Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session R3: Neutron Rich Nuclei in the Laboratory and in the Cosmos |
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Sponsoring Units: DNP Chair: Hendrik Schatz, Michigan State University Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), St. Louis E |
Monday, April 14, 2008 10:45AM - 11:21AM |
R3.00001: Observing the signatures of the r-process in metal-poor stars Invited Speaker: In their atmospheres, old metal-poor Galactic stars retain
detailed information about the chemical composition of the
interstellar medium at the time of their birth. Extracting such
stellar abundances enables us to reconstruct the beginning of the
chemical evolution shortly after the Big Bang. About 5\% of
metal-poor stars with $\mbox{[Fe/H]}<-2.5$ display in their
spectrum a
strong enhancement of neutron-capture elements associated with
the rapid (r-) nucleosynthesis process that is responsible for
the production of the heaviest elements in the Universe. This
fortuity provides a unique opportunity of bringing together
astrophysics and nuclear physics because these objects act as
``cosmic lab'' for both fields of study. The so-called r-process
stars are thought to have formed from material enriched in heavy
neutron-capture elements that were created during an r-process
event in a previous generation SN. It appears that the few stars
known with this rare chemical signature all follow the scaled
solar r-process pattern (for the heaviest elements with $56 |
Monday, April 14, 2008 11:21AM - 11:57AM |
R3.00002: New Views of the $r$-Process Invited Speaker: Nucleosynthesis via rapid neutron capture, the $r$-process, is responsible for approximately half of the solar abundances of the nuclei with mass numbers $A > 100$. Five decades after this process was proposed, two outstanding issues remain: (1) which astrophysical environments can provide the physical conditions required for the $r$-process? and (2) what is the detailed nuclear physics input that governs the yield pattern of nuclei from an $r$-process? Both issues are crucial for a full understanding of the $r$-process. This talk will mainly address the issue of the astrophysical sites. While there are no self-consistent models that can produce a robust $r$-process, observations of elemental abundances in old stars of the Galactic halo over the past decade have provided important guidance to the overall nucleosynthetic characteristics of astrophysical $r$-process sources. For example, these observations strongly suggest that the source for the heaviest $r$-process nuclei produces none or very little of the Fe group and lighter nuclei. On the theoretical front, several new mechanisms other than rapid ($r$) or slow ($s$) neutron capture were found to produce the nuclei with $60 < A < 100$ that were thought to be made dominantly by the $r$ and $s$-processes. Major results from the stellar observations will be highlighted. Their implications for astrophysical models of the $r$-process will be discussed. Existing models and possible improvements will be reviewed based on the observational implications. [Preview Abstract] |
Monday, April 14, 2008 11:57AM - 12:33PM |
R3.00003: Discovery of $^{40}$Mg and $^{42}$Al Invited Speaker: Although very neutron rich nuclei do not exist on earth due to their short lifetimes, they do exist in the cosmos where conditions are met that can produce them. This is the case in the crust of accreting neutron stars, where the high gravitational pressure causes electron capture reactions that form neutron rich nuclei up to the drip line. They also can be formed and detected in the laboratory. The neutron rich nuclei $^{40}$Mg and $^{42}$Al have now been observed for the first time.\footnote{Baumann et al. Nature 449 (2007) 1022.} While $^{40}$Mg has long been predicted by many leading nuclear models to be particle bound, the odd-odd neighbor $^{42}$Al was believed to be unbound until now. The discovery was made at the National Superconducting Cyclotron Laboratory, where a primary beam of $^{48}$Ca was fragmented on a tungsten target and the very rarely formed isotopes of $^{40}$Mg and $^{42}$Al, among others, were separated and identified in flight using a two-stage fragment separator. The findings in the laboratory have a direct impact on our knowledge about the cosmos, for these isotopes now have to be included in the composition of the crust of accreting neutron stars. This might have an effect on the crust heating which influences the rate of X-ray superbursts,\footnote{H. Schatz, K. E. Rehm, Nucl. Phys. A 777 (2006) 601.} for which only recently data has become available. The new discoveries are also consequential for theoretical mass predictions, where the uncertainties are still too large for astrophysical applications. Current global mass models differ significantly in the prediction of the neutron drip line in this region. The comparison of the observed isotopes---especially the odd-odd $^{42}$Al---to established theoretical model calculations suggests that the drip line lies further out to heavier isotopes. [Preview Abstract] |
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