In radiotherapy, treatment beams require precise margins to ensure the preservation of surrounding healthy tissue. Clinical studies have shown that to mitigate range deviations of the Bragg peak, safety margins of typically less than 5\% around the target volume are employed. Consequently, real-time or online diagnostic techniques should be designed to minimize beam perturbation to the greatest extent possible. A minimally-invasive gas jet beam profile monitor for medical treatment facilities is being developed at the Cockcroft Institute (UK) to provide online monitoring. The monitor operates a thin, low-density, gas jet curtain, transecting with the beam. A proof-of-concept experimental study was carried out to quantify the degree of perturbation the gas jet has on a beam, using a 10 keV electron gun with a maximum current of ~100 μA. Any changes in beam profile and current were measured via a scintillator screen and Faraday cup respectively in path of the beam after the gas curtain. In the future, a simulation study will also be carried out using BDSIM, a Beam Delivery Simulation program built on GEANT4, with the experimental beam parameters along with medical hadron beams. This contribution provides the details of an experimental study into the perturbation experienced by an electron beam from a gas jet monitor.

In proton FLASH therapy the beam monitoring is crucial to ensure the conformal dose deposition to the tumour and effective Organ at Risk (OAR) sparing. A non- invasive real time beam monitoring improves the efficacy as the dose is delivered in shorter time scales. To achieve this, gas-jet based Ionization Profile Monitor (IPM) is developed with potential capability towards real time beam monitoring. It detects ions produced by the interaction of primary beam with a thin (<1 mm) gas-curtain without perturbing the beam. This work presents the simulation of IPM to study the ion extraction under different configuration for accurate reconstruction of the beam shape. The role of electric field in the IPM on the trajectory of the ions and the inhomogeneity in their energy distribution affecting the beam profile are studied. The study also investigates the effect of beam misalignment, relative contribution of individual ion states generated due to interaction, and the gas-curtain density distribution. Future work will address configuration required to accommodate broader range of beam relevant to clinical application.