Design of a compact spectrometer for high-flux MeV gamma-ray beams

Experimental Demonstration of a Tunable Energy-Selective Gamma-Ray Imaging System Based on Recoil Electrons

The domain of gamma-ray imaging necessitates technological advancements to surmount the challenge of energy-selective imaging. Conventional systems are constrained in their dynamic focus on specific energy ranges, a capability imperative for differentiating gamma-ray emissions from diverse sources. This investigation introduces an innovative imaging system predicated on the detection of recoil electrons, addressing the demand for adjustable energy selectivity. Our methodology encompasses the design of a gamma-ray imaging system that leverages recoil electron detection to execute energy-selective imaging. The system's efficacy was investigated experimentally, with emphasis on the adaptability of the energy selection window. The experimental outcomes underscore the system's adeptness at modulating the energy selection window, adeptly discriminating gamma rays across a stipulated energy spectrum. The results corroborate the system's adaptability, with an adjustable energy resolution that coincides with theoretical projections and satisfies the established criteria. This study affirms the viability and merits of utilizing recoil electrons for tunable energy-selective gamma-ray imaging. The system's conceptualization and empirical validation represent a notable progress in gamma-ray imaging technology, with prospective applications extending from medical imaging to astrophysics. This research sets a solid foundation for subsequent inquiries and advancements in this domain.

In Situ Measurement of Electron Energy Evolution in a Laser-Plasma Accelerator

We report on a novel, noninvasive method applying Thomson scattering to measure the evolution of the electron beam energy inside a laser-plasma accelerator with high spatial resolution. The determination of the local electron energy enabled the in-situ detection of the acting acceleration fields without altering the final beam state. In this Letter we demonstrate that the accelerating fields evolve from (265±119)  GV/m to (9±4)  GV/m in a plasma density ramp. The presented data show excellent agreement with particle-in-cell simulations. This method provides new possibilities for detecting the dynamics of plasma-based accelerators and their optimization.

High-dose femtosecond-scale gamma-ray beams for radiobiological applications

Objective. In the irradiation of living tissue, the fundamental physical processes involved in radical production typically occur on a timescale of a few femtoseconds. A detailed understanding of these phenomena has thus far been limited by the relatively long duration of the radiation sources employed, extending well beyond the timescales for radical generation and evolution. Approach. Here, we propose a femtosecond-scale photon source, based on inverse Compton scattering of laser-plasma accelerated electron beams in the field of a second scattering laser pulse. Main results. Detailed numerical modelling indicates that existing laser facilities can provide ultra-short and high-flux MeV-scale photon beams, able to deposit doses tuneable from a fraction of Gy up to a few Gy per pulse, resulting in dose rates exceeding 1013 Gy/s. Significance. We envisage that such a source will represent a unique tool for time-resolved radiobiological experiments, with the prospect of further advancing radio-therapeutic techniques.

Spectral characterization of flash and high flux x-ray radiographic sources with a magnetic Compton spectrometer

In this work, we present a new analysis method applied to revitalize permanent magnet Compton spectrometers used to measure photon energy spectra in the MeV range. The inversion of the measured electron distribution to determine the original photon distribution is achieved via a method of consistent coupled radiation transport and magnetic field mapping of the input photon spectra to the measured electron distribution. The method of linear least squares was used to perform the unfolding of the electron distribution to the initial photon spectra, without any assumptions made regarding the electron distribution. We present an application of this method to data from a nominal 19.4 MeV flash radiographic source (the first axis of the Dual Axis Radiographic Hydro-Test Facility) capable of generating 500 R @ 1 m in ∼60 ns and a medical therapy source (a Scanditronix M22, Microtron) capable of variable energies with nominal endpoints of 6, 10, 15, and 20 MeV and an output of ∼1000-2000 R/min @ 1 m. The results provide agreement between the modeled and unfolded experimentally measured photon spectra as quantified by statistical tests, from 1.5 to 20 MeV. Experimental results are presented as well as a discussion of the novel MCNP6-based simulations and methods for reconstruction of the spectra.

