I have a short animation running for 70 frames, for frames 20 - 30 I would like to have a particle emitter.... emit some particles

I can get it to start emitting at frame 20, but it keeps emitting all the way through to frame 70. Is there a way I can stop the emitter at frame 30?

example
The look i'm going for is a bullet hitting the ground and sending up a finite amount of dust/earth. At the moment the dirt just keeps on coming for the whole length of the animation.

Kind of an urgent problem (needs to be solved by 2morrow)!

Thanks guys