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Objective. A reliable determination of the instantaneous dose rate (I-DR) delivered in FLASH radiotherapy treatments is believed to be crucial to assess the so-called FLASH effect in preclinical and biological studies. At present, no detectors nor real-time procedures are available to do that in ultra high dose rate (UH-DR) electron beams, typically consisting of μ s pulses characterized by I-DRs of the order of MGy/s. A dosimetric system is proposed possibly overcoming the above reported limitation, based on the recently developed flashDiamond (fD) detector (model 60025, PTW-Freiburg, Germany). Approach. A dosimetric system is proposed, based on a flashDiamond detector prototype, properly modified and adapted for very fast signal transmission. It was used in combination with a fast transimpedance amplifier and a digital oscilloscope to record the temporal traces of the pulses delivered by an ElectronFlash linac (SIT S.p.A., Italy). The proposed dosimetric systems was investigated in terms of the temporal characteristics of its response and the capability to measure the absolute delivered dose and instantaneous dose rate (I-DR). A 'standard' flashDiamond was also investigated and its response compared with the one of the specifically designed prototype. Main results . Temporal traces recorded in several UH-DR irradiation conditions showed very good signal to noise ratios and rise and decay times of the order of a few tens ns, faster than the ones obtained by the current transformer embedded in the linac head. By analyzing such signals, a calibration coefficient was derived for the fD prototype and found to be in agreement within 1% with the one obtained under reference 60 Co irradiation. I-DRs as high as about 2 MGy s -1 were detected without any undesired saturation effect. Absolute dose per pulse values extracted by integrating the I-DR signals were found to be linear up to at least 7.13 Gy and in very good agreement with the ones obtained by connecting the fD to a UNIDOS electrometer (PTW-Freiburg, Germany). A good short term reproducibility of the linac output was observed, characterized by a pulse-to-pulse variation coefficient of 0.9%. Negligible differences were observed when replacing the fD prototype with a standard one, with the only exception of a somewhat slower response time for the latter detector type. Significance. The proposed fD-based system was demonstrated to be a suitable tool for a thorough characterization of UH-DR beams, providing accurate and reliable time resolved I-DR measurements from which absolute dose values can be straightforwardly derived., (Creative Commons Attribution license.)