Which type of material is encased in a capsule designed to prevent leakage or escape?

A sealed source is a type of radioactive material that is securely encased in a protective capsule or container to prevent any leakage or escape of radioactivity. This encapsulation is crucial for ensuring safety, as it minimizes the risk of exposure to radiation by containing the radioactive material. The design and material of the capsule are specifically chosen to withstand environmental factors and maintain its integrity over time, further enhancing safety measures when handling or using the source in various applications.

In contrast, loose material, unsealed sources, and open sources lack such protective containment. Loose materials can easily be disturbed, leading to potential contamination. Unsealed sources may be in a form that is not confined, making it possible for radioactive particles to escape. Open sources typically refer to materials that are not contained within a sealed environment, further increasing the risks associated with radiation exposure. Thus, a sealed source is the correct answer as it represents a fundamental safety feature in the handling of radioactive materials.