The alpha particle emitted in this type of radioactivity is a doubly charged ion of helium, 4He2+. All alpha particles emitted by a given nuclide either have the same energy or have at most a few different energy values. Energies are in the range of 2 to 8 million electron volts (MeV), with higher energies associated with nuclides of shorter half-life.

In passing through matter, alpha particles give up their energy and become neutral helium atoms. Their range in sohds and liquids is very short; an ordinary sheet of paper will stop alpha particles; the range-energy curve for air at standard conditions is shown in Fig. 2.1. Because of their short range, alpha particles do not constitute an external hazard to human beings. They are absorbed in the outer layers of the skin before they cause injury. On the other hand, if alpha-emitting elements are taken internally, they are very toxic, because of the large amount of energy released in a short distance within living tissue. For example, 1 X 10"7 g of radium is the maximum amount that may safely be allowed to accumulate in the human body.

Alpha radioactivity is found principally among elements beyond bismuth in the periodic table. All the nuclides important as fissionable or fertile material are alpha emitters, with half-lives and decay energies given in Table 2.1. These half-lives are so long that depletion of these fuel species by radioactive decay is not important, but all these nuclides are toxic, especially plutonium, which is even more toxic than radium.

tGram-atom is that quantity of material whose mass in grams is equal to its atomic mass. Similarly, the mass in grams of 1 gram-mole of material is numerically equal to the molecular weight, and Avogadro’s constant is also the number of molecules per gram-mole.

Figure 2.1 Range of alpha particles in air at 0 C, 760 Toit.