# Review of Radiation-Induced Effects in Polyimide
## 3 Results
### Changes in Electrical Properties
The presented paper is not intended to provide comparison between physical parameters extracted from the theoretical simulations of build-up and pre-transit regions of the surface potential versus decay time curve. Instead, attention is focused on the changes in bulk resistivity, or its inverse, conductivity, of the irradiated material. The irradiation-induced enhancement of conductivity in polymers has been observed by numerous different research groups (e.g., [74; 75; 76; 77]). Figure 3 presents conductivity of 76 \(\upmu\)m PI material mounted on rotating aluminum carousel irradiated with 90 keV electrons (maximum penetration depth 83 \(\upmu\)m) [6] versus energetic radiation dose measured under vacuum (pressure \(<\)1 \(\times\) 10\({}^{-6}\) Torr) with the SPD method [55]. Irradiation was performed at an accelerated dose rate of 94.8 Gy/s. The average electron flux received by PI film during irradiation was 7.8 nA/cm\({}^{2}\). Each SPD measurement was performed using irradiation with 5 keV electrons. During charging, a flux of \(\sim\)20 nA/cm\({}^{2}\) was applied for less than 10 s to a fully discharged sample which had been under vacuum for a minimum of 24 hours. The initial applied surface potential was 2.5-3 kV for all tests. The dark conductivity values were derived from the slope of the SPD curve after it became linear on a log/linear plot. This regime of the SPD curve represents the regime in which the front of the charge body has already reached the grounded backing plane and the decay of surface potential may be modeled as a leaky capacitor [78]. The conductivity of electron-irradiated polyimide showed an increase of two orders of magnitude after irradiation with an initial dose of 20 MGy and further increases for larger doses, as shown in Figure 3. Similar results have been reported by other research groups [29; 79; 80]. The interpretation of the physical processes that cause an increase of conductivity in irradiated PI is still questionable. Generally, charge transport in disordered materials like PI occurs via incoherent electron hopping among transport sites. Bulk conductivity is influenced by both energetic and geometric disorder of electron trap sites. That is to say, the facility with which an electron can travel through a disordered material in the presence of a strong electric field is dependent on both the energetic distribution of transport sites within the material and the geometric distribution of the transport sites. The latter dependency arises due to the variation of intersite electronic wavefunction overlap originating from the positional and orientational distribution of these hopping sites [81]. Authors of the present manuscript attribute the observed dose-dependent bulk conductivity increase in irradiated PI to the radiation-induced formation of electron hopping sites and the formation of extended pi-bonded chemical structures which are not present in the pristine material. These sites are created as a result of bond-specific rupture during electron bombardment. This phenomenon is considered to be distinct from RIC which is temporary and is the result of increased energy in the system and therefore the excitation of electrons to excited states, forming electron-hole pairs that facilitate electron mobility. This must not be confused with the creation and destruction of transport sites within the material as discussed here. Other researchers attribute observed long-term conductivity increases after irradiation to the delayed RIC (DRIC) induced by aging [78; 82; 83; 84; 85]. At lower irradiation doses, trapped charge carriers are easily de-trapped in the conduction and valence bands, therefore a high conductivity is maintained for a long period of time without any degradation. At higher doses (\(>\)10\({}^{6}\) Gy), the structural damage in PI prevails over the ionization one thus causing the recovery of the conductivity values to the level of the pristine material.