As weaker types of radiation are often not able to pass through the metal body of the geiger counter tube, often a thin window of light weight material is provided. The mineral mica is often used. Very weak radiation, even alpha rays, can pass though mica. The mica is of course more fragile than the metal, and must be protected from damage. Even just touching a mica window can cause permanent damage to the tube, as the window will usually immediately burst.
(«Телесистемы»: Конференция «Микроконтроллеры и их применение»)
