PROJECT SUMMARY Ultrafast FLASH radiation therapy dosimetry is an unsolved clinical and engineering problem. The goal of this research is to develop a first-of-its-kind pulse-resolved ultrafast radiation detector prototype based on innovative low-gain-avalanche-detector (LGAD) technology and ultrafast electronics for FLASH radiation therapy dosimetry. Our recent work published in Physics in Medicine and Biology demonstrates that, for the first time, LGAD sensors with associated ultrafast readout electronics (International patent number WO2019232172A1) developed at the University of Kansas (KU) can measure charged particle fluences with high speed and spatial precision. The system has been tested with a medical LINAC where single particles were measured with a time resolution of 50 picoseconds. The integrated response is as accurate as a standard ionization chamber but with a spatial resolution twenty times finer and a temporal resolution over 100 million times better with the capability to measure the charges deposited by a single LINAC pulse. The unprecedented resolving power allows the structure of the commonly known 3-microsecod LINAC pulses to be viewed and, most strikingly, the 350-picosecond sub-pulses in the train to be reconstructed. This project is in response to PAR-19-150 which seeks to encourage quantitative and physical scientists to work with biomedical researchers to catalyze the use of bioengineering approaches for their potential to open new areas of biomedical investigation. We aim to 1) design and build a LGAD dosimetry prototype, both a single-element and a 16-element array with a 1-cm pitch, and 2) determine if the prototypes can function as an ultrafast and proportional dose measurement device. This proof-of-concept study provides an innovative method for FLASH dosimetry that has the potential to expand dose measurement capability from milliseconds to microseconds and even to nanoseconds. The ultrafast detectors can be used by the radiation biologists for accurate dose estimations in preclinical animal studies, by FLASH system vendors to allow their use in a safety feedback system that could stop a beam during treatment, and by the clinical medical physicists to function as a two- dimensional detector array for both FLASH delivery commissioning and patient-specific quality assurance and quality control.