There are four important characteristics of radiation that everyone must recognize:

(1) Radioactive decay is spontaneous: the decay of radioisotopes (also called radionuclides) is spontaneous, and it cannot be changed by physical or chemical means. 

(2) Radiation is affected by electromagnetic field: If the radiation is charged, it will be deflected by the electromagnetic field when it travels. However, gamma radiation and X-rays are not charged, so they are not affected by electric fields when traveling.

(3) The radiation intensity decreases with the increase of time: the decay rate (or radiation intensity) of the radioisotope will decrease with the increase of time. The time required for each half of the radiation intensity is called the half-life. The half-life of each radioisotope is fixed and different, just like a human fingerprint. For example, the half-life of cobalt 60 is 5.26 years, and the half-life of radon 222 in the air is 3.82 days.

(4) Different radiations have different penetrating capabilities: As can be seen from the figure, alpha-ray penetrating capabilities are the weakest, and a piece of paper can block it all. The beta-ray penetrating ability is slightly stronger. It can penetrate paper but cannot penetrate aluminum plates. Gamma and X-rays have the strongest penetrating ability and require concrete or lead plates of appropriate thickness to effectively block them.

Therefore, materials for shielding γ-rays or X-rays need high-density metal materials. The thickness of the container using lead as the cobalt 60 radioactive source is the smallest, iron needs to be thicker, and concrete needs to be thicker. If water is used for shielding, it needs to be thicker to achieve the same shielding effect.