English: Schematic illustration of a linear collision cascade and a heat spike. The thick line illustrates the position of the surface, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material. The purple circle is the incoming ion. Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively. In between the ballistic collisions the ions move in a straight path, but in the middle the region of collisions has become so dense that multiple collisions occur simulatenously, which is called a heat spike. In this region the ions move in complex paths, and it is not possible to distinguish the numerical order of recoils - hence the atoms are colored with a mixture of red and blue.