Alpha reactions

Becausethe4He nucleus does not bind to either proton or neutron, it cannot be destroyed by interacting uuth either in stars. Another mechanism, theso-called triple alpha reaction alloius 4He to/use sloiuly into12 C at the cores 0/ red-giant stars. 

Itis ofinterestthatLi is easily destroyed by (p, alpha) reactions atT = 2.5 MK or greater, so that when newly born stars arrive on the Main Sequence, they have already destroyed Li throughout most of their interiors by these nuclear interactions in the deep convection of the pre-Main-Sequence evolution. The observed Li has survived only in the outermost few percent by mass of the interior of the stars. 

A. Sachpatzidis, C. Dealwis, J.B. Lubetsky, P.H. Liang, K.S. Anderson, and E. Lolis. 1999. Crystallographic studies of phosphonate-based alpha-reaction transition-state analogues complexed to tryptophan synth asQ Biochemistry 38 12665-12674. (PubMed)... 

Stellar helium burning proceeds by means of the triple-alpha reaction in which three " He nuclei... 

In subsequent experiments, Biersack, et al. (29) used the boron (n,alpha) reaction to show the effect of pre- and post-irradiation damage on boron implantation profiles. By post-irradiating a boron implant in silicon with 200 keV H2, a migration of the boron to the Induced damage sites was observed. In the same paper, diffusion and trapping of lithium ions in niobium were reported. Using the lithium (n,alpha) reaction, they mapped irradiation Induced crystal defects through a depth of several micrometers with respect to several sample treatment conditions. 

A few informative properties of life come from easy category distinctions, such as the fact that all known life makes essential use of carbon and carbon-oxygen-nitrogen molecules in liquid water solution. The seemingly trivial observation that such carbaquist chemistry is ruled out if astrophysical carbon abundance lies below a certain threshold enabled Hoyle [1] to predict the 7.6 MeV carbon-12 ( C) nuclear resonance with remarkable precision because the discovery of the triple-alpha reaction synthesis of in stars happens to be a bottleneck for stellar nucleosynthesis of all the heavy elements. The pragmatic information in this prediction is easy to measure because it guided experimental characterization of nuclear structure where the existing computational capabilities could not. Similar sensitive dependence of the physical state of water has been used to define a habitable zone in planetary physics [10], which is not predictive in the same sense as carbon abundance (we already knew where the earth s orbit lies), but which creates a useful filter in the search for extraterrestrial life. 

Thus the triple alpha reaction begins with the formation of 8 Be that has a lifetime of only 1 x 10 16 s (this is found from the width / = 6.8 eV of the... 

The triple alpha reaction has a very strong temperature dependence near a value of temperature To, one can show that the energy generation rate is ... 

The product of the triple-alpha reactions 12 C, is burned into 16O by a-capture... 

A cracial aspect of helium burning is that there is no stable mass eight isotope. This means when two He nuclei combine to form a Be nucleus, the Be falls apart quickly into the two He reactants. The consequence is that helium burning must proceed by a three-body reaction. In particular, if the Be nucleus interacts with another He before falling apart, an excited state of C, denoted C, may result. Decay of the excited C nucleus to its ground state then finishes the so-called triple-alpha reaction. The reaction sequence is thus... 

The two-stage nature of the triple-alpha reaction was recognized by Salpeter in the 1950s. Neverttieless, shortly after Salpeter s insight, Sir Fred Hoyle recognized the triple-alpha reaction would be too slow to explain the abundances of the heavy elements unless the reaction Be + He occurred resonantly in stars. Hoyle pointed out that this required a 0 excited state (that is, a state with spin zero and positive parity) in C at 7.6 MeV, and Cook, Fowler, and collaborators demonstrated the existence of this state experimentally (6)- The prediction of the... 

Carbon. Carbon has two stable isotopes, and which have distinct nucleosynthetic origins. The formerj abundant, isotope is produced during He-buming by the triple-alpha reaction ( 3 He + y) whereas requires seed nuclei and hence... 

The basic detection process uses the neutron-alpha reaction in Li. The 4.78-MeV reaction energy produces an easily detected alpha particle with low sensitivity to gamma radiation and good electrical noise immunity. A prototype neutron detection system using a 20-mm gold window surface barrier detector coupled to a LiF foil was tested at the Los Alamos National Laboratory (LANL) Critical Test Facility in the fall of 1968. The detector unit produced an alarm when placed 580 m from a metal assembly producing 5 X 10 fissions in 33 ps. This detector is presently installed in tlie 11 neutron criticality alarm systems at Rocky Flats. 

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