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Time-resolved spectrometry for the characterization of a reference field for pulsed radiation

The investigation of the spectral change during a radiation pulse is of high interest. The aim of the work presented is to prove that the emitted X-ray spectrum of a novel X-ray tube [1], developed at PTB as a reference field for pulsed photon radiation, has no time dependency of the spectral distribution during the pulse. Nowadays X-ray tubes are pulsed by switching on and off the HV generator. Therefore, they have a rising edge of about 600 μs until the pre-set acceleration voltage (e.g. 60 kV) is reached. In contrast, the newly developed X-ray tube reduces the rise and fall time by up to a factor of 10 or 60, respectively. This was achieved by designing the X-ray tube as a triode. The X-ray emission of this tube can be switched on and off by switching only the quite low voltage of the additional grid while the high voltage remains constantly on. For the measurements we used a Timepix detector [2] operated in Time of Arrival (ToA) mode. Additionally, the adjustable analogue threshold was swept through. With such a measurement over multiple X-ray pulses we obtained information about the timing and the energy distribution of the emitted photons during the rise time of the HV generator. The real impinging spectrum was gained by the deconvolution of the time-resolved measured threshold scan. Such a deconvolution is necessary due to the blurring of the energy information in the pixelated sensor by effects like charge sharing or Compton scattering. For the deconvolution the Bayesian approach [3] was chosen as it turned out to be very stable even for low statistics in the measurements. For the novel X-ray tube it is now possible to confirm that the full acceleration voltage is reached quite instantaneously and therefore no spectral change can be observed.