Interaction of Radioactivity with Material - Ionizing Radiation

Interaction of Radioactivity with Material

In addition to nuclear reactions caused by alpha particles and neutrons, high-energy particles and photons also cause ionization of atoms and molecules. These interactions enable us to measure radioactivity, but they also lead to undesirable consequences. Thus, radioactivity is considered hazardous.

Neutrinos do not interact with material in any significant way. They pass through the atmosphere, the earth crust, and the planet as if they do not exist. Because of the low probability of interaction, their detection is very difficult. The escape detection for more than 50 years after they were know to be emitted simultaneously with beta particles.

The major interaction of neutron with matter is nuclear reaction. Neutron absorption usually induces beta-emitting nuclides in a material. Thus neutrons are also hazardous to living organisms.

The discovery of Ionization by Radioactivity

An electroscope consists of two gold leaves suspended from a metallic conductor in a glass jar. Touching the conductor with electric charges causes the leaves stay apart - a charged state due to repulsion of like charges. When electricity is conducted away from the leaves, or neutralizied, the leaves collapse into a discharged state. A charged and a discharged electroscope are shown here.

When radioactive and X-ray sources are brought near a charged electroscope, it became discharged, observed Curie, Rutherford and others soon after their discoveries.

Due to ionization by radioactivity or X-rays, the air contains ions and electrons and becomes conducting. The electroscope discharges as a result.

Ionizing Radiation

Charged particles having high kinetic energies strip electrons off molecules in a material. High energy photons of X- and gamma-rays also knock electrons off atoms or molecules. This process is called ionization, O₂ + energy ® O₂⁺ + e^-
N₂ + energy ® N₂⁺ + e^-
Ar + energy ® Ar⁺ + e^-

Ionization Potential (IP in eV) of Some Substances
Material	IP eV
Air	35
Xenon	22
Helium	43
Ammonia	39
Ge crystal	2.9

Ionization requires energy, and the amount of energy required is called ionization energy. The energy is often expressed in eV, hence it is also called ionization potential. The products O₂⁺ - e^-, N₂⁺ - e^-, and Ar⁺ - e^- are called ion pairs.

High-energy charged particles and photons able to ionize molecules are called ionizing radiation. Some examples are:

Heavy charged particles such as protons, alpha particles, energetic nuclei, mesons, and baryons.
Light charged particles such as electrons and positrons.
Electromagnetic radiation such as X-rays and gamma rays.
High energy neutral particles such as neutrons.

Radiation such as visible light, infrared, microwave, laser, etc. that cause no ionization are non-ionizing radiation.

Tracks of Ionizing Particles

A gas at 273 K and 1.0 atm has 2.7x10²² molecules per liter, or 2.7x10¹⁹ molecules per ml (=cm³). Gas molecules on average travel a short distance (2x10^-7 m or 0.2 micrometer) between collisions. This distance is called mean free path.

A liter of water contains 3.3x10²⁵ molecules, 1,200 times denser than that of a gas. So are atomic densities of other solids and liquid.

Thus, ionizing radiation interacts with many atoms per unit length on its path.

The production of ion pairs by a high-energy particle on its path is depicted in a diagram here. Electrons directly removed from atoms and molecules by radiation are called primary electrons. Some of these electrons carry a very high kinetic energy, and they, cause further ionization. Electrons knocked out by primary electrons are secondary electrons.

Example

At room temperature and pressure, a mole of gas has a volume of 22.4 L. Estimate the molecular density of air.
Solution

We carry out the estimate as follows:

1 mol -------- 22.4 L	6.022e23 molecule ----------- 1 mol	= 2.69x10²² molecules/L

Example 2

From the fact that water has a density of 1.00 kg per L, estimate the molecular density of water.
Solution

We carry out the estimate as follows:

1000 g / L	1 mol ----- 18 g	6.022e23 molecule ----------- 1 mol	= 3.3x10²⁵ molecules/L

Example 3

Silicon has a density of 2330 kg m^-3, estimate the molecular density of water.
Solution

We carry out the estimate as follows:

2.330e6 g --------- 1 m³	1 mol ----- 28.1 g	6.022e23 molecule ----------- 1 mol	= 4.99x10²⁸ molecules m^-3	= 4.99x10²⁵ molecules cm^-3 or molecules/L

The molecular densities of solid and liquid are similar.

Example 1

On average, 35 eV is required to produce an ion pair. How many ion pairs are produced by a alpha particle with a 1.0 MeV kinetic energy? How much total charge is produced?
Solution

The number of pairs and the total positive and negative charges are estimated below:

1.0e6 eV --------- 35 eV	= 28600 pairs.
2.86e4 pairs	2 x 1.6022e-19 C --------- pair	= 9.16e-15 C (positive and negative)

The ion density is very high.