Neutrons nuclear capture

For nuclear fission to result in a chain reaction, the sample must be large enough so that most of the neutrons are captured internally. If the sample is too small, most of the neutrons escape, breaking the chain. The critical mass of uranium-235 required to maintain a chain reaction in a bomb appears to be about 1 to 10 kg. In the bomb dropped on Hiroshima, the critical mass was achieved by using a conventional explosive to fire one piece of uranium-235 into another. 

The moderator component of a reactor slows neutrons without capturing them. Moderators are used because the neutrons released in fission have such high kinetic energies that they are difficult to capture. The critical mass of a nuclear fuel is much smaller for slow neutrons than for fast neutrons, so considerably less fuel is needed in a... 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

The abundances are a consequence of how the elements were synthesized by atomic fusion in the cores of stars with heavy elements only made in supernovae. Synthesis of heavier nuclei requires higher temperature and pressures and so gets progressively harder as the atomic number increases. The odd/even alternation (often referred to as the Oddo-Harkins rule) is again general, and reflects the facts that elements with odd mass numbers have larger nuclear capture cross sections and are more likely to take up another neutron, so elements with odd atomic number (and hence odd mass number) are less common than those with even mass number. Even-atomic-number nuclei are more stable when formed. 

In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium-239 is mixed with uranium-238 so that breeding takes place within the core. For every uranium-235 (or plutonium-239) nucleus undergoing fission, more than one neutron is captured by uranium-238 to generate plutonium-239. Thus, the stockpile of fissionable material can be steadily increased as the starting nuclear fuels are consumed. It takes... 

Nuclear fission is the splitting of a large nucleus into two smaller nuclei and one or more neutrons. When the free neutrons are captured efficiently by other nuclei, a chain reaction can occur. 

Meanwhile, in the outer regions of the star, nuclei may be subjected to intense streams of neutrons, which have been generated by the nuclear processes in the core. These neutrons are captured by the nuclei in collisions, and several additional neutrons may accumulate in a given nucleus. At some stage of neutron accumulation, the nucleus becomes so unstable that it spits out an electron— a sign that, in effect, a neutron has collapsed into a proton and a new, heavier element has been formed. Thus, the kingdom gradually extends beyond iron, and elements up to uranium (and perhaps beyond) are formed. 

The cross sections for neutron capture increase for all atoms for thermal energy neutrons. As a result, even though low cross section materials are used some neutrons are captured by the structural and moderator materials. The probability for the non-capture of thermal neutrons in this fashion is signified by/, the thermal utilization factor, which in our case can be assumed to be 0.9. Thus of the original N neutrons 112 thermal neutrons remain in the second generation to cause fission in the nuclear fuel. 

Neutrons, being uncharged, do not interact electromagnetically with electrons or nuclei in matter. Instead, the nuclear interaction with nuclei is the most common interaction, but this can occur only if the neutron comes within 1 fm of the nucleus. Hence, the attenuation coefficient for neutrons is small and neutrons can penetrate large amounts of matter. The main interaction processes are elastic scattering A n,ri)A, inelastic scattering A(n,n )A, radioactive capture [A(n,y)A+1, and other nuclear captures A(n,2n)A - 1, A(n,p)A(Z - 1), A(n,np)A - 1(Z - 1), A(n,a), A(n,f). ... 

In the reactor, the process of decay of a heavy nucleus supersaturated with neutrons is initiated by the nuclear-neutron collision because of electrical neutrality even a low-energy neutron can reach the nucleus vicinity, where it can be captured by the short-acting nuclear potential. The result is the creation of a compound nucleus that shortly decays with mission of more neutrons. Nuclear reactors operate by the forced fission of uranium nuclei. There is a number of possible ways in which fission can occur, e.g., decay with the formation of Rb and Cs nuclei and emission of four neutrons instead of a single neutron captured by the U nucleus ... 

Excitation of bound states by radiative capture of slow neutrons. Radiative capture is mainly important for thermal neutrons for which there is the technical advantage that very large intensities are available from reactors. The energy spectmm of thermal capture radiations has been observed by Kinsey and his collaborators [37] for a large number of nuclei. In this work the neutron capturing sample was placed in a high flux near the core of a nuclear reactor, and collimated... 

The only nuclear process that has been formd practicable for energy production so far is the fission of a radioactive atomic nucleus, either naturally occirrring or artificially generated ( Pu), to release two or three neutrons whose capture by other nuclei induces further energy-releasing fission events in a chain reaction ... 

An ingenious approach to this problem is called boron neutron capture therapy (BNCT). This technique brings together two components, each of which separately has minimal harmful effects on the cells. The first component uses a compound containing a stable boron isotope ( °B) that can be concentrated in tumor cells. The second component is a beam of low-energy neutrons. Upon capturing a neutron, the following nuclear reaction takes place ... 

Table 38.1 lists some examples of reactor-produced radionuclides important to nuclear medicine. There are three general reaction types used neutron radiative capture (n,y) neutron capture followed by particle emission, e.g., (n,n ), (n,p), and (n,a) and fission (n, f). The radionuclides in Table 38.1 are arranged in order of mass numbers. 

Radioactive nuclei emit a particles, 13 particles, positrons, or y rays. The equation for a nuclear reaction includes the particles emitted, and both the mass numbers and the atomic numbers must balance. Uranium-238 is the parent of a natural radioactive decay series. A number of radioactive isotopes, such as and C, can be used to date objects. Artificially radioactive elements are created by the bombardment of other elements by accelerated neutrons, protons, or a particles. Nuclear fission is the splitting of a large nucleus into smaller nuclei plus neutrons. When these neutrons are captured efficiently by other nuclei, an uncontrollable chain reaction can occur. Nuclear reactors use the heat... 

A neutron is captured by a nucleus, producing an excited compound nucleus with a mass of (M2 + 1) which is unstable and decays over a very short time period ( 10 s), resulting in the emission of neutrons, protons and y photons or fission products. The nuclear recoil caused by the emission of the decay products can lead to atom displacement, just as in the previous case. In contrast to PKAs, these atoms are termed recoil atoms [44]. 

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