Radioactive events come from radioactive particles penetrating the semiconductor material and upsetting the voltage in the bit cell latch. These events are rare under normal circumstances, but can be higher in a radioactive medical environment and/or very high altitudes where there is less of an atmosphere to reduce the particle counts.

Depending upon the angle of the penetration and the intensity of the particle, one or more bit values can be flipped. This will happen in a straight line. If the straight line goes across rows then the multiple failing bits are guaranteed to be in different words. This works for vertically oriented penetration or for diagonal.

If the penetration orientation is down the row, then it must go beyond the mux factor to create more than one bit in the same word.

The smaller the geometries, the more bits can be disturbed by a particle penetration. At a 90 nm process node a penetration of 5 cells is extremely rare. At 65 nm a penetration of 6 cells is extremely rare. Low leakage process nodes are less susceptible to these events than high performance process nodes.

Radioactive particle penetration can, and occasionally does, physically damage SRAM cells. This is more common in process nodes with more aggressive structural geometries.