Scintillator-based calorimetry of multi-MeV X-rays from GeV laser wakefield accelerators

We reconstruct spectra of >100 MeV Bremsstrahlung x-rays from a GeV laser wakefield electron accelerator driven by the Texas Petawatt Laser. We measure from single-shot X-ray depth-energy measurements in a compact (7.5 x 7.5 x 10 cm), modular X-ray calorimeter made of alternating layers of absorbing materials and plastic scintillators [1]. Geant4 simulations of energy deposition of single-energy X-rays in the stack generate an energy-vs-depth response matrix for a given stack configuration. An iterative reconstruction algorithm based on Kramers’ law photon energy distribution was used to unfold X-ray spectra within a minute. The unfolded X-ray spectra agree with GEANT4 simulations using independently-measured electron spectra. The results demonstrate the viability of stack calorimetry for unfolding spectra of secondary plasma-accelerator-based x-rays of photon energies well beyond the range previously demonstrated [1]. We discuss uncertainties and limitations of the unfolding process at multi-MeV photon energies, and the performance of slab- and fiber-based scintillators that enable prompt data readout

[1] A. Hannasch et al., Sci. Rpts. 11, 14368 (2021).