The Physics of the Atomic Bomb (Level 2)

Level 2: Nuclear Emissions and their Penetrating Powers

As mentioned in Level 1 of Physics, radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. There are 3 ways in which radioactive decay can occur. 

Radiation can be absorbed by objects in its path and becomes less intense the further they are from the radioactive material. This is because the nuclear emissions become more spread out. The thicker the substance, the greater the amount of radiation absorbed.

1. Alpha Particles

When an alpha particle is ejected from an unstable nucleus, the nucleus loses 2 protons and 2 neutrons. Alpha particles have the largest mass among the 3 types of nuclear emissions. They collide with O₂ and N₂ molecules in the air, losing some of their energy in ionizing the air molecules until eventually they give up all of their energy and are absorbed. [Ionization is the process in which an atom or molecule loses or gains electrons. Since electrons have a relative charge of -1, the atom or molecule can become Ions which are positively or negatively charged particles.] Hence, alpha particles have poor penetrating power, and can be absorbed by a thin sheet of paper or a few centimeters of air. They are the slowest type of nuclear emission and have a speed 1/10 of the speed of light (3x10⁸ m/s)

2. Beta Particles

In beta decay, the unstable nucleus converts a neutron into a proton and an electron. The beta particle is an electron that is ejected from the nucleus at a speed 9/10 of the speed of light. Beta particles carry less charge (-1) compared to alpha particles (+2), and thus react less with the atoms and molecules of objects in its path. Beta particles can be stopped by a thin sheet of aluminium.

3. Gamma Rays

After emitting an alpha or beta particle, the remaining nuclei may be at a higher energy level. When it returns to its 'normal' state, gamma radiation is emitted in the form of gamma rays. Gamma rays travel at the speed of light, are the most penetrating and can only be stopped by many centimetres of lead, or many metres of concrete.

Content adapted from: 

http://www.bbc.co.uk/schools/gcsebitesize/science/21c_pre_2011/energy/nuclearradiationrev2.shtml

Q: Suppose you are given 3 radioactive cookies: The 1st is an alpha emitter, the 2nd is a beta emitter and the 3rd is a gamma emitter. You must eat one, hold one in your hand, and put the third in your pocket. What can you do to minimize your exposure to radiation?

Ans: Eat the gamma emitter, hold the alpha emitter in your hand, and put the beta emitter in your pocket. 

Q: What is the significance of knowing about the types of nuclear emissions?

Ans: As you will learn in the biology section, different types of nuclear emissions have different penetrating powers and thus have different health effects.