High-resolution Neutron Capture and Transmission Measurements and the Stellar Neutron Capture Cross Sections Of[sup 116,120]n


Book Description

Improved astrophysical reaction rates for[sup 116,120]Sn(n, [gamma]) are of interest because nucleosynthesis models have not been able to reproduce the observed abundances in this mass region. For example, previous s-process calculations have consistently underproduced the s-only isotope[sup 116]Sn. Also, these studies have resulted in residual reprocess abundances for the tin isotopes which are systematically larger than predicted by reprocess calculations. It has been suggested that these problems could be solved by reducing the solar tin abundance by 10-20%, but there is no experimental evidence to justify this renormalization. Instead, it is possible that the problem lies in the (n, T) cross sections used in the reaction network calculations or in the s-process models. One reason to suspect the (n, [gamma]) data is that previous measurements did not extend to low enough energies to determine accurately the Maxwellian-averaged capture cross sections at the low temperatures (kT=6-8 keV) favored by the most recent stellar models of the s process. Also, the two most recent high-precision measurements of the[sup 120]Sn(n, [gamma]) cross section are in serious disagreement. Because of its small size, this cross section could affect (via the s-process branching at[sup 121]Sn) the relative abundances of the three s-only isotopes of Te.



















Neutron Cross Sections


Book Description

Neutron Cross Sections presents the principles of cross-section measurement and use, as well as sufficient theory so that the general behavior of cross sections is made understandable. This compilation is a direct result of experiences connected with the collection and evaluation of cross-section data during the past eight years at ""Sigma Centre"", Brookhaven National Laboratory. Here, experimental results received from laboratories throughout the world are carefully evaluated and compiled in the curves and tables of the large volume Neutron Cross Sections, The most recent version of the compilation, known as BNL 325, appeared 1 July 1955, and Supplement 1 to BNL 325 was published on 1 January 1957. The compilation itself consists almost completely of cross sections at specific energies, shown in the form of curves or tables, with only brief explanatory texts. The text opens with discussions of the general properties of cross sections and principles of nuclear structure that are important to the understanding of cross-section behavior. Separate chapters follow that describe specific techniques for measuring cross sections along with experimental results involving fast neutrons, resonance neutrons, resonances in fissionable materials, and thermal neutrons.




Neutron Capture Cross Sections for 86Sr and 87Sr at Stellar Temperatures. Revision 1


Book Description

Recent work on s-process nucleosynthesis has focused attention on the investigation of capture cross sections for nuclei in the mass region near the N = 50 closed neutron shell. Of special astrophysical interest are (i) the analysis of the s-process branching through 85Kr as a monitor of stellar neutron density and temperature and (ii) the investigation of the possible chronometric pair 87Rb-87Sr as an independent measure of the age of the galaxy. For both problems the capture cross sections of the two pure s-process nuclei 86Sr and 87Sr have to be known to an accuracy of 5% or better. The current investigation of the neutron capture cross sections for 86Sr and 87Sr was undertaken to extend recent measurements by Walter and Beer to energies below 3.5 keV, where strong resonances are known to exist, and to explore the discrepancy in the results of the Maxwellian averaged capture cross section of 87Sr at kT = 30 keV as reported by previous investigators. 9 refs., 1 fig.




Atlas of Neutron Resonances


Book Description

Atlas of Neutron Resonances: Resonance Properties and Thermal Cross Sections Z= 1-60, Sixth Edition, contains an extensive list of detailed individual neutron resonance parameters for Z=1-60, as well as thermal cross sections, capture resonance integrals, average resonance parameters and a short survey of the physics of thermal and resonance neutrons. The long introduction contains: nuclear physics formulas aimed at neutron physicists; topics of special interest such as valence neutron capture, nuclear level density parameters, and s-, p-, and d-wave neutron strength functions; and various comparisons of measured quantities with the predictions of nuclear models, such as the optical model. As in the last edition, additional features have been added to appeal to a wider spectrum of users. These include: spin-dependent scattering lengths that are of interest to solid-state physicists, nuclear physicists and neutron evaluators; calculated and measured Maxwellian average 5-keV and 30-keV capture cross sections of importance to astrophysicists involved in nucleosynthesis modeling; s-, p-, and d-wave average radiative widths; and, nuclear level density parameters. - Provides a comparison of average resonance parameters with optical model calculations and with the generalized Landau-Fermi model - Presents scattering radii for various partial waves from the analysis of total neutron cross sections in the keV to MeV energy region - Includes a brief review of sub-threshold fission - Examines consistent treatment of average neutron parameters with values from the resolved resonance region




Neutron Cross Sections


Book Description