How Do You Weigh a Particle Moving at Half the Speed of Light?

1) You can slow down and stop the particles in a series of detectors that measure the rate that the particles are slowing down (their energy loss), or their velocity. The rate at which a particle slows down is a strong function of the charge of the particle (which, since these are bare nuclei with only protons and neutrons, tells you the number of protons, i.e. the element). The energy loss is a much weaker function of the mass (the number of protons + neutrons) of the particle, but from the small differences in slowing down, you can determine the mass. Typically, in order to measure the difference between Iron-56 and Iron-57 for example, which differ by less than 2% in mass, you need to get your error (sigma) in the mass measurement down to about .25% or one in four hundred. The CRIS (http://www.srl.caltech.edu/ACE/CRIS_SIS/cris.html) instrument on ACE is an example of this type of detector system.

2) You can use a strong magnetic field to bend the track of the charged particle (cosmic ray). If you measure the curvature of the track (with drift chamber detectors for example) and have an independent measure of the particle velocity (with a time-of-flight system or Cerenkov detector), you can determine the mass. ISOMAX (http://lheawww.gsfc.nasa.gov/docs/gamcosray/hecr/ISOMAX/) uses this method. It has a pair of superconducting magnets that generates a magnetic field that is more than 1 Tesla in strength.