Measurements of the Birks' coefficient of GAGG:Ce using hard X-rays

TL;DR

This study quantifies the Birks' coefficient $k_B$ for GAGG:Ce using mono-energetic 20–80 keV X-rays to explain nonlinearity near the K-edge in hard X-ray detectors. By comparing POLAR-2–style GAGG:Ce crystal array measurements at the LARIX-A beamline with Geant4 simulations that incorporate Birks' law, the authors determine a best-fit $k_B \approx 0.075$ mm/MeV (with ~0.07–0.08 mm/MeV across plausible uncertainties). The analysis shows Birks' quenching accounts for the observed drop in pulse height above the K-edge and the accompanying jump in energy resolution, while the result is sensitive to the Ga:Al ratio in the crystal. These measurements provide a critical parameter for accurate hard X-ray spectrometry and polarization measurements in future missions using GAGG:Ce, and highlight the influence of crystal chemistry on scintillator nonlinearity.

Abstract

Inorganic scintillators continue to be widely used within astrophysical X-ray and gamma-ray detectors. This is in part thanks to the development of new scintillators, such as GAGG:Ce, as well as the availability of new scintillator readout sensors such as Silicon Photomultipliers and Silicon Drift Detectors. In order to use such scintillator materials for spectrometry or polarimetry, a detailed understanding of their response is important. One parameter that can affect the scintillator performance, particularly at lower photon energies, is their Birks' coefficient, which correlates the relative light yield to the ionization energy density. While for many high-Z inorganic scintillators this effect can be ignored, for GAGG:Ce this appears to not be the case. Here we provide a measurement of the Birks' coefficient for GAGG:Ce using data from a detector irradiated in the 20-80~keV energy range at the LARIX-A X-ray beam in Ferrara, Italy. While the effects due to Birks' law are visible below 30 keV, they also significantly influence the performance of GAGG:Ce performance near one of the K-edges, affecting both the measured gain and the energy resolution. Here, we use beam test data to derive the Birks' coefficient from GAGG:Ce. The results indicate that for usage in hard X-ray and soft gamma-ray missions, this coefficient has a significant effect on the measurements.

Figures (10)

• Figure 1: Left: The mean pulse height versus beam energy as measured by the POLAR-2 spectrometer prototype. A clear jump between 50 and 51 keV can be observed. Right: The energy resolution from the same detector as a function of beam energy. Here, an increase in the energy resolution can be observed at the same energy.
• Figure 2: The mean pulse height divided by the beam energy versus the beam energy. The clear drop in efficiency of the scintillator above the K-edge remains visible, while non-linear effects at low energies also become visible.
• Figure 3: Left:The array of $8\times8$ GAGG:Ce crystals placed in the $\mathrm{BaSO}_4$ mechanics as used during this test. Taken from Kole2025_a. Right: The schematic design of the full detector. The GAGG crystals are shown as teal blocks placed below a 1 mm-thick carbon fiber mechanical frame. The thin $\mathrm{BaSO}_4$ housing is now shown in this schematic. The SiPM array is shown in green below the crystals, with below it the readout PCB and further aluminum mechanics in gray.
• Figure 4: The energy spectrum from the beam as measured using the HPGe detector for a beam energy of 50 keV. A shift of 0.7 keV in the mean can be observed while the energy resolution corresponds to $0.75\%$.
• Figure 5: An example of the pulse height spectrum in ADC taken with a beam energy of 75 keV. The noise region can be observed at low ADC values along with 2 photo-peaks. The higher of these two corresponds to the 75 keV photo-peak while the lower is a 32 keV peak induced by the $\mathrm{BaSO}_4$ mechanical housing.