Flash X-ray measurements with a time projection Compton spectrometer

Abstract

We have developed a “time projection Compton spectrometer” (TPCS) to measure the time-integrated photon energy spectrum between 0.1 and 2 MeV of short-pulse, intense bremsstrahlung sources. A target in the TPCS produces Compton electrons when illuminated by the flash X-ray source. Target electrons are trapped magnetically by current in a rod on the axis of an evacuated drift tube. The curvature and gradient of the magnetic field cause the electrons to drift parallel to the spectrometer axis. The time of electron arrival at the end of a 1 m drift encodes the energy spectrum of the X-ray burst. The detector is a plastic scintillator coupled to a fast photomultiplier tube. Despite heavy shielding, enough prompt X-rays penetrate to give a time fiducial. Background is measured on shots with the X-ray target removed. The response of the TPCS is calculated using the TIGERP computer code to model photon/electron transport in the target and a sophisticated algorithm to compute electron drift velocity. The response functions and signal are the known components of a matrix equation, which is solved to determine the unknown spectrum. We tested the TPCS with flash X-ray sources on the Saturn accelerator. Signals have been analyzed, spectra unfolded and results compared with other methods.