Reactions Involving Neutrons

Most synthetic isotopes used in medicine and scientific research are made using neutrons as the bombarding particles. Because neutrons are neutral, they are not repelled by the nucleus. Consequently, they do not need to be accelerated to cause nuclear reactions. The neutrons are produced in nuclear reactors. For example, cobalt-60, which is used in cancer radiation therapy, is produced by neutron capture. Iron-58 is placed in a nuclear reactor and bombarded by neutrons to trigger the reactions sequence 

Can an electrostatic or magnetic fieid be used to accelerate neutrons in a particle accelerator Why or why not  

---

Additional interactions of neutrons with nuclei include die release of charged particles by neutron-induced nuclear disintegration, Commonly known reactions are n-p. n — d. and n—ct. In these cases, the incident neutrons may contribute part of their kinetic energy to the target nucleus to effect the disintegration. Hence, more than mere neutron capture is involved, Then, there is usually a lower threshold for the neutron energy below which the reaction fails to occur, Another important reaction involving neutrons is fission, which may occur under different conditions for eidier slow or fast neutrons with appropriate fissionable material. 

Two nuclear reactions involving neutron capture are responsible for the production of the majority of nuclides. The s-process, in which (n,y) reactions occur on a time-scale of 10 to 10 years per neutron capture, gives rise to most of the nuclides in the mass-range 23-46 which are not formed in the a-process, together with a large proportion of those in the range 63 to 209. The s-process is responsible for peaks in the abundance curve at mass numbers 90, 138 and 208. The r-process also involves (n,y) reactions but with a time-scale of 0.01 to 10 sec per neutron capture. It is responsible for very... 

I first heard the possibility of nuclear power seriously discussed in the spring of 1934 when I saw Szilard during a visit to London. The efficiency of collisions between neutrons and nuclei was realized by Szilard simultaneously with and independently from Fermi. He visualized the possibility of chain reactions involving neutrons even at this time. His interest was centered aroimd the reaction... 

Describe the role of a moderator in nuclear reactions involving neutrons. 

The nuclear reactor is a device in which a controlled chain reaction takes place involving neutrons and a heavy element such as uranium. Neutrons are typically absorbed in uranium-235 [15117-96-17, or plutonium-239 [15117 8-5], Pu, nuclei. These nuclei spHt, releasing two fission fragment nuclei... 

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

The most familiar fission reactions involve the splitting of uranium atoms. In these reactions, a uranium-235 atom is bombarded with neutrons. The uranium nucleus then splits apart into various product nuclei. Two examples of fission reactions that involve uranium-235 are shown in Figure 5.10. 

Up to this point, we have been describing single atoms and their electrons. Chemical reactions occur when electrons from the outer shells of atoms of two or more different elements interact. Nuclear reactions involve interactions of particles in the nucleus (mainly protons and neutrons) of atoms, not the atoms electrons. This distinction is fundamental. The former is atomic chemistry (or electron chemistry), and the latter is nuclear chemistry (or nuclear physics). 

The nuclear reaction involving the bombardment of curium with calcium that directly produced element 116 occurred on December 6, 2000, at the Joint Institute for Nuclear Research in Dubna, Russia, in cooperation with personnel of the Lawrence-Livermore Berkeley Group. This nuclear reaction resulted in the production of a few atoms of the isotope ununhexium-292, which has a half-life of 0.6 milliseconds and emits four neutrons. Uuh-292 is also the most stable isotope of element 116 as it continues to decay into elements with Z numbers of 114, 112, 110, 108, and 106, plus emitting four alpha particles for each transmutation. (Z numbers are the number of protons in the nuclei of atoms.)... 

Just to reiterate what we have said, neutron capture is the only valid channel towards the extreme complexity of gold (Z = 79). Reactions involving charged particles are energetically unfavourable and moreover inhibited by insurmountable electrical barriers. Because of the strong electrical repulsion between heavy nuclei (which thus contain many protons), the classic thermonuclear fusion reactions are ineffective, and we are forced to accept the idea that nuclear species beyond iron are produced by a process other than thermonuclear fusion. This process is neutron capture. 

The identification of an isotope of element 95, by Seaborg, Ghiorso, James, and Leon Morgan in late 1944 and early 1945, followed the identification of this isotope of element 96 (242Cm) as a result of the bombardment of 7j Pu with neutrons in a nuclear reactor. The production reactions, involving multiple neutron capture by plutonium, are... 

J.R. Ward J J. Rocchio, Effect of Neutron Radiation on the Rate of Ammonium Perchlorate Decomposition Reactions Involved in Combustion , Ibid, paper 6.4, pp 271—80 (1975) 246) Ibid, Effect of Neutron Radia-... 

Troth (32) has studied the polymerization of vinyl acetate in the presence of triphenylmethane and observed the effects discussed above. In practice, there are complications resulting from reactions involving the initiator radicals and the transfer agent. These complications were found also when carbon tetrabromide was used as a transfer agent in the polymerization of styrene in this case, the bromine contents of the polymers were determined by neutron activation analysis (17). 

Fusion is what powers the Sun and stars. One type of fusion reaction involves the combination of two "heavy" isotopes of hydrogen. Isotopes of an element have the same number of protons, but a different number of neutrons. For example, hydrogen and its isotopes—deuterium and tritium—all have one proton in their nuclei. Remember that the number of protons plus the number of neutrons make up the mass of an atom. Because they have different numbers of neutrons, hydrogen, deuterium, and tritium have different masses. Deuterium has one proton and one neutron. It has a mass of 2 atomic mass units (amu). Deuterium can also be written as hydrogen-2. The number following the element s name is the isotope s mass. Tritium has one proton and two neutrons. So, tritium has a mass of 3 amu. Tritium can be written as hydrogen-3. 

Through the use of alpha particles, neutrons, and other particles, many nuclear reactions have been induced since Rutherford s demonstration of the first transmutation. Examples of several different types of nuclear reactions follow, but no effort is made here to illustrate all the types that are known to occur. An additional example of a nuclear reaction involving bombardment by means of alpha particles is found in the production of radioactive carbon of mass 14 ... 

Because nuclear reactions involve protons and neutrons, collectively called nucleons, chemists use the notation in Figure 14.1 to show the composition of the nucleus. 

One potential problem facing the nuclear power industry is the supply of " fU. Some scientists have suggested that we have nearly depleted those uranium deposits rich enough in U to make production of fissionable fuel economically feasible. Because of this possibility, breeder reactors have been developed, in which fissionable fuel is actually produced while the reactor runs. In a breeder reactor the major component of natural uranium, nonfissionable -9zU, is changed to fissionable Pu. The reaction involves absorption of a neutron, followed by production of two particles ... 

---