-empirical estimates for isotopes without
experimental information. Concerning stellar cross sections of the
32 stable, proton-rich isotopes produced by the process experimental
information is only available for 20 isotopes, but 9 of them have rather large
uncertainties of 9%. The first part of a systematic study of stellar
cross sections of the -process isotopes Se, Sr,
Pd, Te, Ba, Ba, Dy, and Hf is
presented. In another application \textsc{KADoNiS} v0.2 was used for an
modification of a reaction library of Basel university. With this modified
library -process network calculations were carried out and compared to
previous results.Comment: Proceedings "International Conference on Nuclear Data for Science and
Technology 2007", Nice/ Franc
Stellar (n,gamma) cross sections of p-process isotopes PartI: 102Pd, 120Te, 130,132Ba,and 156Dy
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.Comment: 13 pages, 4 figure
First tests of the applicability of -ray imaging for background discrimination in time-of-flight neutron capture measurements
In this work we explore for the first time the applicability of using
-ray imaging in neutron capture measurements to identify and suppress
spatially localized background. For this aim, a pinhole gamma camera is
assembled, tested and characterized in terms of energy and spatial performance.
It consists of a monolithic CeBr scintillating crystal coupled to a
position-sensitive photomultiplier and readout through an integrated circuit
AMIC2GR. The pinhole collimator is a massive carven block of lead. A series of
dedicated measurements with calibrated sources and with a neutron beam incident
on a Au sample have been carried out at n_TOF, achieving an enhancement
of a factor of two in the signal-to-background ratio when selecting only those
events coming from the direction of the sample.Comment: Preprint submitted to Nucl. Instr. and Meth.
Conceptual design of the BRIKEN detector: A hybrid neutron-gamma detection system for nuclear physics at the RIB facility of RIKEN
BRIKEN is a complex detection system to be installed at the RIB-facility of the RIKEN Nishina
Center. It is aimed at the detection of heavy-ion implants, Ăź-particles, Âż-rays and Ăź-delayed neu-
trons. The whole detection setup involves the Advanced Implantation Detection Array (AIDA), two
HPGe Clover detectors and a large set of 166 counters of 3He embedded in a high-density polyethy-
lene matrix. This article reports on a novel methodology developed for the conceptual design and
optimisation of the 3He-tubes array, aiming at the best possible performance in terms of neutron
detection. The algorithm is based on a geometric representation of two selected parameters of merit,
namely, average neutron detection efficiency and efficiency flatness, as a function of a reduced num-
ber of geometric variables. The response of the detection system itself, for each configuration, is
obtained from a systematic MC-simulation implemented realistically in Geant4. This approach has
been found to be particularly useful. On the one hand, due to the different types and large number
of 3He-tubes involved and, on the other hand, due to the additional constraints introduced by the
ancillary detectors for charged particles and gamma-rays. Empowered by the robustness of the al-
gorithm, we have been able to design a versatile detection system, which can be easily re-arranged
into a compact mode in order to maximize the neutron detection performance, at the cost of the
gamma-ray sensitivity. In summary, we have designed a system which shows, for neutron energies up
to 1(5) MeV, a rather flat and high average efficiency of 68.6%(64%) and 75.7%(71%) for the hybrid
and compact modes, respectively. The performance of the BRIKEN system has been also quantified
realistically by means of MC-simulations made with different neutron energy distributions.Postprint (published version
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