In rotational tomotherapy a high energy (6 MeV) photon beam irradiates the patient. A CT detector placed behind the patient is used to establish the position of the patient and the dose delivered. A possible detector design uses amorphous selenium (a-Se) as the x-ray to charge conversion medium requiring a detailed investigation of the change in x-ray sensitivity due to exposure to radiation in a-Se. Our novel experimental method called x-ray time of flight provides instantaneous measurements of x-ray sensitivity and charge transport parameters in a-Se films. The method analyzes the current from a-Se samples in response to single short pulses of LINAC radiation. X-ray sensitivity was observed to decrease substantially with large exposures (e.g. more than 50% after 4 Gy accumulated over ~5min) and to completely recover over <48h. The mechanisms responsible were studied from the kinetics of the measured current waveforms. On the basis of experimental data, a model for dose dependent sensitivity was formulated taking into account carrier trapping, re-distribution of electric field in the a-Se film due to space charge and evolving recombination processes. In principle quantitative comparison of experimental and theoretical characteristics will permit the determination of the main material parameters (carrier mobility, deep trapping lifetime), as well as the generation rate of carriers by x-rays. Thus a basis for the development of a practical a-Se based megavoltage CT detector has been investigated.