Objective: To develop a bremsstrahlung target and megavoltage (MV) x-ray irradiation platform for ultrahigh dose-rate (UHDR) irradiation of small-animals for the Advanced Rare Isotope Laboratory (ARIEL) electron linac (e-linac) at TRIUMF. Approach: An electron-to-photon converter design for UHDR radiotherapy (RT) was centered around optimization of a tantalum-aluminum (Ta-Al) explosion-bonded target. Energy deposition within a homogeneous water-phantom and the target itself were evaluated using EGSnrc and FLUKA MC codes, respectively, for various target thicknesses (0.5-1.5 mm), beam energies (Ee-=8,10 MeV) and electron (gaussian) beam sizes (2σ=2-10 mm). Depth dose-rates in a 3D-printed mouse phantom were also calculated to infer the compatibility of the 10 MV dose distributions for FLASH-RT in small-animal models. Coupled thermo-mechanical FEA simulations in ANSYS were subsequently used to inform the stress-strain conditions and fatigue life (N) of the target assembly. Main Results: Dose-rates of up to 128 Gy s-1 at the phantom surface, or 85 Gy s-1 at 1-cm depth, were obtained for a 1x1 cm2 field size, 1-mm thick Ta target and 7.5 cm source-to-surface distance using the nominal treatment-beam configuration (Ee-=10 MeV,2σ=5 mm,P=1 kW); furthermore, removal of the collimation assembly and using a shorter (3.5 cm) SSD afforded dose-rates >600 Gy s-1, albeit at the expense of field conformality. Target temperatures were maintained below the tantalum, aluminum and cooling-water thresholds of 2000, 300 and 100 °C, respectively, while the aluminum strain behavior remained everywhere elastic and helped ensure N>3000 thermal cycles could be tolerated over the prescribed 5 yr life of the target. Significance: Effective design iteration, target cooling and failure mitigation have thus culminated in a robust target compatible with intensive transient (FLASH) and steady-state (diagnostic) applications. The ARIEL UHDR photon source will facilitate FLASH-RT experiments concerned with sub-second, pulsed or continuous beam irradiations at dose rates in excess of 40 Gy s-1.
A series of irradiation tests have been performed at TRIUMF to investigate different material pairings to act as highpower electron-to-gamma converter for the ARIEL Electron Target East (AETE). The bulk of the converter body will be made out of an aluminum alloy with a sub-millimeter high-Z metal layer bonded to the surface facing the incoming electron beam. This contribution presents the approach chosen to select the optimal material for the high-Z layer, describes the tests performed and shows result which led to the successful selection of a specific tantalum-aluminum pairing as the future ARIEL converter material.
A detailed analytical model has been developed to simulate isotope-releasecurves from thin-foils ISOL targets. It involves the separate modeling of diffusion and effusion inside the target.The former has been modeled using both first and second Fick'slaw.The latter,effusion from the surface of the target material to the end of the ionizer, was simulated with the Monte Carlo code Mol Flowþ. The calculated delay-time distribution for this process was then fitted using a double-exponential function. There lease curve obtained from the convolution of diffusion and effusion shows good agreement with experimental data from two different target geometries used at ISOLDE. Moreover, the experimental yields are well re- produced when combining there lease fraction with calculated in-target production
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