Experimental characterization of a single-shot spectrometer for high-flux, GeV-scale gamma-ray beams

We report on the first experimental characterization of a gamma-ray spectrometer designed to spectrally resolve high-flux photon beams with energies in the GeV range. The spectrometer has been experimentally characterized using a bremsstrahlung source obtained at the Apollon laser facility during the interaction of laser-wakefield accelerated electron beams (maximum energy of 1.7 GeV and overall charge of $207\pm 62$ pC) with a 1 mm thick tantalum target. Experimental data confirms the possibility of performing single-shot measurements, without the need for accumulation, with a high signal-to-noise ratio. Scaling the results to photons in the multi-GeV range suggests the possibility of achieving percent-level energy resolution as required, for instance, by the next generation of experiments in strong-field quantum electrodynamics.

Figure 1

Top-view schematic of the experimental setup.

Figure 2

Examples of spectral intensity of laser-wakefield electrons generated within the gas cell target for eight consecutive shots (blue dashed), their mean spectral intensity (solid orange) and one standard deviation from the mean (shaded orange).

Figure 3

Simulated energy spectra per pC of electron beam charge of (a) photons exiting the converter located (blue solid), and photons incident on the spectrometer converter (orange dashed), and (b) $e^{-}{e}^{+}$ pairs (green solid and pink dashed, respectively) produced within the spectrometer converter which reach the LANEX detector.

Figure 4

(a) Example of single-shot background-subtracted data recorded by the scintillator screen in the gamma-ray spectrometer. The figure evidences the positron and electron signal region on either side of the spectrometer axis. (b) Average over eight consecutive shots. An artefact induced by the bottom edge of the collimator aperture of the spectrometer is visible in both images close to (0,0). This artefact is due to a nonideal background subtraction around the collimator edges, due to shot-to-shot fluctuations in the pointing of the gamma beam.

Figure 5

Energy spectrum of $e^{+}{e}^{-}$ pairs measured at the back of the spectrometer with corresponding uncertainty (shaded). Experimental results are compared to the positron spectrum (green) extracted from simulations.

Figure 6

Reconstructed photon spectra obtained by applying the deconvolution algorithm to the experimental (orange) and simulated (green) positron spectra in Fig. 5. The overlaid dashed line shows the photon spectrum incident on the spectrometer from simulation, as in Fig. 3. Shaded bands represent the 95% HPDI calculated by the algorithm.

Figure 7

Result of deconvolving (a) a Gaussian (mean = 5 GeV, $\sigma =1.2$ GeV), and (b) a piecewise step-like photon spectrum. Black is the original spectrum and the shaded blue is the 95% HPDI of the reconstruction in each case.