Investigations of radiation shielding, diagnostic methods and imaging for conventional and laser-driven radiation sources

Englbrecht, Franz Siegfried (2022): Investigations of radiation shielding, diagnostic methods and imaging for conventional and laser-driven radiation sources. Dissertation, LMU München: Fakultät für Physik

Abstract

The interaction of highly intense petawatt-class (PW) laser pulses with matter can cause the emission of a large number of secondary particles with large kinetic energy. Over the past years, many developments enabled the transition from proof-of-principle studies generating a spray of particles such as protons, neutrons, light-ions, electrons and X-rays towards laser-based particle sources of biomedical relevance. Bunch-charges beyond 1 nC, broadband proton bunch energy spectra with maximum energies up to 100 MeV and electron bunches beyond 8 GeV are soon to be realized. The whole bunches are impinging onto experiments at shot repetition rates of 1 Hz within a few ns and present a fluence of 10^7 p/cm2, originating from μm source sizes of X-rays and protons. The named source parameters, together with the intrinsically mixed radiation field of large divergence and the intense co-propagating broadband electromagnetic pulse (EMP), enable not only novel applications exploiting these distinct features, but also pose challenges. They hinder the direct transfer of established experimental concepts from conventional electron-, photon-, proton- or light ion-beam accelerator facilities. This work is dedicated to identify limits as well as to develop computational tools to overcome some of these transfer challenges. Knowledge of the secondary radiation field around accelerators is of key relevance for radiation protection and can enable novel beam monitoring in both conventional and laser-driven acceleration facilities. The systematic correlation between 75-250 MeV scanned therapeutic proton beams and the neutrons in the secondary radiation field as function of the angle of observation at a conventional proton treatment facility has been studied. FLUKA Monte Carlo (MC) simulations were compared to GEANT4 simulations and Extended-Range Bonner Sphere Spectrometer (ERBSS) neutron spectra between 10^-9-10^2 MeV. The modelling of the gantry and the treatment room are relevant to reproduce the ERBSS data, especially in the thermal energy region (10^-2-10^1 MeV) of increased biological effectiveness. Additionally, the Centre for Advanced Laser Applications (CALA) was modelled in FLUKA to evaluate the energy spectra, dose rates and spatial distributions of secondary electrons, neutrons, pions, X-rays and γ-ray photons generated by laser-accelerated bunches of protons, electrons, carbon and gold ions. The dose rates from the intense radiation bunches, most caused by beamlines using the 2.5 PW, 800 nm, 25 fs laser pulses from the ATLAS laser at 1 Hz repetition frequency were brought into agreement with the regulatory limits. Dose rates for unclassified zones could be kept < 0.5 μSv/h, while supervised zones below < 2.5 μSv/h were successfully realized, as the existence of controlled zones was not possible due the lack of adequate personal dose monitors for the ns short radiation bunches. Neutron energy spectra from 10-75 MeV protons from the near-future operation of the Laser-driven Ion Acceleration (LION) experiment were simulated to provide an estimate on the correlation of the laser-based proton and electron bunches to the secondary neutron radiation field. The detection of laser-accelerated ion bunches is transitioning from offline towards online electronic systems with a few Hz repetition rates. The characterisation of the energy spectrum, particle number and divergence of the bunches is key to the exploration of the optimal source parameters and the development of a stable proton source for applications. Control software development for the 5 cm × 10 cm RadEye CMOS sensor with 48 μm pixel size successfully enabled several applications: the triggered main diagnostic of the LION experiment in a combined electron and proton magnet spectrometer, as readout of a stack of scintillators, as well as a dynamic range extension by tacking multiple consecutive frames of a single bunch impact. FLUKA simulations of large electron fields were used to accurately reproduce depth-dose distributions and lateral field profiles of a Siemens ONCOR medical electron linac. This well defined electron source together with measurements of the electron fields helped to identify the radiation-induced measurement background in the magnetic spectrometer. In a following step, a MC based electron spectra reconstruction algorithm was developed and tested. Finally, in view of future developments, the CM49 CMOS sensor was tested as a successor candidate of the RadEye for the LION experiment. As last challenge the envisioned application of the laser-driven sources for imaging was investigated. FLUKA simulations were combined with data from experimental campaigns at the Laboratory for Extreme Photonics (LEX) and Texas Petawatt laser (TPW), where basic imaging experiments were performed using protons alone as well as simultaneous X-rays and protons. The results allow to assess the gap that needs to be bridged in order to take full advantage of the novel sources for imaging. Summarizing, this thesis work addresses several developments which help pave the way towards the use of laser-driven radiation for a broader set of applications.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 15. Juni 2022
• 1. Bericht­erstatter:in: Parodi, Katia