Measuring Energetic Electrons - What Works and What Doesn't

Abstract

This paper is a tutorial on the interaction of energetic electrons (∼10 keV to ∼10 MeV) with matter and the difficulties associated with making absolute energy measurements of them. Energetic electrons interacting with matter degrade in energy through a cascade of electron-electron and electron-nucleus scatterings which produce lower energy electrons and photons. The photons, in turn, interact with other electrons to produce lower energy photons and perhaps low energy electrons. During the cascade process, the electrons undergo large angle scatterings. From 15% to 70% of electrons incident on a surface will backscatter from that surface, retaining some or most of their original energy. The photons also may escape from the local vicinity without interacting. These effects have serious consequences when the matter is a sensor detector. Pulse height analysis of an electron energy deposit in a detector will not provide an accurate identification of the original particle energy. Furthermore, because of the uncertainties in the portion of the original energy that is deposited in the detector and the magnitude of the backscattering effects, the efficiency of detection as a function of incident energy is usually poorly known. Absolute measurement techniques have been available since early in this century, but are seldom used in satellite instruments. The interaction of electrons with matter, measurement techniques, typical flight instruments, and what works and what doesn't are discussed. Data of use in instrument design are also provided.