Abstract: An ion implanter for implanting ions into a semiconductor wafer comprises means for generating an ion beam, means for receiving charged particles ejected from the ion beam, first means for rejecting those of the received charged particles below a first predetermined energy, second means for rejecting those of the received charged particles above a second predetermined energy, the first predetermined energy being lower than the second predetermined energy. The ion implanter further comprises means to count the number of received charged particles between the first and second predetermined energies, and scanning means for scanning the first and second predetermined energies to determine the energy at which the number of received charged particles between the first and second predetermined energies is a maximum.