Compact broadband Compton spectroscopy used for intense laser-driven gamma rays

A compact broadband Compton spectrometer is designed to measure the continuous spectrum of gamma-ray sources driven by an intense laser. The incident gamma rays are converted into electrons in low-Z materials by Compton scattering. Produced by a pair of stepped magnets, a weaker-front-stronger-rear nonuniform magnetic field in the electron magnetic spectrometer is used to spectrally resolve the scattered electrons, leading to a broadband gamma-ray spectral coverage of 2-20 MeV in a compact volume. Flat imaging-plate detectors are placed near the focused imaging points of the magnetic spectrometer to record the dispersed electrons, thereby achieving an optimal spectral resolution of 6%-13% in the energy range of 3-20 MeV. The spectrometer is used successfully to measure the gamma-ray spectrum generated by the high-energy electron beams produced by a femtosecond-laser-driven wakefield.

Conceptual Design of a High-flux Multi-GeV Gamma-ray Spectrometer

We present here a novel scheme for the high-resolution spectrometry of high-flux gamma-ray beams with energies per photon in the multi-GeV range. The spectrometer relies on the conversion of the gamma-ray photons into electron-positron pairs in a solid foil with high atomic number. The measured electron and positron spectra are then used to reconstruct the spectrum of the gamma-ray beam. The performance of the spectrometer has been numerically tested against the predicted photon spectra expected from non-linear Compton scattering in the proposed LUXE experiment, showing high fidelity in identifying distinctive features such as Compton edges and non-linearities.

Gradient magnet design for simultaneous detection of electrons and positrons in the intermediate MeV range

We report the design and development of a compact electron and positron spectrometer based on tapered neodymium iron boron magnets to characterize the pairs generated in laser-matter experiments. The tapered design forms a gradient magnetic field component allowing energy dependent focusing of the dispersed charged particles along a chosen detector plane. The mirror symmetric design allows for simultaneous detection of pairs with energies from 2 MeV to 500 MeV with an accuracy of ≤10% in the wide energy range from 5 to 110 MeV for a parallel beam incident on a circular aperture of 20 mm. The energy resolution drops to ≤20% for 4-90 MeV range for a divergent beam originating from a point source at 20 cm away (i.e., a solid angle of ∼8 milli steradians), with ≤10% accuracy still maintained in the narrower energy range from 10 to 55 MeV. It offers higher solid angle acceptance, even for the divergent beam, compared to the conventional pinhole aperture-based spectrometers. The proposed gradient magnet is suitable for the detection of low flux and/or monoenergetic type electron/positron beams with finite transverse sizes and offers unparalleled advantages for gamma-ray spectroscopy in the intermediate MeV range.

A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV

We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

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.

Making spectral shape measurements in inverse Compton scattering a tool for advanced diagnostic applications

Interaction of relativistic electron beams with high power lasers can both serve as a secondary light source and as a novel diagnostic tool for various beam parameters. For both applications, it is important to understand the dynamics of the inverse Compton scattering mechanism and the dependence of the scattered light’s spectral properties on the interacting laser and electron beam parameters. Measurements are easily misinterpreted due to the complex interplay of the interaction parameters. Here we report the potential of inverse Compton scattering as an advanced diagnostic tool by investigating two of the most influential interaction parameters, namely the laser intensity and the electron beam emittance. Established scaling laws for the spectral bandwidth and redshift of the mean scattered photon energy are refined. This allows for a quantitatively well matching prediction of the spectral shape. Driving the interaction to a nonlinear regime, we spectrally resolve the rise of higher harmonic radiation with increasing laser intensity. Unprecedented agreement with 3D radiation simulations is found, showing the good control and characterization of the interaction. The findings advance the interpretation of inverse Compton scattering measurements into a diagnostic tool for electron beams from laser plasma acceleration.

Ultrahigh Brilliance Multi-MeV γ-Ray Beams from Nonlinear Relativistic Thomson Scattering

We report on the generation of a narrow divergence (θ_{γ}<2.5  mrad), multi-MeV (E_{max}≈18  MeV) and ultrahigh peak brilliance (>1.8×10^{20}  photons s^{-1} mm^{-2}  mrad^{-2} 0.1% BW) γ-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a_{0}≈2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.