Radiation Shielding

Introduction

Alpha, beta, gamma, and X-rays can pass through matter, but can also be absorbed or scattered in varying degrees, depending on the material and on the type and energy of the radiation. Medical X-ray images are possible because bones absorb X-rays more than soft tissues. Strongly radioactive sources are often stored in lead boxes to shield the local environment from the radiation.

Some materials absorb beta rays. A sheet of common cardboard will absorb some of the betas, but will allow most to pass through. You can measure this absorption by fixing a beta source and a radiation monitor so their positions do not change, and then inserting layers of cardboard between them.

When an absorber is in the path of beta rays, it will allow a certain fraction, ƒ, to pass through. The fraction depends on the density and thickness of the absorber, but will be a constant for identical absorbers and fixed beta-ray energy. If the number of counts detected in a count interval is N₀ when no absorber is in place, then the counts, N, with the absorber, will be N = ƒ N₀. In the preliminary questions, you will develop a more general expression for additional layers of cardboard absorbers, and then test it against real data.

You will use a small source of beta radiation for this experiment. Beta rays are high-energy electrons. Follow all local procedures for handling radioactive materials.

Objectives

In this experiment, you will

• Develop a model for the absorption of beta radiation by matter.
• Use a radiation counter to study how the radiation emitted by a beta source is absorbed by cardboard.
• Test the model against experimental data to determine its validity.