Slow neutron reactions

Slow neutron reactions general. An elementary extrapolation of the parameters of slow neutron resonances in heavy elements hows that for energies up to 2 Mev and more the levels of the compound nucleus are discrete and do not overlap to any great extent. At low energies, then, there can be little doubt that the interaction of a neutron with a nucleus results in the formation of a compound nucleus and the decay of this nucleus does not reflect its mode of 

3 See ref. 3, p- 246 the measured temperatures are all consistent with S = kElA with kf 9, as predicted by Bethe s formula (57-4), provided only heavy nuclei are considered. 

A simple expression may be readily obtained for the capture cross section, for 5-wave neutrons. To a first approximation the capture cross section is given by the one-level Breit-Wigner formula  

The weighted mean value of g is near and for most purposes it is sufficient to put g—i. The energy dependence of the quantity in parenthesis in (6I.3) is determined in part by the relative values of and for 7 + =-T. At low 

Since X oci/e and the neutron width is proportional to the neutron velocity, 0 ) must be proportional to e, i.e. (t is proportional to the reciprocal of 

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Although a number of different reactions can be initiated when an element is irradiated with neutrons, this discussion will be restricted essentially to slow neutron reactions of the (n,y) type. These reactions produce a mass increase of unity in any isotope involved and, consequently, cause a change in the isotopic composition. The magnitude of these changes will be determined for the various isotopes by the neutron capture cross section, the neutron flux, and the irradiation time. 

There is one more very important development which I would like to report on. Turner drew attention in a proposed Letter to the editor of the Physical Review in about May 1940 to the fate of the U239. This nucleus is obtained by neutron capture from U238 and this neutron capture forms the most important hindrance to the slow neutron reaction. Turner pointed out that the U239 goes over by -emission into element 93 and this element may be susceptible to fission itself. If it is, this would improve the slow neutron reaction. If it is not, it will absorb an additional neutron until finally a slow neutron sensitive nucleus is produced. 

The cross sections for slow neutron reactions are often on the order of several bams and can in some cases be as large as 10 barn. The predominant reaction for slow neutrons is the (n, y) reaction. 

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...

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... 

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. 

The most sensitive method for determining trace amounts of technetium is the neutron activation . The Tc sample is irradiated by slow neutrons. The radioactive isotope Tc with a half-life of 15.8 s is formed by the reaction Tcfn, y) Tc, the neutron capture cross section of which is comparatively large (20 bams), so that it is possible to determine amounts < 2x 10 " g of Tc. However, the method is not widely used since the half-life of Tc is very short. Moreover, this method is only convenient when a reactor or a neutron source is available. 

Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with its strength and resistance to corrosion, makes it superior to cadmium, which is also used for making control rods for nuclear reactors. This use is of particular importance for the type of nuclear reactors used aboard submarines. By moving the control rods in and out of a nuclear reactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metal of the rods. The drawback to hafnium control rods is their expense it costs approximately one million dollars for several dozen rods for use in a single nuclear reactor. 

This, in turn is produced by successive slow neutron irradiation of curium-244 Californium-254 may be produced by thermonuclear explosion resulting in the reaction of uranium-238 with intense neutron flux followed by a sequence of p- decays (Cunningham, B. B. 1968. In Encyclopedia of Chemical Elements, ed. Clifford A. Hampel, New York Reinhold Book Co.)... 

Your results are very startling. A reaction with slow neutrons that supposedly leads to harium . .. At the moment the assumption of such a thoroughgoing breakup seems very difficult to me, hut in nuclear physics we have experienced so many surprises, that one cannot unconditionally say it is impossible. 

Martin (Ref 5) obtd an expin temp of 245° for 0,02 g of the subst which detond violently after 5 sec, but this compd could not be detond by impact. The photochemical decompn of Na, K Li azides in solns irradiated by UV light of 2537-Xwave length was studied by Bonnemay (Refs 13), For low concns the reactn was homogenous and decompn proceeded at a vel proportional to the conen, but independent of the cation. At high concns the vel of decompn was not explained by a simple law (for example Beer s Law) but showed, after an induction period, that reaction proceeded by chains which formed at the start of photolysis. Crystalline Li azide can be initiated to expln by intense electron streams but not by slow neutron bombardment (Ref 16)... 

The isotope 235 U has great importance because it undergoes the nuclear fission reaction with slow neutrons, and it has been separated in substantial amounts in nearly 100% isotopic composition. 

For slow neutron-induced reactions that do not involve resonances, we know (Chapter 10) that ct ( ) °c 1 /vn so that (ctv) is a constant. For charged particle reactions, one must overcome the repulsive Coulomb force between the positively charged nuclei. For the simplest reaction, p + p, the Coulomb barrier is 550 keV. But, in a typical star such as the sun, kT is 1.3 keV, that is, the nuclear reactions that occur are subbarrier, and the resulting reactions are the result of barrier penetration. (At a proton-proton center-of-mass energy of 1 keV, the barrier penetration probability is 2 x 10-10). At these extreme subbarrier energies, the barrier penetration factor can be approximated as ... 

The measurement of neutron fluxes by foil activation is more complicated because the neutrons are not monoenergetic and the monitor cross sections are energy dependent. The simplest case is monitoring slow neutron fluxes. Radiative capture (ivy) reactions have their largest cross sections at thermal energies and are thus used in slow neutron monitors. Typical slow neutron activation detectors are Mn, Co, Cu, Ag, In, Dy, and Au. Each of these elements has one or more odd A isotopes with a large thermal (n,y) cross section, 1-2000 barns. The (n,y)... 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

Although thermal (slow) neutrons derived from nuclear reactors are the most practical source of particles for nuclear excitation and generally provide the more useful reaction, other excitation sources, such as 14-MeV (fast) neutrons from commercially available accelerators or generators, have also been applied to coal analysis. 

After 1933 Fermi turned increasingly to experimental physics. Inspired by recent work in which artificial radioactive substances were produced by a-particle bombardment, Fermi and several collaborators used neutron bombardment to create several transuranic elements heavier than uranium, including plutonium. This work, and his finding that slow neutrons produce nuclear reactions more efficiently than fast ones, earned Fermi wide acclaim and the 1938 Nobel Prize in physics. After accepting the prize in Sweden, Fermi and his Jewish wife immigrated to the United States to escape the Nazis. 

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