Slow neutrons

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 23su while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 

Fig. 1. Nuclear reactions for the production of heavy elements by intensive slow neutron irradiation. The main line of buildup is designated by heavy...

The First Reactor. When word about the discovery of fission in Germany reached the United States, researchers thereafter found that (/) the principal uranium isotope involved was uranium-235 (2) slow neutrons were very effective in causing fission (J) several fast neutrons were released and (4) a large energy release occurred. The possibiUty of an atom bomb of enormous destmctive power was visualized. 

Neutron economy in graphite occurs because pure graphite has a neutron capture cross section of only 0.0032 0.002 x lO " cm. Taking into account the density of reactor grade graphite (bulk density 1.71 g/cm ), the bulk neutron absorption coefficient is 0.0003/cm. Thus a slow neutron may travel >32 m in graphite without capture. 

S2-4 Helium burning as additional process for nucleogenesis 19S4 Slow neutron absorption added to stellar reactions 195S-7 Comprehensive theory of stellar synthesis of all elements in observed cosmic abundances 196S 2.7 K radiation detected... 

E. Fermi (Rome) demonstration of the existence of new radioactive elements produced by neutron irradiation and for the related discovery of nuclear reactions brought about by slow neutrons. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

Our research at Berkeley has resulted in the discovery of element 94, demonstration of the slow neutron fissiona-bility of its isotope 94239, discovery and demonstration of the slow neutron fissionability of U23 3, spontaneous fission measurements on these isotopes, discovery of 93237, isolation of and nuclear measurements on U23, study of the chemical properties and methods of chemical separation of element 94, demonstration of the presence of small concentrations of 94 in nature and much related information. 

See Molten salt extraction Slow neutron fission and isotopes... 

Induced nuclear fission is fission caused by bombarding a heavy nucleus with neutrons (Fig. 17.23). The nucleus breaks into two fragments when struck by a projectile. Nuclei that can undergo induced fission are called fissionable. For most nuclei, fission takes place only if the impinging neutrons travel so rapidly that they can smash into the nucleus and drive it apart with the shock of impact uranium-238 undergoes fission in this way. Fissile nuclei, however, are nuclei that can be nudged into breaking apart even by slow neutrons. They include uranium-235, uranium-233, and plutonium-239—the fuels of nuclear power plants. 

State whether the following statements are true or false. If false, explain why. (a) The dose equivalent is lower than the actual dose of radiation because it takes into account the differential effects of different types of radiation, (b) Exposure to 1 X 1 ()x Bq of radiation would be much more hazardous than exposure to 10 Ci of radiation, (c) Spontaneous radioactive decay follows first-order kinetics, (d) Fissile nuclei can undergo fission when struck with slow neutrons, whereas fast neutrons are required to split fissionable nuclei. 

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... 

Identification of the isotope 239Np, which is generated by slow-neutron bombardment of 238U and subsequent beta decay. 

It is known that 10B collects in brain tumors to a greater extent than in normal tissue. Research has been conducted on the use of the isotope 10B for treating brain tumors. Bombardment of the tumor with slow neutrons leads to the production of alpha particles (4He2+) and lithium nuclei that have enough energy to destroy the abnormal tissue. 

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. 

Middle-sized stars, between about 1 and 8 M , undergo complicated mixing processes and mass loss in advanced stages of evolution, culminating in the ejection of a planetary nebula while the core becomes a white dwarf. Such stars are important sources of fresh carbon, nitrogen and heavy elements formed by the slow neutron capture (s-) process (see Chapter 6). Finally, small stars below 1 M have lifetimes comparable to the age of the Universe and contribute little to chemical enrichment or gas recycling and merely serve to lock up material. 

When a slow neutron is captured by the nucleus of element X, another isotope of the same element is instantaneously formed, in an excited state because of the impact (labelled compound nucleus in Figure 2.13), which then de-excites by the emission of a gamma particle (and possibly other particles) from the nucleus to produce a radioactive nucleus. For example, when 23Na captures a neutron (signified by on, since neutrons have a mass of one unit, but no electrical charge), it becomes the radioactive nucleus 24Na, as follows ... 

Two different types of information can be obtained by bombarding soil with neutrons. Fast neutrons are slowed when they interact with water and thus can be used to measure the amount of water present. This type of analysis is most often conducted in the field rather than in the laboratory. Figure 8.2 illustrates the use of a fast neutron source and a slow neutron detector to measure the moisture content of soil. This method depends on the interaction of neutrons with hydrogen and so it is not as useful in soils with significant or highly variable organic matter contents. 

Figure 8.2. Neutron probe for determining soil moisture, where nf is fast neutrons and ns is slow neutrons.

In neutron activation analysis, the sample in a suitable container, often a pure polyethylene tube, is bombarded with slow neutrons for a fixed time together with standards. Transmutations convert analyte elements into radioactive elements, which are either different elements or isotopes of the original analyte. 

Intensity Slow Neutron Sources. I. The Szilard-Chalmers Enrichment of 35-9h. 82Br. J. Amer. chem. Soc. 74, 237 (1952). 

The extraction of deuterium from natural water feed forms an excellent case study of the application of large scale distillation and exchange distillation to isotope separation. The principal historical demand for deuterium has been as heavy water, D20, for use in certain nuclear reactors. Deuterium is an excellent neutron moderator, and more importantly, it has a low absorption cross section for slow neutrons. Therefore a reactor moderated and cooled with D20 can be fueled with natural uranium thus avoiding the problems of uranium isotope enrichment. This was the... 

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