Calorimeter with Bayesian unfolding of spectra of high-flux broadband x rays.

We report the development of a multipurpose differential x-ray calorimeter with a broad energy bandwidth. The absorber architecture is combined with a Bayesian unfolding algorithm to unfold high energy x-ray spectra generated in high-intensity laser-matter interactions. Particularly, we show how to extract absolute energy spectra and how our unfolding algorithm can reconstruct features not included in the initial guess. The performance of the calorimeter is evaluated via Monte Carlo generated data. The method accuracy to reconstruct electron temperatures from bremsstrahlung is shown to be 5% for electron temperatures from 1 to 50 MeV. We study bremsstrahlung generated in solid target interaction showing an electron temperature of 0.56 ± 0.04 MeV for a 700 µm Ti titanium target and 0.53 ± 0.03 MeV for a 50 µm target. We investigate bremsstrahlung from a target irradiated by laser-wakefield accelerated electrons showing an endpoint energy of 551 ± 5 MeV, inverse Compton generated x rays with a peak energy of 1.1 MeV, and calibrated radioactive sources. The total energy range covered by all these sources ranges from 10 keV to 551 MeV.

Compact spectroscopy of keV to MeV X-rays from a laser wakefield accelerator.

We reconstruct spectra of secondary X-rays from a tunable 250-350 MeV laser wakefield electron accelerator from single-shot X-ray depth-energy measurements in a compact (7.5 × 7.5 × 15 cm), modular X-ray calorimeter made of alternating layers of absorbing materials and imaging plates. X-rays range from few-keV betatron to few-MeV inverse Compton to > 100 MeV bremsstrahlung emission, and are characterized both individually and in mixtures. 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 analytic models of betatron, inverse Compton and bremsstrahlung photon energy distributions then unfolds X-ray spectra, typically within a minute. We discuss uncertainties, limitations and extensions of both measurement and reconstruction methods.

Compact high energy x-ray spectrometer based on forward Compton scattering for high intensity laser plasma experiments.

This article describes the design and presents recent results from testing and calibration of a forward Compton scattering high energy X-ray spectrometer. The calibration was performed using a bremsstrahlung source on the photon scattering facility at the γ Electron linac for beams with high brilliance and low emittance accelerator at Helmholtz-Zentrum Dresden-Rossendorf, which provides high energy X-ray photons with energies up to 18 MeV. The calibration was conducted at different bremsstrahlung end point energies-10.5, 13, 15, and 18 MeV. Experimental spectra show a systematic increase in the maximum energy, photon temperature, and flux. The spectrometer is effective for an energy range of 4-20 MeV with 20%-30% energy resolution. The spectrometer operates in low vacuum with pressure less than 0.1 mbar. Experimental tests showed that operating such a spectrometer in air causes a spuriously enhanced high energy signal due to Compton scattering of photons within air. The article also describes the design and shielding considerations which helped to achieve a dynamic range greater than 30 with this spectrometer. The comparison between the experimental results and Monte Carlo simulations are also presented.