# Review of Radiation-Induced Effects in Polyimide
## 3 Results  
### Changes in Electrical Properties  
The electrical properties that are of major importance to the operation and lifetime of a GEO satellite include the surface and volume resistivities of its surface materials. Figure 2 presents a plot of the surface potential decay rate as a function of dark (volume) resistivity [51]. When the charge decay time of the surface materials exceeds the orbital period (one day for GEO), the material can differentially charge throughout the entire mission and the likelihood of discharge increases. The red star in Figure 2 shows the resistivity of pristine polyimide. As shown in Figure 3, the conductivity of polyimide can increase by orders of magnitude after electron irradiation. The irradiated material can become far less susceptible to charge accumulation. Thus, an understanding of the charge conduction and dissipation mechanisms is important for the satellite designers to understand the behavior of a satellite throughout its operational lifetime. It should be noted that the method of conductivity measurement of inhomogeneous insulators is important and depends on the application. The measurement method must be chosen carefully in order to avoid misleading results [52; 53]. In order to measure the most conductive pathway through a disordered insulator, for applications such as shielding high voltage wires or microprinting electronics, where possible current leakage is the relevant property, then the ASTM standard [54] produces the relevant resistivity value. However, for spacecraft charging applications in which understanding residual surface charge due to charged particle implantation is the goal, the result given by the surface-potential decay (SPD) method is more appropriate as this will measure the least conductive domains in the inhomogeneous material [55]. Figure 3: Conductivity of PI material irradiated with different doses. Figure 2: Plot of charge decay time versus resistivity value for PI. The red star indicates the resistivity of pristine PI. Note, that after electron irradiation the resistivity can decrease by several orders of magnitude. Adapted with modifications from [56]. The evolution of surface potential during electron irradiation (charge build-up) and after charging (potential decay) [55; 57; 58] and pulsed electro-acoustic (PEA) [59; 60; 61; 62] measurements are considered the most significant laboratory methodologies used to investigate the PI's charging and discharging behavior in a space environment. Several different factors need to be taken into account for the development of reliable theoretical models of charge distribution and dissipation in electron-irradiated material. For example, charging involves simultaneous charge transport and electron scattering mechanisms, whereas potential decay is an even more complex process superimposing electronic excitation assisted charge transport, termed radiation induced conductivity (RIC) [63], charge capture and release by radiation-induced deep and shallow traps, surface conduction, and, possibly, polarization effects [64]. Charge build-up in electron-irradiated PI films has been extensively studied by [PERSON] et al. [65; 66] and [PERSON] et al. [67; 68; 69]. Excellent reviews of mechanisms accounting for the potential decay in electron-irradiated PI as well as various charge carriers transport models developed over the past several decades have been recently published by [PERSON] [70] and [PERSON] [71]. The various approaches for the potential build up and decay differ significantly with respect to the underlying assumptions concerning charge trap distribution, carrier mobility, charge injection methods, etc. [72; 73]. A number of physical parameters related to charge carrier transport may be extracted from the fitting of experimental surface potential build up/decay curves. These parameters include, but are not limited to, the density of charge trap states, charge capture cross-section, the charge trap release and re-trap probabilities, and dark resistivity of the irradiated material.