The Radiological Research Accelerator Facility has modified a decommissioned Varian Clinac to deliver ultra-high dose rates: operating in 9 MeV electron mode (FLASH mode), samples can be irradiated at a Source-Surface Distance (SSD) of 20 cm at average dose rates of up to 600 Gy/s (3.3 Gy per 0.13 µs pulse, 180 pulses per second). In this mode multiple pulses are required for most irradiations. By modulating pulse repetition rate and irradiating at SSD = 171 cm, dose rates below 1 Gy/min can be achieved, allowing comparison of FLASH and conventional irradiations with the same beam. Operating in 6 MV photon mode, with the conversion target removed (SuperFLASH mode), samples are irradiated at higher dose rates (0.2–150 Gy per 5 µs pulse, 360 pulses per second) and most irradiations can be performed with a single very high dose rate pulse. In both modes we have seen the expected inverse relation between dose rate and irradiated area, with the highest dose rates obtained for beams with a FWHM of about 2 cm and ± 10% uniformity over 1 cm diameter. As an example of operation of the ultra-high dose rate FLASH irradiator, we present dose rate dependence of dicentric chromosome yields.

Objective: Proton pencil beam scanning (PBS) treatment fields needs to be verified before treatment deliveries to ensure patient safety. In current practice, treatment beam quality assurance (QA) is measured at a few selected depths using film or a 2D detector array, which is insensitive and time-consuming. A QA device that can measure all key dosimetric characteristics of treatment beams spot-by-spot within a single beam delivery is highly desired. Approach: We developed a multi-layer strip ionization chamber (MLSIC) prototype device that comprises of two layers of strip ionization chambers (IC) plates for spot position measurement and 64 layers of plate ionization chambers for beam energy measurement. The 768-channel strip ion chamber signal are integrated and sampled at a speed of 3.125 kHz. It has a 25.6 cm × 25.6 cm maximum measurement field size and 2 mm spatial resolution for spot position measurement. The depth resolution and maximum depth were 2.91 mm and 18.6 cm for 1.6 mm thick IC plate, respectively. The relative weight of each spot was determined from total charge by all IC detector channels. Main results: The MLSIC is able to measure ionization currents spot-by-spot. The depth dose measurement has a good agreement with the ground truth measured using a water tank and commercial 1D multi-layer plate chamber. It can verify the spot position, energy, and relative weight of clinical PBS beams and compared with the treatment plans. Significance: MLSIC is a highly efficient QA device that can measure the key dosimetric characteristics of proton treatment beams spot-by-spot with a single beam delivery. It may improve the quality and efficiency of clinical proton treatments.

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Distributed x-ray sources enable novel designs of x-ray imaging systems. However, the x-ray power of such sources is limited by the focal spot power density of the fixed anode. To further improve x-ray output, we have designed and evaluated a diamond-W transmission target for multi-pixel x-ray sources. The target features a thin layer of tungsten deposited on a diamond substrate. The thickness of tungsten layer was optimized for maximum fluence through Monte Carlo simulations. Finite element thermal simulations were performed to evaluate focal spot temperature in the target under different power loadings and dwell duration. The results showed that the optimal thickness of the tungsten layer in the W-diamond transmission target is linearly proportional to the electron energy. A 5-6 μm tungsten thickness is suitable for the kVps ranges from 60 kVp to 140 kVp. A W-diamond transmission target produces up to 20% more x-ray fluence than a traditional W reflection target in the beam center depending on the kVp settings. The x-ray spectrum of the transmission target shows less characteristic x-rays than that of reflection target. The thermal performance of W-diamond targets for peak power is significantly better than that of reflection targets. The maximum focal spot power densities of W-diamond transmission and W reflection targets are both strongly dependent on the dwell duration. For longer pulse durations, the W-diamond target allows as much as a four-fold increase in power and an eight-fold increase in power density in comparison to a traditional W reflection target for the same temperature spikes. The stability of the W-diamond bond needs to be tested experimentally. Nevertheless, the W-diamond transmission target is an appealing target that can significantly simplify the design and improve the performance of distributed x-ray sources.

Background Emerging multi‐pixel X‐ray source technology enables new designs for X‐ray imaging systems. The power of multi‐pixel X‐ray sources with a fixed anode is limited by focal spot power density. Purpose The purpose of this study is to optimize the W‐diamond target and predict its performance in multi‐pixel X‐ray sources. Methods X‐ray intensity and energy deposition in the W‐diamond target with different thicknesses of tungsten film and incident electron energies was calculated with the Geant4 Monte Carlo toolkit. COMSOL Multiphysics software was used to analyze the transient and stationary heat transfer in the thin‐film W‐diamond target. The maximum tube power and X‐ray output intensity were predicted for both transmission and reflection target configurations. Results The maximum focal spot power density was limited by either the graphitization of the diamond substrate or the melting point of the W target. With optimal W‐target thickness, the maximum transmission X‐ray intensities are about 40%–50% higher than the maximum reflection intensities. Thin‐film W‐diamond targets allow four to five times more maximum power input and produce six to seven times higher transmission X‐ray intensity in continuous mode compared with conventional reflection W thick targets. Depending on the focal spot size, reducing the X‐ray pulse duration can further enhance the tube power. Conclusions Multi‐pixel X‐ray sources using this W‐diamond target design can produce significantly higher X‐ray output than traditional thick tungsten targets without major modification of the tube design.