11.4 Nuclear radioactivity  (Page 3/12)

Like x rays, nuclear radiation in the form of $\alpha$ s, $\beta$ s, and $\gamma$ s has enough energy per event to ionize atoms and molecules in any material. The energy emitted in various nuclear decays ranges from a few $\text{keV}$ to more than $\text{10 MeV}$ , while only a few $\text{eV}$ are needed to produce ionization. The effects of x rays and nuclear radiation on biological tissues and other materials, such as solid state electronics, are directly related to the ionization they produce. All of them, for example, can damage electronics or kill cancer cells. In addition, methods for detecting x rays and nuclear radiation are based on ionization, directly or indirectly. All of them can ionize the air between the plates of a capacitor, for example, causing it to discharge. This is the basis of inexpensive personal radiation monitors, such as pictured in [link] . Apart from $\alpha$ , $\beta$ , and $\gamma$ , there are other forms of nuclear radiation as well, and these also produce ionization with similar effects. We define ionizing radiation     as any form of radiation that produces ionization whether nuclear in origin or not, since the effects and detection of the radiation are related to ionization.

The range of radiation    is defined to be the distance it can travel through a material. Range is related to several factors, including the energy of the radiation, the material encountered, and the type of radiation (see [link] ). The higher the energy , the greater the range, all other factors being the same. This makes good sense, since radiation loses its energy in materials primarily by producing ionization in them, and each ionization of an atom or a molecule requires energy that is removed from the radiation. The amount of ionization is, thus, directly proportional to the energy of the particle of radiation, as is its range.

Radiation can be absorbed or shielded by materials, such as the lead aprons dentists drape on us when taking x rays. Lead is a particularly effective shield compared with other materials, such as plastic or air. How does the range of radiation depend on material ? Ionizing radiation interacts best with charged particles in a material. Since electrons have small masses, they most readily absorb the energy of the radiation in collisions. The greater the density of a material and, in particular, the greater the density of electrons within a material, the smaller the range of radiation.