THE IMPACT OF SPECTROMETER EFFICIENCY ON THE TRACE-ELEMENT SENSITIVITY OF TIME-OF-FLIGHT MEDIUM-ENERGY BACKSCATTERING

The intrinsic efficiency of a time-of-flight spectrometer which derives timing information from the passage of an ion through a carbon foil is governed primarily by secondary electron emission and multiple scattering in the foil and by the intrinsic responses of the detectors which generate the time markers. For all ions except hydrogen, the efficiency is a monotonically increasing function of energy in the range up to a few hundred keV used for medium energy backscattering. Combining expressions for spectrometer efficiency and trace element sensitivity obtained previously, we show that for a given ion the product of scattering cross section and spectrometer efficiency determines an optimum energy for trace element analysis by backscattering. The optimum choice of ion is a function of the mass of the trace element under investigation. If the limit of sensitivity is determined by sputtering, then light elements such as He appear to be unconditionally superior to heavy projectiles such as C or N. However, the numerical value of the limiting sensitivity for a given trace element is strongly influenced by the details of the sputtering and the degree of beam induced mixing at the surface. For elements in the region of Fe to Cu reasonable assumptions about sputtering lead to the conclusion that sensitivities of about 3 X 10(10) cm(-2) are achievable using 100 keV He+ projectiles and a spectrometer which subtends a solid angle of 1 msr.