Abstract: An implantable medical device in the form of a cardiac stimulation device such as a pacemaker or defibrillator includes a hermetically sealed housing having an inner peripheral surface, an electronic component such as an integrated electrical circuit sensitive to bombardment by alpha particles and located within the housing spaced from the inner peripheral surface, and blocking material intermediate the inner peripheral surface of the housing and the electronic component capable of suppressing alpha particles having an energy level up to about 15 mev directed towards the electronic component. The blocking material has a thickness of at least 0.010 inches; it may be sheet material; it may be congruently attached to the inner peripheral surface of the housing; or it may be a liquid coating applied to the inner peripheral surface and subsequently cured.