Uranium-238 decay series

Alpha Decay - An ALPHA PARTICLE is an ionizing radiation that consists of two protons and two neutrons. The neutrons and protons give the alpha particle a relatively large mass as compared to other ionizing radiation particles. Because of this large size, the alpha particle has a relatively low speed and low penetrating distance (one or two inches in air). The particle tends to travel in a straight line, causing a large number of ionizations in a small area. Alpha particles are easily shielded (or stopped) by a thin sheet of paper or the body’s outer layer of skin. Since they do not penetrate the outer (dead) layer of skin, they present little or no hazard when they are external to the body. However, alpha particles are considered to be an internal hazard, because they can be in contact with live tissue and have the ability to cause a large number of ionizations in a small area. INTERNAL and EXTERNAL HAZARDS refer to whether the radioactive material is inside the body (internal) or outside the body (external).

Beta Decay - A BETA PARTICLE is a high speed ionizing radiation particle that is usually negatively charged. The charge of a beta particle is equal to that of an electron (positive or negative), and its mass is equal to about 1/1800th of that of a proton or neutron. Due to this relatively low mass and charge, the beta particle can travel through about 10 feet of air and can penetrate very thin layers of materials (for example, aluminum). However, clothing will stop most beta particles. The beta particle can penetrate into the live layers of the skin tissue and is considered both an internal and an external hazard. Beta particles can also be an external hazard to the lens of the eye. Beta particles are best shielded by thin layers of light metals (such as aluminum or copper) and plastics.

Gamma Decay - A GAMMA RAY is an ionizing radiation in the form of electromagnetic energy (no rest mass, no charge) similar in many respects to visible light (but far more energetic). Due to the high energy, no charge, and no rest mass, gamma rays can travel thousands of feet in air and can easily pass through the human body. Because of their penetrating capability, gamma rays are considered both an internal and external hazard. The best shielding materials for gamma rays are very dense materials such as lead, concrete, and uranium.

Neutron Decay - The NEUTRON PARTICLE is an ionizing radiation emitted by nuclear fission and by the decay of some radioactive atoms. Neutrons can range from high speed, high energy particles to low speed, low energy particles (called thermal neutrons). Neutrons can travel hundreds of feet in air and can easily penetrate the human body. Neutrons are considered both an internal and external hazard, although the likelihood of an internal, neutron emitting, radioactive material is extremely unlikely. The best shielding materials for neutrons would be those that contain hydrogen atoms, such as water, polyethylene, and concrete. The nucleus of a hydrogen atom contains a proton. Since a proton and a neutron have almost identical masses, a neutron hitting a hydrogen atom gives up a great amount of its energy, and therefore, the distance traveled by the neutron is limited. This is like a cue ball hitting another billiard ball. Since they are the same size, the cue ball can be made to stop and the other ball will start moving. But, if a ping pong ball is thrown against a bowling ball, the ping pong ball will bounce off with very little change in velocity, only a change in direction. Therefore, heavy atoms, like lead, are not good at stopping neutrons.