Proton capture

When hydrogen is burned up in the nuclear furnace of a star, helium burning takes over, forming carbon, which in turn leads to oxygen, etc. Subsequent emission processes releasing a-particles, equilibrium processes, neutron absorption, proton capture, etc. lead to heavier elements. 

Proton capture processes by heavy nuclei have already been briefly mentioned in several of the preceding sections. The (p,y) reaction can also be invoked to explain the presence of a number of proton-rich isotopes of lower abundance than those of nearby normal and neutron-rich isotopes (Fig. 1.5). Such isotopes would also result from expulsion of a neutron by a y-ray, i.e. (y,n). Such processes may again be associated with supernovae activity on a very short time scale. With the exceptions of " ln and " Sn, all of the 36 isotopes thought to be produced in this way have even atomic mass numbers the lightest is Se... 

Since a small trace of water suffices to produce a large effect, the equilibrium of (43) evidently lies far in favor of the right-hand side (see also Sec. 115), indicating that a water molecule dissolved in ethyl alcohol provides a vacant level for an additional proton that lies lower than the level occupied by the protons in the OH2 group of the (C2H3OH2)+ ion. A proton captured in this lower level of (HaO)+ will have to wait until it receives the necessary energy before it can move back to an alcohol molecule. In the meantime the (H30)+ ion can merely contribute to the electrical conductivity by drifting slowly in the field only when the proton has returned to an alcohol molecule can the rapid proton jumps be resumed. 

For the Sgr dSph we present the UVES DIC1 spectra for 12 giants. Complete analysis of two of them has already been published [2], while for the other ten only iron and a-elements abundances have been published so far (see [3]). Details on the reduction and analysis procedures, and physical parameters for the stars are provided in [3], but they can be briefly resumed here the spectra have been analyzed by means of LTE, one dimensional atmosphere models, using ATLAS, WIDTH and SYNTHE codes (see [7] and [10]). Te// for the stars are in the range 4800 - 5050 K, log g between 2.3 and 2.7. We analyzed abundances of proton capture (Na, Al, Sc, V), a (Mg, Si, Ca, Ti), Iron-peak (Cr, Fe, Co, Ni, Zn) and heavy neutron-capture (Y, La, Ce, Nd) elements. 

The first reaction is a fusion of two protons to produce a 2H nucleus, a positron (e+) and a neutrino (ve). The second reaction is a proton capture with the formation of 3He and a y-ray. In the third reaction two 3He nuclei fuse to give 4He and two protons. The total energy released in one cycle is 26.8 MeV or 4.30 x 10-12 J. An important product of this process is the neutrino and it should provide a neutrino flux from the Sun that is measurable at the surface of the Earth. However, the measured flux is not as big as calculated for the Sun - the so-called neutrino deficit... 

The Hot Big Bang theory of the Universe was pioneered by George Gamow, R. A. Alpher and R. C. Herman in the late 1940s and early 50s. They supposed that during the first few minutes of the (then radiation-dominated) Universe, matter was originally present in the form of neutrons and that, after some free decay, protons captured neutrons and successive captures, followed by /3-decays, built up all the elements (Alpher Herman 1950). 

In stars with masses greater than 1.2 times the solar mass, hydrogen fusion proceeds via another channel, the so-called CNO cycle. This process tags together proton captures and () decays in the following chain of reactions ... 

The proton capture probability might be measured by the proton affinity. For H20 the latter is 182 kcal/mole while for NH3 a value of 207 kcal/mole has been proposed (13). The (ethyl) namines would probably have intermediate values, considering their pKb. The probability that Reaction 11 could go from left to right would not be too different from that of Reaction 12 going from right to left. Therefore, from Reactions 11 and 12... 

We will make the following assumptions 1) the accreted material is of a solar composition 2) the original white dwarf material is either He or C/0 or 0/Ne/Mg 3) during an outburst the material is well mixed such that the ejected material and the remnant material initially have the same abundance 4) the H and C of any remnant material are burned to He and N before next accretion period 5) all of the excess N comes from proton capture unto C and little 0 is created or destroyed 6) He depletion is negligible 7) the ejected material is composed of white dwarf material, remnant material and accreted material (hereafter referred to as the components of the ejecta) and 8) two sequential out-... 

PROTON-PROTON REACTION. A thermonuclear reaction in which two protons collide at very high velocities and combine to form a deuteron. The resultant deuteron may capture another proton to form tritium and the latter may undergo proton capture to form helium. The proton-proton reaction is now believed to be the principal source of energy within the sun and other stars of its dass. A temperature of the order of five million degrees Kelvin and high hydrogen (proton) concentrations are required for this reaction to proceed at rates compatible with energy emission by such stius. 

Note that this EC decay process does not involve capture of the orbital electron of the 7Be since it is fully ionized in a star but rather involves capture of a free continuum electron. As a consequence, the half-life of this decay is 120 d rather than the terrestrial half-life of 77 d. The resulting 7Li undergoes proton capture as ... 

A small fraction of the 7Be from the 3He + 4He reaction can undergo proton capture, leading to... 

A process that is sometimes related to the p process is the rp process, the rapid proton capture process. This process makes proton-rich nuclei with Z = 7 — 26. This process involves a set of (p, y) and (3+ decays that populate the p-rich nuclei. The process starts as a breakout from the CNO cycle, that is, a side... 

The sulfone (18) (see Section VI,P) has been resolved.214,215 Unlike open-chain a-sulfonyl carbanions, whose generation and proton capture proceeds with high retention of configuration, Corey et Z.214, 216 found that the carbanion generated by base-catalyzed decarboxylation of (+ )-18 gave a completely racemic sulfone (19). It was concluded that the lack of stereospecificity of the reaction is evidence for a planar cyclic a-sulfonyl carbanion intermediate. Cram and Whitney216,218 have studied this reaction in some detail their results indicate that symmetrical (planar) a-sulfonyl carbanions in asymmetric environments are involved as discrete reaction intermediates in the decarboxylation reaction. 

As the temperature and density continue to increase, the 0(a,y) Ne and 0(a,p) F reactions lead to break out from the CNO cycle to a process of rapid proton capture (rp-process) which involves sequential proton captures out to the proton drip line or until the Coulomb barrier becomes too large. Each of these transitions to higher-temperature reactions lead to orders-of-magnitude increases in the rates of energy production. Thus, in addition to effects on nucleosynthesis, the dynamics of the various high temperature environments are intimately coupled to the cross sections for proton and alpha-particle capture reactions on unstable nuclei. In a few cases [WAL81] even the question of whether the next proton or aphha capture leads to a bound nuclear state can have a dramatic effect on the evolution of the environment. 

Electron capture A proton captures an electron (beta particle). The result is a change into a neutron. The capture is in the first shell of the proton, the K orbit, and may be called K-electron capture. (Note This is essentially the reverse of beta decay.)... 

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