For example hydrogen atoms can have zero, one, or two neutrons. Since the 1930s hundreds of other sub-atomic particles have also been discovered. The resulting positively charged particles are called ions. Ions and free electrons can exist as a plasma, the fourth state of matter. The instruments on the ACE spacecraft study electrons and a variety of isotopes.
Scientists classify cosmic ray particles depending on their source.
The particles ACE can measure may be positive or negative. SIS and CRIS measure mass as well. The SI unit for energy is the joule. An eV is defined as the energy acquired by an electron as a result of moving through a potential difference of 1 volt.