Neutron liberation

The neutron-capture cross section of any isotope represents the probability with which it is able to capture free neutrons passing by it. This quantity is important for the s process of nucleosynthesis. This process was named s owing to the need to patiently make the s isotopes of the heavy elements by the slow capture of free neutrons, neutrons liberated by other nuclear reactions within the gas in stellar interiors. It was one of the first nucleosynthesis processes identified historically. The capture of a free neutron by a nucleus increases its mass number A by one unit. As the captures continue, each nucleus in the gas is rendered heavier, little by little, capture by capture. When an isotope of mass number A of an element with atomic number Z captures a neutron, the compound nucleus formed from their union becomes an isotope of the same element but having mass number greater by one unit (i.e. A + l). 

The other main site for the s process occurs in massive stars, more than twelve times more massive than the Sun. They burn their helium in a large convective ( boiling ) core, usually comprising about40% of the entire mass of the star. A portion of the 22Ne reacts there with He nuclei and liberates neutrons by the same (alpha, n) reaction that occurs in AGB stars. There results a weaker fluence of neutrons liberated... 

This is illustrated in Fig. 11.1. Fission of may be taken as an example v is the average number of neutrons liberated in nuclear fission of by thermal neutrons and F[/Z a is the ratio of the macroscopic cross sections for fission and for absorption of neutrons. For pure nuclides (e.g. pure iTf/iTa = The fission factor... 

Average number of neutrons liberated in thermal neutron fission 3.13 2.43 2.87... 

Cl is produced in the atmosphere by (n,p) reaction with Ar and (p,na) reaction with Ar. It also comes down with the precipitations. Appreciable amounts of C1 have been formed in the atmosphere by the neutrons liberated by nuclear explosions. Consequently, the C1 deposition by precipitations increased from about 20 to about 5000 atoms per m per s in the years 1955 to 1962, and since then it has decreased slowly to the original value. 

Six groups of fission products are particularly important to the control engineer. These decay to form daughter nuclei that are in a sufficiently excited state to throw off a neutron on formation. Such neutrons are called delayed neutrons, and the six groups of fission products are known as delayed neutron precursors. Delayed neutrons make up less than 1% of the total number of neutrons liberated by fission, but the fact that they are released on a much slower timescale than the prompt neutrons liberated at the time of fission renders the control of nuclear reactors relatively easy. 

The high speed neutrons liberated by fission of U(235) mostly escape or are neutralised by foreign bodies during natural disintegration. As, however, usually one to three secondary high speed neutrons are liberated for each fruitful collision it is clear that, if sufficient of these could be slowed down to thermal velocities and themselves allowed to combine fruitfully with further U(235) atoms, the process might be made continuous or chain-wise. This is shown diagrammatically in Fig. io. 

The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons. This one, Number of neutrons liberated in the nuclear fission of uranium, rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fissioa The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions, the three men wrote, was already mentioned in our previous letter. Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied. That is, uranium would most probably chain-react. 

H. Halban et al., Number of Neutrons Liberated in the Nuclear Fission of Uranium, Nature 43, 680 (1939). 

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