Here's how a chopper works. The beam of ions is passed through two doors - first one door, then the other. Each door is open for only a short period of time. First one door is open, then closed. Then a certain time later, a certain distance down the beam line, another door is opened, then closed. Suppose an ion makes it through the first door. If it is moving too fast, it collides with the chopper's second door that hasn't opened yet; if it is moving too slow, it collides with the chopper's second door that has already opened and closed. If its velocity is just right, it makes it through the second door. In that way, the chopper selects atoms moving at one particular velocity.