  Using the multi-channel Transient Current Technique the currents induced by
electron-hole pairs, produced by a focussed sub-nanosecond laser of 660 nm
wavelength close to the Si-SiO2 interface of p+n silicon strip sensors have
been measured, and the charge-collection efficiency determined.
  The laser has been operated in burst mode, with bursts typically spaced by 1
ms, each consisting of 30 pulses separated by 50 ns.
  In a previous paper it has been reported that, depending on X-ray-radiation
damage, biasing history and humidity, situations without charge losses, with
hole losses, and with electron losses have been observed.
  In this paper we show for sensors before and after irradiation by X-rays to 1
MGy (SiO2), how the charge losses change with the number of electron-hole pairs
generated by each laser pulse, and the time interval between the laser pulses.
  This allows us to estimate how many additional charges in the accumulation
layers at the Si-SiO2 interface have to be trapped to significantly change the
local electric field, as well as the time it takes that the accumulation layer
and the electric field return to the steady-state situation.
  In addition, results are presented on the change of the pulse shape caused by
the plasma effect for high charge densities deposited close to the Si-SiO2
interface.
