Spectral Analysis of 50–100 MeV Thomson Backscatter Gamma-rays from GeV Laser-Plasma Accelerator

Abstract

We present experimental results of a method that measures high flux, MeV scale x-rays produced via Thomson backscatter (TBS) of an intense laser pulse (ao ≈ 1) interacting with a counter-propagating Laser wakefield acceleration (LWFA) electron bunch. We predict the spectra of broadband 50–100 MeV TBS gamma-ray pulses in a single shot by combining two diagnostics - a pair-producing magnetic spectrometer and a cerium doped lutetium-yttrium oxyorthosilicate (LYSO(Ce)) scintillator - to capture two independent, cross-confirming signals. By combining these two diagnostics in a single shot, we are able to compare signal originating from pair production in the gamma-conversion spectrometer with the transverse electromagnetic shower profile in the LYSO scintillator, to deconvolve a photon spectrum. We make use of Geant4 [1] simulations and Thomson scattering theory [2] to theoretically reconstruct potential candidate spectra based on the LWFA electron energies and laser parameters, and, using a chi-squared fitting method, fit the signal simulated in Geant4 to signal measured on both detectors obtaining the most probable spectra. Below, we report a preliminary most probable spectrum with a mean energy of 60 MeV.