radioactive sources for use in school experiments are always stored in a container lined with which material give the full name not the symbol

Lead (pb)

Explanation:

There are 3 types of radiation:

1) Alpha radiation: has a strong ionization energy and easily absorbed by light material like paper

2) Beta particles: moderate ionization, and can be absorbed by thin aluminum foil.

3) Gamma rays: weak ionization energy but extremely dangerous as it can go through several thickness of lead wall.

Most radioactive materials radiate all three radiation so, in order to be on the safe side, the walls of the container for storing radioactive materials are usually lined with thick lead wall.