The Nature of Radioactive Emissions

While the early separation experiments were in progress, an understanding was slowly being gained about the nature of the spontaneous emissions from various radioactive elements. Becquerel thought at first that they were simply X-rays. However, three different kinds of radioactive emissions, now called alpha particles, beta particles, and gamma rays, were soon found. We now know that alpha particles are positively charged particles of helium nuclei, beta particles are streams of high-speed electrons, and gamma rays are high-energy radiation similar to X-rays. The emissions of these three types of radiation are depicted in Figure 2.8 below.

Figure 2.8  Deflection of Radioactive Emissions

The important characteristics of each type of radiation can be summarized as follows:

Tip

Know the types of radiation and the characteristics of each.

Alpha Particle

(helium nucleus ) Positively charged, 2+

1. Ejection reduces the atomic number by 2 and the mass number by 4.
2. High energy, relatively low velocity.
3. Range: about 5 cm in air.
4. Shielding needed: stopped by the thickness of a sheet of paper, skin.
5. Interactions: produces about 100,000 ionizations per centimeter; repelled by the positively charged nucleus; attracts electrons but does not capture them until its speed is much reduced.

6. An example: thorium-230 has an unstable nucleus and undergoes radioactive decay through alpha emission. The nuclear equation that describes this reaction is:

   

   In a decay reaction like this, the initial element (thorium-230) is called the parent nuclide and the resulting element (radium-226) is called the daughter nuclide.

Beta Particle     

(fast electron) Negatively charged, 1–

1. Ejected when a neutron decays into a proton and an electron.
2. High velocity, relatively low energy.
3. Range: about 12 m.
4. Shielding needed: stopped by 1 cm of aluminum or the thickness of the average book.
5. Interactions: weak because of high velocity, but produces about 100 ionizations per centimeter.

6. An example: protactinium-234 is a radioactive nuclide that undergoes beta emission. The nuclear equation is:

   

Gamma Rays

(electromagnetic radiation identical with light; high energy) No charge

1. Beta particles and gamma rays are usually emitted together; after a beta particle is emitted, a gamma ray follows.
2. Arrangement in nucleus is unknown. Same velocity as visible light.
3. Range: no specific range.
4. Shielding needed: about 13 cm of lead.
5. Interactions: weak of itself; gives energy to electrons, which then perform the ionization.