# NUCLEAR REACTIONS

NUCLEAR REACTIONS

In the preceding sections we studied the decay of unstable nuclei, especially spontaneous emission of an a or f3 particle, sometimes followed by r emission. Nothing was done to initiate this decay, and nothing could be done to control it. This section examines some nuclear reactions. rearrangements of nuclear components that result from a bombardment by a particle rather than a spontaneous natural process. Rutherford suggested in 1919 that a massive particle with sufficient kinetic energy might
penetrate a nucleus. The result would be either a new nucleus with greater and mass number or a decay of the original nucleus. Rutherford bombarded C with a particles and obtained an oxygen C 10) nucleus and a proton: Rutherford used alpha particles from naturally radioactive sources. In  we’ll describe some of the particle accelerators that are used nowadays to reactions.

Nuclear reactions are subject to several conservation laws. The principles for charge, momentum, angular momentum, and energy  energies) are obeyed in all nuclear reactions. An additional conservation  palpated by classical physics, is conservation of the total number of nucleons.  of protons and neutrons need not be conserved separately; we have seen  neutrons and protons change into one another. We’ll study the basis of the  of nucleon number in Chapter 46. When two nuclei interact, charge conservation requires that the sum atomic numbers must equal the sum of the final atomic numbers. Because of nucleon number, the sum of the initial mass numbers must also the final mass numbers. In general, these are not elastic collisions, and, the total initial mass does not equal the total final mass.

REACTION ENERGY

The difference between the masses before and after the reaction energy, according to the mass-energy relation E = me’, If initial interact to produce final particles C and D, the reaction energy Q is Q = (MA. + MB – Mc – MD)C2 (reaction energy). To balance the electrons, we use the neutral atomic masses in Eq. use the mass of [H for a proton, iH for a deuteron, ;He for an a particle, positive, the total mass decreases and the total kinetic energy is called an reaction. When Q is negative, the mass . kinetic energy decreases, and the reaction is called an and borrowed from chemistry, are also used. reaction the reaction cannot occur at all unless the initial kinetic energy mass reference frame is at least as great as IQI. That is, there is a minimum kinetic energy to make an endothermic reaction go. Ordinarily the endoergic reaction of Example 45-12 would be produced by bombarding stationary I~ nuclei with alpha particles from an accelerator. In this case an alpha’s kinetic energy must be greater than 1.191 MeV. If all the alpha’s kinetic energy went solely to increasing the rest energy, the final kinetic energy would be zero, and momentum would not be conserved. When a particle with mass m and kinetic energy K collides with a stationary particle with mass M, the total kinetic energy Kern in the center-of-mass coordinate system (the energy available to cause reactions) Even though the reaction is exoergic, the proton must have a minimum kinetic energy of about 1.2 MeV for the reaction to occur; unless the proton tunnels through the barrier (see Section 42-5).

NEUTRON ABSORPTION

Absorption of neutrons by nuclei forms an important class of nuclear reactions. Heavy nuclei bombarded by neutrons in a nuclear reaction can undergo a series of neutron absorption alternating with beta decays, in which the mass number A increases by as much as 25. Some of the transuranic elements, elements having Z larger than 92, are produced in this way. These elements have not been found in nature. At this writing, 19 transuranic elements, having Z up to Ill, have been identified. The analytically technique of neutron activation analysis uses similar reactions. When bombarded by neutrons, many stable nuclides absorb a neutron to become unstable and then undergo fJ decay. The energies of the {r and r emissions depend on the unstable nuclide and provide a means of identifying it and the original stable nuclide. Quantities of elements that are far too small for conventional chemical analysis can be detected in this way.