Radioactivity: Introduction and History

Michael F. L'Annunziata

• Publisher: Elsevier Science
• Year: 2007
• ISBN:
  9780080548883
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Radioactivity: Introduction and History provides an introduction to radioactivity from natural and artificial sources on earth and radiation of cosmic origins. This book answers many questions for the student, teacher, and practitioner as to the origins, properties, detection and measurement, and applications of radioactivity. Written at a level that most students and teachers can appreciate, it includes many calculations that students and teachers may use in class work. Radioactivity: Introduction and History also serves as a refresher for experienced practitioners who use radioactive sources in his or her field of work. Also included are historical accounts of the lives and major achievements of many famous pioneers and Nobel Laureates who have contributed to our knowledge of the science of radioactivity.* Provides entry-level overview of every form of radioactivity including natural and artificial sources, and radiation of cosmic origin.
* Includes many solved problems to practical questions concerning nuclear radiation and its interaction with matter
* Historical accounts of the major achievements of pioneers and Nobel Laureates, who have contributed to our current knowledge of radioactivity

Part I

Radioactivity Hall of Fame

Outline

Chapter 1: Alpha Radiation

1

Alpha Radiation

Publisher Summary

This chapter discusses various aspects of alpha radiation, which is made up of alpha particles. An alpha particle, structurally equivalent to the nucleus of a helium atom, consists of two protons and two neutrons. During the process of nuclear decay, the liberated energy (decay energy) is shared between the daughter nucleus and the alpha particle. The two neutrons of an alpha particle give it additional mass that further facilitates ionization by coulombic interaction or even direct collision of the alpha particle with atomic electrons. Alpha particles as well as other types of charged particles dissipate their energy during these collisions mainly by two mechanisms: ionization and electron excitation. The high mass and charge of an alpha particle, relative to other forms of nuclear radiation, give it greater ionization power but a poorer ability to penetrate matter. However, electron excitation occurs when the alpha particle fails to impart sufficient energy to an atomic electron for it to be ejected from the atom. Rather, the atoms or molecules of a given material absorb a portion of the alpha-particle energy and become elevated to a higher energy state. Depending on the absorbing material, the excited atoms or molecules of the material immediately fall back to a lower energy state or ground state by dissipating the absorbed energy as photons of visible light. The chapter also presents the formulas for deriving the range of alpha particles in liquids, solids, and air.

1.1 INTRODUCTION

The alpha particle, structurally equivalent to the nucleus of a helium atom and denoted by the Greek letterα, consists of two protons and two neutrons. It is emitted as a decay product of many radionuclides predominantly of atomic number greater than 82. For example, the radionuclide americium-241 (²⁴¹Am) decays by alpha-particle emission to yield the daughter nuclide ²³⁷Np according to the following equation:

${}_{95}^{241}\text{A}\text{m}\to {}_{93}^{237}\text{N}\text{p}+{}_2^4\text{H}\text{e}+5.63\text{MeV}$ (1.1)

(1.1)

The loss of two protons and two neutrons from the americium nucleus results in a mass reduction of four and a charge reduction of two on the nucleus. In nuclear equations such as the preceding one, the subscript denotes the charge on the nucleus (i.e., the number of protons or atomic number, also referred to as the Z number) and the superscript denotes the mass number (i.e., the number of protons plus neutrons, also referred to as the A number). The 5.63 MeV of eq. (1.1) is the decay energy, which is described subsequently.

1.2 DECAY ENERGY

The energy liberated during nuclear decay is referred to as decay energy. Many reference books report the precise decay energies of radioisotopes. The value reported by Holden (1997a) in the Table of Isotopes for the decay energy of ²⁴¹Am illustrated in eq. (1.1) is 5.63 MeV. Energy and mass are conserved in the process; that is, the energy liberated in radioactive decay is equivalent to the loss of mass by the parent radionuclide (e.g., ²⁴¹Am) or, in other words, the difference in masses between the parent radionuclide and the product nuclide and particle.

We can calculate the energy liberated in the decay of ²⁴¹Am, as well as for any radioisotope decay, by accounting for the mass loss in the decay equation. Using Einstein’s equation for equivalence of mass and energy

$E=m{c}^2$ (1.2)

(1.2)

we can write the expression for the energy equivalence to mass loss in the decay of ²⁴¹Am as

$Q=\left(M_{{241}_{\text{Am}}}-M_{{237}_{\text{Np}}}-M_{\alpha }\right)c^2$ (1.3)

(1.3)

where Q is the disintegration energy released in joules, $M_{{241}_{\text{Am}}},M_{{237}_{\text{Np}}},$

and M_α are the masses of ²⁴¹Am, ²³⁷Np, and the alpha particle in kg and c is the speed of light in a vacuum, 3.00 × 10⁸ m/sec). When the nuclide masses are expressed in the more convenient atomic mass units (u) the energy liberated in decay equations can be calculated in units of megaelectron volts according to the equation

$Q=\left(M_{{241}_{\text{Am}}}-M_{{237}_{\text{Np}}}-M_{\alpha }\right)\left(931.494\text{Me}\text{V}/\text{u}\right)$ (1.4)

(1.4)

The precise atomic mass units obtained from reference tables (Holden, 1997a) can be inserted into eq. (1.4) to obtain

$\begin{array}{l}Q=\left(241.056822\text{u}-237.048166\text{u}-4.00260325\text{u}\right)\left(931.494\text{Me}\text{V}/\text{u}\right)\\ =\left(0.00605275\text{u}\right)\left(931.494\text{Me}\text{V}/\text{u}\right)\\ =5.63\text{MeV}\end{array}$

The energy liberated is shared between the daughter nucleus and the alpha particle. If the parent nuclide (e.g., ²⁴¹Am) is at rest when it decays, most of the decay energy will appear as kinetic energy of the liberated less-massive alpha particle and on...