NATURAL RADIOACTIVITY

19.2. NATURAL RADIOACTIVITY

The nuclei of the atoms of some isotopes are inherently unstable, and they disintegrate over time to yield other nuclei (Sec. 3.5.). The process is called radioactive decay, and the decay of each nucleus is called an event. There is nothing that humans can do about this type of radioactivity; as long as these isotopes exist, the nuclei will decompose. Depending on the isotope involved, some number of existing atoms decompose over a period of time (Sec. 19.3). Three main types of small particles are emitted from the nuclei during natural radioactive decay; they are named after the first three letters of the Greek alphabet. Their names and properties are listed in Table 19-1. When an alpha or beta particle is emitted from the nucleus, the identity of the element is changed because the atomic number is changed,

A stream of alpha particles is sometimes called an alpha ray. A stream of beta particles is called a beta ray.

A gamma ray is composed of a stream of gamma particles. We can denote the charge and mass number of these small particles as we denoted atomic numbers and mass

numbers in Sec. 3.5.

CHAP. 19]

NUCLEAR REACTIONS

Table 19-1 Products of Natural Radioactivity

Symbol

Name

Mass Number

Charge

Identity

Helium nucleus β

α Alpha particle

High-energy electron γ

Beta particle

Gamma ray 0 0 High-energy particle of light

Mass number

2 a b 0 −1 0 0 g

Charge

The superscripts refer to the mass numbers of the particles; the subscripts refer to their charges. Nuclear equations are written with both the total charge and the total of the mass numbers unchanged from

reactants to products. That is, the total of the subscripts of the reactants equals the total of the subscripts of the products, and the total of the superscripts of the reactants equals the total of the superscripts of the products. The subscripts of isotopes may be omitted because the symbol of the element gives the atomic number.

EXAMPLE 19.1. Show that the mass number and the total charge are both conserved in the natural disintegration of 238 92 U:

238 U −→ 234 Th + α

Ans. The equation may be rewritten including all atomic numbers and mass numbers:

Adding the 234 + 4 superscripts of the products gives the superscript of the reactant. Adding the 90 + 2 subscripts of the products gives the subscript of the reactant. The nuclear equation is balanced.

EXAMPLE 19.2. Complete the following nuclear equation:

233 Pa −→ 233 U+?

Ans. Inserting the proper subscript and superscript indicates that the product is a particle with 1− charge and 0 mass number:

91 Pa −→ 233 92 U+ − 0 1 ?

The missing particle is a beta particle (Table 19-1).

91 Pa −→ 233 92 U+ − 0 1 β

Note that a beta particle has been emitted from the nucleus. This change has been accompanied by the increase in the number of protons by 1 and a decrease in the number of neutrons by 1. In effect, a neutron has been converted to a proton and an electron, and the electron has been ejected from the nucleus.

The emission of a gamma particle causes no change in the charge or mass number of the original particle. (It does cause a change in the internal energy of the nucleus, however.) For example,

50 Sn −→ 119 50 Sn + 0 0 γ

The same 119 50 Sn isotope is produced, but it has a lower energy after the emission of the gamma particle.

NUCLEAR REACTIONS

[ CHAP. 19