Nucleus neutron capture

When a nucleus is placed in a flux of neutrons, it may capture another neutron. It thus is often unstable toward further decay by j3 -emission. The induced radioactivity is critical to the study of chemical consequences of neutron capture, since so few of these new nuclei are produced that generally they cannot be observed by any other means. This radioactivity is not, however, a part of the phenomenon which we wish to observe and, moreover, is occasionally a distraction. 

The energy balance in neutron capture is easily accounted for by use of the law of conservation of mass-energy. Where a nucleus captures a neutron to become we have the reaction energy, Q, given by... 

One way to create unstable nuclides is by neutron capture. Because neutrons have no electrical charge, they readily penetrate any nucleus and may be captured as they pass through a nucleus. The sun emits neutrons, so a continuous stream of solar neutrons bathes the Earth s atmosphere. The most abundant nuclide in the atmosphere,... 

Almost every nuclide undergoes neutron capture if a source of neutrons is available. Unstable nuclides used in radiochemical applications are manufactured by neutron bombardment. A sample containing a suitable target nucleus is exposed to neutrons coming from a nuclear reactor (see Section 22-1). When a target nucleus captures a... 

Neutron capture always is exothermic, because the neutron is attracted to the nucleus by the strong nuclear force. Consequently, neutron capture generates a product nuclide in a metastable, excited state. These excited nuclei typically lose energy by emitting either y rays or protons ... 

Schematic view of fission. Neutron capture produces a highly unstable nucleus that distorts and then splits into smaller nuclei and a few free neutrons. ...

C22-0054. Identify the compound nucleus and final product resulting from each of the following nuclear reactions (a) carbon-12 captures a neutron and then emits a proton (b) the nuclide with eight protons and eight neutrons captures an a particle and emits a y ray and (c) curium-247 is bombarded with boron-11, and the product loses three neutrons. 

Electron capture (EC). In this type of decay, an electron from outside the nucleus is captured by the nucleus. Such a decay mode occurs when there is a greater number of protons than neutrons in the nucleus. 

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy 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... 

Fig. 2.2-1. A neutron capture event seen in relation to the size of the target. Electron microscopic image of uncontrasted tumor tissue, stained for boron by antibodies. The smaller structure surrounded by clusters of dots is the nucleus. The thin structure lined with dots is the cell membrane. The dots are gold particles attached to the antibodies which are specifically directed against the...

Similarly, a series of hydroxy-terminated poly(ether) dendrimers, 35, with a single carborane nucleus at their core, were observed to have water solubilities comparable to that of chloroacetic acid, or D,L-valine. Again this is in direct contrast to the starting carborane nucleus which is insoluble in aqueous solutions and permitted the use of 35 in neutron capture therapy. Similar effects have also been observed with dendrimers containing calixarenes [66] and porphyrins [67] as the central units. 

Beyond iron, nucleosynthesis proceeds via neutron capture by iron and its neighbours. Two types of neutron capture, slow denoted by s and rapid denoted by r, come into play depending on the intensity and duration of neutron irradiation. Once the neutron has been absorbed, the resulting product depends on whether the neutron has time to convert into a proton inside the nucleus before a further neutron is absorbed. If the transmutation occurs before further capture, we have an s process, otherwise an r process. 

Fig. 5.6. Path of s and r processes across the Z, N) plane. Everything begins with iron. The s process follows roughly along the valley of statrility, flowing like a river along the banks it defines. It ends with the a decay of bismuth-209. The r process takes matter far out of the valley on the neutron-rich side, whilst the weak interaction brings it back to the fold. In this case neutron capture continues until the nucleus undergoes fission. The climb to neutron-rich summits is indeed vertiginous.

Heavy nucleus abundances in ancient stars are determined by rapid neutron capture, very probably associated with type II supernovas. 

The effective neutron capture radius of a nucleus having a cross section by 1.0 barn, calculated ... 

Electron capture The final form of beta decay, electron capture, occurs when an inner electron — one in an orbital closest to the atomic nucleus — is captured by an atomic proton (see Chapter 4 for info on orbitals). By capturing the electron, the proton converts into a neutron and emits a neutrino. Here again, the atomic number decreases by 1 ... 

One radioactive decay pathway, electron capture, has been found to have a minor dependence on external conditions of relevance to cosmochemistry. The rate at which an electron in the cloud surrounding the nucleus is captured by a proton to make a neutron is... 

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. 

Additional interactions of neutrons with nuclei include die release of charged particles by neutron-induced nuclear disintegration, Commonly known reactions are n-p. n — d. and n—ct. In these cases, the incident neutrons may contribute part of their kinetic energy to the target nucleus to effect the disintegration. Hence, more than mere neutron capture is involved, Then, there is usually a lower threshold for the neutron energy below which the reaction fails to occur, Another important reaction involving neutrons is fission, which may occur under different conditions for eidier slow or fast neutrons with appropriate fissionable material. 

Let us consider stripping reactions first and, in particular, the most commonly encountered stripping reaction, the (d, p) reaction. Formally, the result of a (d, p) reaction is to introduce a neutron into the target nucleus, and thus this reaction should bear some resemblance to the simple neutron capture reaction. But because of the generally higher angular momenta associated with the (d, p) reaction, there can be differences between the two reactions. Consider the A (d, p) B reaction where the recoil nucleus B is produced in an excited state B. We sketch out a simple picture of this reaction and the momentum relations in Figure 10.16. 

Zo-Ao) 1 Transition state nucleus with saddle deformation S and (E - Ef) MeV of excitation, f Lifetime depends on E - Ef and is about 10"5 sec for thermal neutron capture... 

For the slow neutron capture process, there is an equilibrium between the production and loss of adjacent nuclei. Stable nuclei are only destroyed by neutron capture. For such nuclei, we can write for the rate of change of a nucleus with mass number A ... 

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