  The collection of charge carriers generated in p+n strip sensors close to the
Si-SiO2 interface before and after 1 MGy of X-ray irradiation has been
investigated using the transient current technique with sub-nanosecond focused
light pulses of 660 nm wavelength, which has an absorption length of 3.5 um in
silicon at room temperature. The paper describes the measurement and analysis
techniques used to determine the number of electrons and holes collected.
Depending on biasing history, humidity and irradiation, incomplete collection
of either electrons or holes is observed. The charge losses change with time.
The time constants are different for electrons and holes and increase by two
orders of magnitude when reducing the relative humidity from about 80 % to less
than 1 %. An attempt to interpret these results is presented.
