Time-dependent radiation-induced conductivity in electron-irradiated Teflon foils

The radiation-induced conductivity (RIC) of 2.5×10−3 -cm-thick Teflon was measured using partially penetrating electrons with an energy of 40 keV. The behavior of the external currents generated during irradiation is expressed by a ’’generalized box model.’’ We explain, with this model, the experimentally observed buildup and decay of the RIC by using a time-dependent conductivity σ(t)=σ0 (1−et/τs), where σ0 is the steady-state conductivity and τs is the time constant of the RIC buildup. The time sequence in which voltage and irradiation are applied is found to give significantly different time effects; for irradiation first, the induced conductivity reaches a final value when the voltage is applied, while with voltage first, the conductivity increases with time after the radiation is turned on. Measurements are complex because the nonirradiated region of the foil is entirely blocking for electrons while it allows transport of holes. The peak value of the induced current, and thus of the induced conductivity, increases proportional to ‖I0‖0.79, while the initial slope of the current versus time is proportional to the electron beam current I0.