Adv. Space Res. Vol. 13, No. lO,pp. (10)183—(10)187, 1993 0273—1177/93 $24.00 Printed in Great Britain. 1993 COSPAR SECONDARY-ELECTRON YIELDS OF SOLAR SYSTEM ICES David M. Suszcynsky,* Joseph E. Borovsky* and Christoph K. Goertz** * Space Plasma Physics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. ** Deceased, formerly of Departmeiu of Physics and Astronomy, University of Iowa, iowa City, IA 52242, U.S.A. ABSTRACT The charging dynamics of ice particles in plasmas (e.g. for planetary ring dynamics) is highly dependent upon the secondary-electron yields of the ices. To this end, the secondary-electron yields of H 20, C02, NH3 (ammonia) and CH3OH (methanol) ices have been measured in a scanning electron microscope as a function of the electron-beam energy in the 2—30 keV energy range. Estimates are given for the maximum secondary-electron yield Yma~ of each ice and the energy Emax at which this maximum yield occurs and the implications of these estimates are discussed in terms of solar system ices. Based on these results and a general literature review of Ymax values for nonconducting materials, it is suggested that a typical range to quote for the secondary-electron yield of ices should be about 1—10. This is much lower than the range of 1—30 that is presently quoted in the space physics community. INTRODUCTION When an ice grain is immersed in a plasma, as is often the case in the space environment /e.g. 1,2,3,4/ the grain will charge at a specific rate and to a final equilibrium value that depend upon the characteristics of both the ice grain and the plasma /e.g. 5,6/. For most situations, the net charging current to the grain at a particular moment in time is given by the sum of the ambient ion and electron currents impacting the grain, and the backscatter and secondary-electron currents leaving the grain. Under certain conditions, other sources of current such as sputtering or photoelectron production may also significantly contribute to the overall charging current. The charging level and charging rate of an ice grain determine the electrodynamic behavior of the grain /e.g. 7/. The grain charging can be particularly sensitive to the secondary-electron yield of the ice and the magnitude of this yield will often determine whether the grain charges negatively or positively. Consequently, the secondary-electron yield of an ice grain can influence such processes as the rates of grain sputtering and energy transfer from the plasma to the grain /8/ and can produce various collective effects in clouds of ice grains /e.g. 9-13/. This paper describes measurements of the secondary-electron yields of four solar system ices (1120, C02, CH3OH, and NH3) and were made by irradiating the ice samples with a 2—30 keV electron beam in a scanning electron microscope. The results have been reported elsewhere in greater detail /14,15/. SECONDARY-ELECTRON EMISSION PROCESS When an energetic charged particle impacts a target material, secondary electrons are emitted from the surface of that material in a three-step process: (1) target electrons are liberated from the material through Coulomb-scattering processes between the energetic particle and the target lattice electrons, (2) the lib- erated electrons migrate isotropically in a random walk fashion and, (3) some electrons reach the surface, overcome the surface potential barrier and escape as secondary electrons. The number of secondary elec- trons produced per incident particle is known as the secondary-electron yield of the material, Y. The average depth d3 from which secondary electrons are produced in the tar$et material is approximately equal to the mean free path of a slow electron in the material, about 10—50 A for conducting materials and about 50—500 A for insulating materials. The Sternglass theory /16/ is a particularly simple and physically illustrative theory for secondary-electron emission. This theory was originally developed to explain secondary-electron emission from materials impacted by high-speed ions but can also be applied to the case of electron-induced secondary-electron emission /14,17/. In the Sternglass picture, the secondary-electron yield due to incident energetic electrons can be written as Pd3 (dE\ ~ (1) where P .~ 1/2 is the probability that an electron liberated from a depth d,, will reach the surface and escape, d,, is the mean free path of a slow electron , E~ 25 eV is the average energy deposited by the energetic electron into the target to liberate one electron, and (dE/dx)~ is the electronic stopping power of the target material to energetic electrons and is a function of the velocity of the electrons. (10)183