Neutron capture cross sections for neutron-rich isotopes

Abstract

Average continuum (Hauser-Feshbach) and resonance (Breit-Wigner) neutron capture rates for neutron-rich isotopes, determined on the basis of recent experimental nuclear physics input parameters, are compared to earlier theoretical reaction rates commonly used in astrophysical model computations.Further progress in nuclear astrophysics is related to more experimental and theoretical efforts in nuclear physics. The main difficulty from a nuclear physics point of view in dealing with neutron burning processes in stellar environments such as the n-or the r-process, lies in reliable neutron capture cross sections (On) on very neutron-rich nuclei. The classical assumption of an isotopic (n,y)-(y,n) equilibrium of course enables abundance calculations to be performed without using reaction cross sections. However, a quantitative assessment of the validity of the equilibrium approximation has, after all, to rely on cross section data.Since the direct measurement of On-rates for short-lived radioactive isotopes is not possible, theoretical average continuum cross sections from statistical Hauser-Feshbach (HF) calculations are commonly used /i/. The reliability of these cross sections has suffered so far from the poor knowledge of the relevant nuclear physics input parameters, such as the neutron binding energyBm the level density p(E), the average decay widths F n and F of compound nuclear (CN) states as well as spin ~nd parity of the target JT and of the lowest-energy states of the compound nucleus J~N" Furthermore, HF-calculations may become unreasonable when applied to nuclei with low p(E). In these cases, where O n may be dominated by single resonances in the energy range of astrophysical interest, the Breit-Wigner (BW) resonance formalism has to be used. This formalism requires additional information on each individual CN state, such as resonance energy En, neutron angular momentum i n and partial decay widths F n and Fy, respectively. As has been demonstrated recently at the example of the CN SYKr /2,3/, for quite a number of short-lived isotopes at least a substantial part of the requested nuclear physics parameters can be derived from the 'inverse reaction' to neutron capture, i.e. B-delayed neutron (and y-ray) emission. In this paper we present first results on the application of this method to derive o_-rates for the two stable target 9 88 n 136 nuclez Kr and Xe as well as for the radioactive target isotopes ~2'9~'9~Sr. A schematic description of the inverse relationship between neutron capture and S-delayed neutron emission (Bdn) is shown in Fig.l for the CN 95Sr. Neutron capture on the J~=O + target nucleus 74 s 94Sr will populate states above Bn=4.16 MeV in 9SSr with all possible J~N" The capture cross section is then given by the fraction of those levels which decay by y-ray emission to bound states in 9SSr. Beta decay from 370 ms 9SRb (J~=5/2-), on the other hand, will only populate J~N=(3/2-7/2) -states in 95Sr according to the Gamow-Teller (GT) selection rules. Depending on their partial decay w...

β-Delayed Neutron Spectroscopy Using Trapped Radioactive Ions

“…Delayed-neutron measurements aid in the understanding of the nuclear structure of neutronrich nuclei [12][13][14][15] and are needed to improve nuclearstructure models [16] and empirical predictions [17] used to determine the properties of nuclei for which no data exists. High quality data also has the potential to help determine neutron-capture rates [18,19] for neutron-rich isotopes needed to understand the non-equilibrium phase of r-process nucleosynthesis [20][21][22] and to support the stockpile stewardship mission [23,24].…”

Relevance of β-delayed neutron data for reactor, nuclear physics and astrophysics applications

“…Selected aspects of their applications for reactor control [50,51], nuclear structure far from stability (see, e.g. [15,16,17,19,52,53,54,55,56]) and astrophysics (see, e.g. [18,20,21]) will be presented in the following chapters.…”

“…[18,19,20,21,39,68,69]). The first method comprises neutron-capture and transmission measurements on 86 Kr performed at the Oak Ridge electron linear accelerator ORELA .…”

“…[11,16,18,19]). Together with the use of reliable microscopic β -strength function models, these parameters are essential for calculations of β dn spectra that have not yet been measured.…”