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

A Hannasch,R Zgadzaj,T E Cowan,A Köhler,L Naumann,A Ferrari,U Schramm,M C Downer,M Laberge,M Molodtsova,A Laso Garcia,A Irman,J P Couperus Cabadağ,T Kurz,O Zarini

doi:10.1038/s41598-021-93689-5

Abstract

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.

Highlights

Accelerator-based sources of bright, hard X-rays have enabled decades of advances in materials science1, medicine2,3, geology[4], warm dense matter s cience[5], radiography of high-Z materials[6] and non-destructive testing in industry[7]
We spectrally characterize betatron, bremsstrahlung and inverse Compton scattered (ICS) X-rays from a 250–350 MeV Laser wakefield accelerators (LWFAs) in a single shot, using a single, compact, inexpensive instrument: a modular calorimeter consisting of a stack of absorbers of varying Z and thickness, interlaced with imaging plates (IPs)
A stack calorimeter consisting of 24 absorbing layers interspersed with IPs, recorded the depth-energy distribution of particle cascades initiated by secondary X-rays from the LWFA

Summary

Introduction

Accelerator-based sources of bright, hard X-rays have enabled decades of advances in materials science1, medicine2,3, geology[4], warm dense matter s cience[5], radiography of high-Z materials[6] and non-destructive testing in industry[7]. LWFAs can generate three types of secondary X-rays: betatron radiation, inverse Compton scattered (ICS) radiation, and bremsstrahlung. Secondary X-ray photons from LWFAs span an energy range from several keV to several hundred MeV, enabling a wide range of applications[13,14], but requiring an unusually versatile spectrometer for source characterization[14]. The energy of the secondary Compton electrons and e+e− pairs is related straightforwardly to that of the incident X-rays, provided the converter is thin enough to avoid multiple scattering events. This converter thickness requirement limits signal-to-noise ratio, often necessitating averaging over multiple shots. We reconstruct spectra that are betatron-, ICS- or bremsstrahlung-dominated, as well as spectra containing a mixture of different types of X-rays with widely separated photon energies

Methods

Results

Conclusion