Elements proton-capture process

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. 

The p-process is one of proton capture (p,y) or of gamma-ray absorption with neutron emission (y,n), and is responsible for many proton-rich nuclides of low abundance, generally derived from iron-group elements. Finally, an x-process is responsible for the synthesis of D, Li, Be and B which are unstable at the temperatures reached inside stars, where they are converted to helium by a large number of processes including 2D(p.y) He, Li(p,a) He and iiB(p,a)8Be -> 2 He. 

For heavy elements with atomic masses greater than ss 56 their proton numbers are so large that electrostatic repulsion inhibits charged particle reactions (e.g., proton and a captures) except under very specific conditions. Most heavy nuclei are instead formed by neutron addition onto abundant Fe-peak elements. For this reason, neutron-capture processes are secondary. That is, they require that some Fe-peak nuclei (e.g., 56Fe) is already present in the star. The solar abundance distribution is characterized by peaks that can be explained by... 

One of the most basic questions in nuclear astrophysics is How do the nuclei heavier than iron get produced This question was first answered by Burbidge, Burbidge, Fowler and Hoyle in 1957 [35]. They proposed that these elements are produced through the slow (s) and rapid (r) neutron-capture processes. The words rapid/slow refer to the rate of neutron capture compared to the rate of /3-decay in the astrophysical conditions. Figure 13 shows the nuclei involved in the r- and s-processes. The s-process path stays close to the valley of stability whereas the r-process path moves staying close to the drip line. The figure also shows the nuclei involved in the rp-process these are proton rich nuclei where capture of protons are involved and that the rate is compared to the / + rates. 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Hydrogen atoms and part of He are believed to have been created during the Big Bang by proton-electron combinations. Most nuclides lighter than iron were created by nuclear fusion reactions in stellar interiors (cf table 11.1). Nuclides heavier than the Fe-group elements (V, Cr, Mn, Fe, Co, Ni) were formed by neutron capture on Fe-group seed nuclei. Two types of neutron capture are possible slow (s-process) and rapid (r-process). 

The main mechanism by which nuclides beyond the iron peak are produced is by neutron capture. The basic processes involved in neutron capture were laid out by Burbidge, Burbidge, Hoyle, and Fowler (1957) (this classic paper is commonly known as B2FH). The common ingredient in these processes is the capture of a neutron by a nucleus, increasing the atomic mass by one unit. If the resulting nucleus is stable, it remains an isotope of the original element. If not, the atom P-decays (a neutron emits an electron and becomes a proton) and becomes an isotope of the next heavier element. Any isotope, whether stable or unstable, can capture another neutron. The rate of capture compared to the rate of decay leads to two basic end-member processes, the -process and the r-proccss. The s-process is capture of neutrons on a time scale that is slow compared to the rate of P-decay. The r-process is neutron capture on such a rapid time scale that many neutrons can be captured before P-decay occurs. 

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

The irradiation of a medium by neutrons also leads to formation of charged particles via the secondary processes. The most important among the latter is the elastic scattering of a neutron with formation of a recoil nucleus. This process is most efficient in media consisting of light elements. Besides this, the slow and thermal neutrons are efficiently captured by certain types of nuclei with ejection of either a proton (for... 

The fourth type of emission is called electron capture. In this process, an inner shell electron is pulled into the nucleus, and when this occurs, the electron combines with a proton to form a neutron. In electron capture reactions, the atomic number decreases by one, the mass number remains the same, and the element changes. One difference in this type of reaction is that the electron is written to the left of the arrow to show that it is consumed, rather than produced, in the process. An example of electron capture can be seen in the following reaction ... 

The two other decay processes in Table 5.4 are less common in nature. In K-capture, any orbiting electron (usually in an inner shell) combines with a proton in the nucleus to form a neutron. This relatively rare nuclear transformation process (e + p+ —> n°) is just the opposite of that for P decay, meaning that the formed nucleus also has the same mass but is displaced one element to the left on the periodic table. Conversion of to °Ar by K-capture is an example of the chemical conversion that can attend radioactive decay, in this case leading to transformation of a non-volatile alkali metal into the inert gas Ar, the third most abundant gas in the atmosphere. Although no nuclear particle is emitted by K-capture, the attending cascade of electrons into lower orbitals leads to X-ray emission of characteristic energy that can be measured by the appropriate detectors. The last decay process (also rare) involves emission of a positron (p+), a positively charged electron. The nuclear process (p+ n° + p+) has the same net effect as K-capture and is also characterized by X-ray emission. 

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