To facilitate timing measurements in our experiments, it is necessary to pulse the atomic H beam such that short (~5 ms) bursts of H atoms arrive at the target at known regular intervals. By varying the delay time between the arrival of the atoms at the target, and the moment at which we look for the H 2 product, we should be able to study the entire process as a function of time.

We will pulse the atomic beam by passing it through four tuning fork choppers mounted in series, one after the other. These devices are tuning forks driven by calibrated electromagnetic induction circuits which are tuned to the resonance frequency of the forks, in this case 10, 15, 40 and 80 Hz. This combination of frequencies presents the atom beam with a 5 mm aperture which is open for ~5 ms once every 200 ms.

A tuning fork chopper in action.
Image from Scitec Instruments Ltd (UK) who sell these devices.

The fork vanes oscillate back and forth continuously, and plates welded to the vanes create an aperture which opens and closes at the same frequency as the fork. By pulsing the incident H atom beam we also reduce the mass flow rate into the main experimental chamber, which will aid in keeping the background gas concentration in the target vicinity to a minimum.