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

Abstract

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.

Figure 1

GeV-scale photon interaction with a thin tungsten target (a) Photon attenuation through tungsten as a function of energy: total (orange solid line) and due to pair production in the nuclear field (blue dashed line). Data taken from the NIST database. (b) Simulated electron spectra (solid lines) at the rear surface of a 10 μm tungsten foil irradiated by mono-energetic photon beams of different energies: 5, 10, 15, and 17.5 GeV (dashed lines). (c) Number of electrons/positrons per GeV per primary photon as a function of photon energy.

Figure 2

Sketch (not in scale) of the setup, highlighting the main constituents of the system. The dashed square represents the region where the transverse distribution of the particles, shown in Fig. 3c, is taken.

Figure 3

Simulated noise and signal spatial distribution in the spectrometer Electron (a) and photon (b) spatial distribution after entering the system sketched in Fig. 2. The positron and electron distributions are symmetrical around the longitudinal axis of the spectrometer (positron distribution not shown). Dashed rectangles indicate a possible position for the detector. (c) Corresponding transverse distribution of electrons, positrons, and photons at the back of the spectrometer (dashed rectangle in Fig. 2). Electrons and positrons are detectable with a signal-to-noise ratio larger than 10.

Figure 4

Energy-dependent spectral resolution of the spectrometer for different divergences of the primary photon beam.

Figure 5

Example of reconstruction of the gamma-ray spectrum. (a,c) simulated electron and positron spectra at the detector plane for the incident gamma-ray spectrum shown as a red line in frame (b,d) (b,d) Comparison between the original input spectrum (red line) and the one predicted by the reconstruction algorithm (blue line) using the simulated electron and positron spectrum shown in frame (a,c). Distinctive features in the spectrum are highlighted in frame (b).

Figure 6

Accuracy in yield of the spectrometer (a) Relative difference between the input gamma-ray spectrum and the prediction of the reconstruction algorithm as a a function of energy for the case shown in Fig. 5b and a constant energy bin size of 250 MeV. (b) The distribution of the residuals is reasonably approximated by a Gaussian distribution with a standard deviation of 23%.