High Speed Ratio Helium Beams: Extremely High Precision Time- of-Flight Calibration and Characterization of Experimental Limitations on Resolution.

To obtain very high precision absolute calibration of our helium beam time-of-flight (TOF) measurements, we have developed a novel technique based upon in situ variation of the flight path. We describe this technique briefly and demonstrate that the calibration can utilize TOF signatures other than that of a constant temperature beam, taking as an example selective adsorption signatures in helium scattering from LiF(001). The baseline TOF resolution of the apparatus is considered next. We show that variable flight path measurements allow the TOF spread due to the beam velocity distribution to be separated from that due to a non-negligible detector length (shutter function contributions are evaluated by varying the chopper speed). We discuss and document as well other factors which limit the TOF resolution and/or calibration accuracy, notably the radial (transverse) velocity distribution, real gas effects, and clustering.