the invention provides methods and apparatus for measuring the distribution of cluster ion sizes in a gas cluster ion beam and for determining the mass distribution and mass flow of cluster ions in a gcib processing system without necessitating the rejection of a portion of the beam through magnetic or electrostatic mass analysis . the invention uses time - of - flight measurement to estimate or monitor cluster ion size distribution either before or during processing of a workpiece . the measured information is displayed and incorporated in automated control of a gcib processing system .