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Nuclear Processes Study Guide

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Updated on Sep 25, 2011

Introduction

Nuclear. Just a mention of the word scares many people. However, a nuclear reaction is a reaction at the atomic level where energy and/or mass are released or absorbed in a process. Nuclear medicine is one of the fastest-growing branches of medicine and has proven to be beneficial in the detection and treatment of many diseases, including cancer.

Nuclear Reactions

Nuclear chemistry describes reactions involving changes in atomic nuclei. Some isotopes are radioactive and are broken down by nuclear processes. Radioactivity is the process by which unstable nuclei break down spontaneously, emitting particles and/or electromagnetic radiation (i.e., energy), also called nuclear radiation. Heavy elements (from atomic number 83) are naturally radioactive, and many more (the transuranium elements, atomic numbers 93 to 116) have been generated in laboratories.

The types of nuclear radiation include the following:

  • Alpha emission: An alpha particle (symbol: He or ) corresponds to the nucleus of a helium atom (having two protons and two neutrons) that is spontaneously emitted by a nuclear breakdown or decay. The -particles are of low energy and therefore low penetrating (a lab coat is sufficient to block their penetration) but dangerous if inhaled or ingested.
  • Beta emission: A beta particle (symbol: or is an electron released with high speed by a radioactive nucleus in which neutrons (in excess) are converted into protons and electrons (i.e., β-particles). The β-particles are medium-penetrating radiation requiring dense material and several layers of clothing to block their penetration. They are dangerous if inhaled or ingested.
  • Gamma emission: Gamma rays (symbol: γ) are massless and chargeless forms of radiation (pure energy). They are the most penetrating form of radiation, similar to X-rays, and can only be stopped by barriers of heavy materials such as concrete, lead, and so on. They are extremely dangerous and can cause damage to the human body.

In addition, neutrons ( ) can be used to bombard a nucleus, and neutrons can be products of nuclear processes.

Nuclear transmutation is another type of radioactivity occurring when nuclei are bombarded by other particles (protons or neutrons) or nuclei. By this process, lighter elements can be enriched and thus converted to heavier ones or vice versa with emissions of alpha or beta particles and gamma rays. Lord Rutherford observed the first transformation in the early twentieth century:

Notice in the example that the atomic number (protons) adds to nine on each side of the equation and the mass numbers (protons + neutrons) add to 18 on both sides of the equation. During a nuclear reaction, the following occurs:

  • conservation of mass number (i.e., the same number of protons in the products and reactants)
  • conservation of atomic number (i.e., the same number of protons and neutrons in the products and reactants)

The two principle types of nuclear reactions are fission and fusion. Nuclear fusion is the process in which small nuclei are combined (i.e., fused) into larger (more stable) ones with the release of a large amount of energy. Fusion reactions take place at very high temperatures (thermonuclear reactions) as it occurs in the sun. Examples of fusion include the following:

Nuclear fission is the process in which a heavier nucleus (usually less stable) splits into smaller nuclei and neutrons. The process releases a large amount of energy and neutrons that can set up a chain reaction (or self-sustaining nuclear fission reactions) with more and more uncontrollable releases of energy (highly exothermic reactions) and neutrons. Examples of fission include the following:

A radioactive isotope (radioisotope) is an unstable isotope of an element that decays into a more stable isotope of the same element. They are of great use in medicine as tracers (to help monitor particular atoms in chemical and biological reactions) for the purpose of diagnosis (such as imaging) and treatment. Iodine (-131 and -123) and Technetium-99 are used for their short half-lives.

Half-Life

A radioactive isotope's half-life (symbol: ) is the time required for the concentration of the nuclei in a given sample to decrease to half its initial concentration. The half-life is specific to a radioactive element and varies widely (from three hours for Sr-87 to millions of years for U-238, for example). The mathematical expression for half-life is as follows (k is the rate constant):

Example 1:

Technetium-99 ( Tc) is a common radioisotope used in nuclear medicine. The rate constant for Tc-99 is 1.16 * 10-1 h-1.What is the half-life of Tc-99?

Solution 1:

Example 2:

The half-life of a given element is 70 years. How long will it take 5.0 g of this element to be reduced to 1.25 g?

Solution 2:

The easiest way to solve this problem is to recognize that two half-lives will be needed (5.0 →2.5 →1.25) for this decay: 70 years * 2 = 140 years

Practice problems for these concepts can be found at - Nuclear Processes Practice Questions

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