With the development of the International Thermonuclear Experimental Reactor (ITER) and the China Fusion Engineering Test Reactor (CFETR), the measurement of tritium in low-level tritiated water is indispensable from the viewpoint of assessment and safety of tritium. In our work, CaF 2 (Eu) was chosen as the scintillation material in the research to design a tritium monitor for tritiated water. The effect of particle size of CaF 2 (Eu) powder and chamber thickness on detection efficiency and count rate were assessed by Geant4 simulation. Our results show that reducing the CaF 2 (Eu) particle size can effectively improve the probability of energy deposition in the scintillator. Under the same conditions, the use of CaF 2 (Eu) particles with a radius of 0.5 µm increases the energy deposition probability by a factor of more than 20 compared to particles with a radius of 50 µm. However, the coincidence detection efficiency rapidly decreases as the chamber thickness of the solid scintillation counting (SSC) system increases when the particle size is small. With a fine CaF 2 (Eu) powder (particle radius of 1 µm), the coincidence detection efficiency is greater than 50% when the chamber thickness is 1 mm. Only approximately 35% of tritium decay is coincidentally detected when the chamber thickness is 5 mm. In addition, filling the chamber with CaF 2 (Eu) particles of different sizes leads to different maximum count rates. With the finest CaF 2 (Eu) powder, the SSC system has the highest count rate when the chamber thickness is 5 mm. With such results, the replacement of the scintillation cocktail with CaF 2 (Eu) powder in low-level tritiated water measurements is very promising. The simulation result will help in the design of a SSC system applied to low-level tritiated water measurement, and such a system will be tested in our future work. K: Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc); Very low-energy charged particle detectors 1Corresponding author.

With the development of ITER and CFETR, the measurement of tritium in low-level tritiated water is indispensable from the viewpoint of assessment and safety of tritium. To reduce radioactive waste produced by liquid scintillation counting, we have built a new low-level tritiated water measurement system using CaF 2 (Eu) powders. When an 8 mm thickness sample chamber and M sample (average grain sizes: 59.1 µm) were used, our measurement system responded accurately to tritium concentrations down to approximately 2.6 Bq/mL for 10 min measurements, and the calculated detection limit was about 4.6 Bq/mL. When a 3 mm thickness sample chamber and S sample (average grain sizes: 8.4 µm) were used, our measurement system responded accurately to tritium concentrations down to approximately 1.2 Bq/mL for 10 min measurements, and the calculated detection limit was about 0.64 Bq/mL.

tissues that can be exposed to non-FLASH dose as well as the quantification of the received non-FLASH doses. Materials/Methods: The PBS dose rate was determined by the proton beam energy, the beamline transmission efficiency and the cyclotron beam current. Its spatial distribution is governed by pencil beam's Gaussian distribution. Our investigation was carried out using simulations and experimental measurements. Two independent but complementary measurement setups were used: (i) A radiographic film sandwiched in solid water, along the beam axis, was irradiated to 15 Gy with a FLASH proton beam of 245 MeV and the spatial dose distribution was evaluated. (ii) A volumetric plastic scintillator was irradiated by the same beam and a high-speed camera was used to record the spatial distributions of dose and dose rate at the imaging rate of 100,000 fps. The absolute dose was measured at 2cm water depth using a plane-parallel ionization chamber to calibrate the imaging data from (i) and (ii) which were used to validate the simulations. Results: Based on our simulation which validated using the film and scintillator imaging data, the dose rate is highest at the cyclotron maximum beam energy (250 MeV, »100% transmission), can achieve a maximum of 480 Gy/sec/cm 3 at the central region of the Bragg peak. Then, as result of limited beamline efficiency, it drops to about 10% (47 Gy/sec/cm 3 ) for the next highest beam energy (249 MeV) and exponentially decreases to 0.5 Gy/sec/cm 3 for 70MeV. The dose rate values shift from FLASH to non-FLASH magnitudes depending on the location within the spatial dose distribution. The proportion of volume irradiated by a non-FLASH dose rate can be up to 40% of the total irradiated volume. Therefore, for a treatment using PBS, considerable amount of healthy tissue could receive non-FLASH dose. The received non-FLASH dose could be up to full prescription dose if the treatment is delivered in beam transmission mode and up to 30% of the prescription dose if the treatment is delivered using Bragg peaks. Conclusion: Our results show that the Gaussian distribution of the proton beam can limit the FLASH benefit. In order to fully utilize the proton pencil beam for FLASH treatment, a robust solution to FLASH dose rate falloff is needed.

A solid scintillation counting (SSC) system which uses the mixture of CaF2(Eu) powder and tritiated water inside the chamber room will be developed for the measurement of low-level tritiated water. However, the coincidence detection efficiency and net count rate might be limited by the photon absorption in the chamber of the SSC detector. This paper discusses the feasibility of using light guide fibers as the optical passageway in the chamber room by Geant4 simulation. The simulation results show that the performance of the detector can be improved by using wavelength-shifting fibers as optical passageway in the chamber. The detector can maintain a 4.2% coincidence detection efficiency at a 6 cm chamber thickness, and the detection limit can be as low as 0.47 Bq/mL under 10 min measurement. As a result, using wavelength-shifting fibers as optical passageways in the chamber room of the SSC system is possible and effective.