Neutron chain carrier

If the three product neutrons strike three other fissile nuclei, then after the next round of fission there will be nine neutrons, which can induce fission in nine more nuclei. In the language of Section 13.9, neutrons are carriers in a branched chain reaction (Fig. 17.24). 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

Nuclear energy can be extracted by arranging for a nuclear chain reaction to take place in a critical mass of fissionable material. with neutrons as the chain carriers. A moderator is used to reduce the speeds of the neutrons in a reactor that uses fissile material. 

Perhaps the most striking feature of chain reactions is that some of them can result in detonation. If the propagation mechanism includes a step or steps that produce more chain carriers than they consume, the reaction is self-accelerating. This is called chain branching. The result may be an event much more violent than a thermal explosion, in which self-acceleration stems from temperature increase owing to the inability of heat transfer to keep up with the heat production of the reaction. The detonation of a nuclear bomb can be viewed as a chain reaction with neutrons as carriers and with chain branching. 

Neutrons are chain carriers in a branched chain reaction (see Section 13.12). 

At the time our work was undertaken, the opinions of our colleagues were quite divided concerning the possibility of establishing a self-sustaining fission chain reaction with natural uranium. Most of our responsible physicists were inclined to question this possibility. However, Fermi and Szilard had made their historic decision to explore first the possibility of using thermal neutrons as chain carriers. This decision implied that... 

The detailed features of the chain reaction are determined by the various nuclear processes which can occur between the free neutrons and the materials of the reactor system. As in chemical chain reactions, the rates of the reactions involved in the chain are directly dependent upon the density of the chain carrier, in the present case the neutrons. Thus in order to determine the various properties of a reactor, such as the power-production rate and the radiation-shielding requirements, it is necessary to obtain the fission reaction rate throughout the system and, therefore, the neutron-density distribution. In fact, all the basic nuclear and engineering features of a reactor may be traced back ultimately to a knowledge of these distribution functions. 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

Another major turning point in the history of nuclear science came with the discovery of fission by Otto Hahn and Fritz Strassmann in December 1938 (Hahn and Strassmann 1939a, b). In several laboratories in Rome, Berlin, and Paris, a complex series of P-decay chains resulting from neutron irradiation of uranium had been investigated since 1934, and these chains had been assigned to putative transuranium elements formed by neutron capture in uranium with subsequent P" transitions increasing the atomic numbers (see Sect. 1.2.3). But then evidence appeared that known elements in the vicinity of uranium, such as radium, were produced as well. When Hahn and Strassmaim attempted to prove this by a classical fractional crystallization separation of radium from barium serving as its carrier, the radioactivity turned out to be barium, not radium hence, new and totally unexpected type of nuclear reaction had to be invoked. 

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