Gi Nuclear Reactions

In this section we present the basic terminology and concepts involved in nuclear reactions. We are primarily concerned with two reactions:

(1) The nucleus can be excited to a higher energy state (analogous to promoting an electron to a higher energy state in atomic spectroscopy); the nucleus can then de-excite by y-ray emission (7 is the Greek letter gamma)

(2) A different nucleus Y may be formed as a result of the nuclear reaction between an incident proton or neutron and the target nucleus X.

In most nuclear reactions we have two particles or nuclei interacting to form two different nuclei. Thus a + b ^ c + d Reactants Products

Although there is no theoretical limitation on what a, b, c, and d can be, as a practical matter each side of the equation usually includes a very light particle. If we designate the particle by a lowercase letter, we can write a nuclear reaction as

In a common shorthand notation, one would write

A nucleus with four neutrons and three protons is designated by

3 x, where the subscript is the atomic number Z, the number of protons, and the superscript is the total number of nucleons, which we term the mass number A. More generally, therefore, a nucleus is designated by

A specific nuclear reaction is

0n + 1§B ^ 4He + ¡Li. where 4He is an alpha (a) particle. In shorthand,

The reactant and product light particles are placed in parentheses and separated by a comma. The terms most frequently used are given below.

Nucleon Either a proton or a neutron.

Nuclide A specific nuclear species with a given proton number Z and neutron number N.

Isotopes Nuclides of the same Z and different N.

Isobars Nuclides of the same mass number A, where A = Z + N.

Alpha (a) A helium nucleus: two protons and two neutrons.

Beta (fi) An electron or positron emitted in a reaction (beta decay).

Gamma (7) A high-energy photon.

In our treatment of atomic reactions and the stable nucleus, the four particles are electron, photon, proton, and neutron. In nuclear reactions and radioactive decay we have particle and antiparticle as well as the subnuclear particle the neutrino. The positron, or antielectron, e+, is identical to the electron, but with positive charge. The positron can be produced along with an energetic electron (electron-positron pair production) by interaction of a gamma ray of energy greater than 1.022 MeV (2m c2) with a nucleus or electron. Other particle/antiparticle pairs can be produced, such as proton-antiproton, which requires 1896-MeV pho-

Fig. G.1. Schematic representation of a nuclear reaction in which a proton (p) strikes a carbon-12 (12C) nucleus and forms a radioactive nitrogen-13 (13N*) nucleus, which decays by emission of gamma (y) rays leaving a product nucleus of 13N, or by emission of protons (p) leaving a product nucleus of 12C, or by emission of neutrons (n), deuterons (d), alpha (a) rays, and others.

product nucleus y i3n p p, KC

Fig. G.1. Schematic representation of a nuclear reaction in which a proton (p) strikes a carbon-12 (12C) nucleus and forms a radioactive nitrogen-13 (13N*) nucleus, which decays by emission of gamma (y) rays leaving a product nucleus of 13N, or by emission of protons (p) leaving a product nucleus of 12C, or by emission of neutrons (n), deuterons (d), alpha (a) rays, and others.

tons. Our interest is primarily in beta rays, electrons, and positrons, because of their emission in beta decay. In beta decay, neutrinos and antineutrinos with symbol v and v are required for conservation of energy. The neutrino has such weak interactions with matter and hence is so difficult to detect that as a practical matter we ignore neutrinos in our description of beta decay.

Let us consider the irradiation of nuclei in a flux of protons, specifically proton irradiation of 12C (Figure G.1). If the protons have sufficient energy to overcome the coulomb barrier, they may actually be captured by the nucleus to form a "compound nucleus." The compound nucleus is now in a highly excited state and can now de-excite in many different ways by emitting y-rays, protons, neutrons, alpha particles, etc. The incident protons can, however, also transfer sufficient energy to single nucleons or groupings of nucleons (such as deuterons and alphas) that they may be directly ejected from the nucleus. Examples of such direct interactions are (p,n) (p,a), (a,p), and (a,n) reactions. Compound nucleus reactions are more likely at relatively low energies, whereas the probability for a direct interaction increases with energy.

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