Alpha capture

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

At around 108-109 K alpha capture also occurs, producing a number of nuclei ... 

Big Bang nucleosynthesis (cosmic nucleosynthesis) Proton-proton cycle Triple He collisions Alpha capture CNO cycle Neutron capture High-energy photon collisions produce antimatter-matter pairs. This leads to H,D, He and some Li nuclei Hydrogen burning to produce He 12C production Addition of 4He to the nucleus Production of 13C, 13N, 14N and 150 Post-Fe nuclei... 

Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to 14C dating of water which is more than 40,000 years old. 

Bob nods. This is sometimes called the miracle of alpha capture, because the helium-4 nucleus is also known as an alpha particle. The ant s formula is supposed to represent the final results of combining three helium-4 nuclei, but the formula does not indicate that it takes more than one step to carry this out. ... 

Yes, much more. In order for fife as we know it to eventually arise, the star with triple-alpha capture must explode as a supernova. ... 

One of the cornerstones of this kind of approach is the well established overabundance of the alpha-capture elements relative to Iron in metal-poor stars of the Galactic halo. Mg, Si, Ca, and Ti are generally overabundant by factors of between two and three in stars where Fe is below one tenth solar, i.e. [a/Fe] = +0.3 to +0.5 when [Fe/H] — 1.0 (Ryan et al. 1996). This result can be understood if approximately two thirds of the Fe (and other Fe-peak) elements are produced by Type la supernovae (SN) and released into the ISM with a time lag of about 1 Gyr relative the a-capture elements (and one third of the Fe) manufactured by the massive stars which explode as Type II supernovae. 

One isotope of magnesium does not rely on such exotic processes as those described here, but the process slowly builds up over thousands of years in stellar atmospheres via alpha capture as follows ... 

Mass and charge balance for alpha capture reaction... 

Amine boranes have been examined by a variety of spectroscopic methods (24—29). The boron-substituted alpha-amino acids have been utilized in animal model studies. These compounds along with their precursors and selected derivatives have been shown to possess antineoplastic, antiarthritic, and hypolipidemic activity (30—32). The boron amino acid analogues are also being evaluated for possible utility in boron neutron capture therapy (BNCT) (33). 

There are three common ways by which nuclei can approach the region of stability (1) loss of alpha particles (a-decay) (2) loss of beta particles (/3-decay) (3) capture of an orbital electron. We have already encountered the first type of radioactivity, a-decay, in equation (/0). Emission of a helium nucleus, or alpha particle, is a common form of radioactivity among nuclei with charge greater than 82, since it provides a mechanism by which these nuclei can be converted to new nuclei of lower charge and mass which lie in the belt of stability. The actinides, in particular, are very likely to decay in this way. 

Figure 2. Alpha spectrum for a radium adsorbing manganese-oxide thin film exposed to a groundwater sample, after Surbeck (2000) and Eikenberg et al. (2001b). A 2x2 cm sheet is exposed to O.l-l.O L of sample for 2 days, capturing nearly all of the radium in the sample. These sample discs can be used directly for low-level alpha spectrometry without the need for further separation and preparation methods to produce planar sample sources. Energy resolution is nearly as good as for electroplated sources, and detection limits are typically 0.2 mBqA (6 fg Ra/L) for Ra and " Ra for a one-week counting period. These sensitivities are comparable to traditional methods of alpha spectrometry. [Used by permission of Elsevier Science, from Eikenberg et al. (2001), J Environ Radioact, Vol. 54, Fig. 4, p. 117]...

Radioactivity—Spontaneous nuclear transformations that result in the formation of new elements. These transformations are accomplished by emission of alpha or beta particles from the nucleus or by the capture of an orbital electron. Each of these reactions may or may not be accompanied by a gamma photon. 

McClure, TD and Liebler, DC, 1995. Electron capture negative chemical ionization mass spectrometry and tandem mass-spectrometry analysis of beta-carotene, alpha-tocopherol and their oxidation products. J Mass Spectrom 30, 1480-1488. 

A neutron is captured by a boron atom, and an energetic alpha particle is emitted. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations. 

When the individual insecticides are present in the solution in such a concentration range, the electron capture responds nearly uniformly to all insecticides. A column filled with 1.5% silicone OV-17 plus silicone oil (fluoralchylsiloxane) on Chromasorb W (80-100 mesh) is used for separation of the BHC alpha, beta, gamma and delta isomers (hexachlorocyclohexane), o,p -DDT, p,p -DDE, p,p -DDD, and p,p -DDT. a-BHC and hexachlorobenzene (HCB) have a common peak. They can be separated on a column filled with 2.5% Silicone Oil XE-60 (Is-cyanoethyl-methylsilicone) on Chromosorb W (80-100 mesh). 

Alpha, beta, and gamma emission are the most common types of natural decay modes, but we do occasionally observe positron emission and electron capture. 

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