MEASUREMENT OF TIME-OF-FLIGHT SPECTROMETER EFFICIENCY FOR LIGHT-IONS AT MEDIUM ENERGIES

Medium energy time-of-flight spectrometry is an effective technique for helium backscattering as well as the elastic recoil detection of hydrogen and other light elements. However, the efficiency of a time-of-flight spectrometer, unlike that of a surface barrier detector, depends strongly on the energy and mass of the detected particle. A thorough understanding of this spectrometer quantum efficiency is necessary for quantitative measurements. We report initial measurements of the quantum efficiency of two similar time-of-flight spectrometers in forward and backscattering geometries for hydrogen, helium, and carbon projectiles over the energy range 50-275 keV. The results are compared with predictions of a model which incorporates multiple scattering and secondary electron emission at the carbon start foil along with an energy-independent microchannel plate response. When these processes are taken into account, the efficiency of a spectrometer as a function of projectile species and energy can be described to within experimental error by a single overall calibration constant.