FLASH radiotherapy and particle therapy are performed using ultra-high dose rate radiation (UHDR), specifically a dose rate higher than 40 Gy/sec. The advantage of FLASH compared to conventional therapy (CT) is based on the FLASH effect: improved normal tissue sparing while still maintaining tumor control. Accurate dosimetry and real-time beam monitoring are critical for its clinical translation, but current detectors suffer from saturation effects in the signal production and signal read-out when dealing with UHDR beams. We report on our development and preliminary tests a novel dosimeter for high-dose rate particle therapy, based on advanced Silicon Photomultiplier (SiPM) detectors coupled with scintillating fibers (SciFi). The SciFi detector aims towards an improved time and space resolution, with respect to the present commercial readout system. In our first tests we will develop and compare dosimeters made also with standard optical fibers (exploiting the radiation induced emission) coupled with the SiPM. We will describe the present status of the devices and facilities used for FLASH RT. We will show the preliminary design of the detector that is expected to be tested using a proton UHDR beam with the beam's energy ranging from 70 MeV (CT) to 228 MeV (FLASH RT). The ultimate goal is a detector working for both FLASH and Conventional Radiotherapy, with good sensitivity, as well as spatial and energy resolution