Investigation of Radiation Emitted by Sub GeV Electrons in Oriented Scintillator Crystals

TL;DR

The study addresses how sub-GeV electrons interacting with oriented scintillator crystals can emit enhanced radiation through coherent axial effects. It combines pre-characterization of crystal quality with a beam experiment at MAMI using 855 MeV electrons aligned along crystal axes in PWO, BGO, and CsI to measure photon spectra. The results show clear radiation enhancement for axial orientations, strongest in BGO along the 111 direction, with PWO also exhibiting significant coherence and CsI to a lesser extent due to higher mosaicity. These findings demonstrate the viability of oriented crystal scintillators for compact, directionally selective detectors and gamma detectors in nuclear, particle, and astrophysical contexts.

Abstract

The research investigates coherent interactions between sub-GeV electrons and oriented scintillator crystals, leading to enhanced electromagnetic (EM) radiation. Experiments at Mainz Mikrotron (MAMI) involved PWO, BGO, and CsI crystals oriented along $\langle100\rangle$, $\langle111\rangle$, and $\langle100\rangle$ axes. Enhanced radiation emission was observed when the beam aligned with crystal axes, especially in BGO and CsI for the first time. These findings are crucial for innovative detectors using oriented crystal scintillators, amplifying EM processes along specific crystallographic directions. Potential applications include ultra-compact, highly sensitive electromagnetic calorimeters for high-energy physics and astroparticles, as well as high-performance gamma detectors for nuclear physics and medical imaging.

Figures (5)

• Figure 1: Photographic plate measurements. (a) PWO; (b) BGO; (c) CsI. The crystallographic structure is visible.
• Figure 2: RC measurements. (a) PWO; (b) BGO; (c) CsI. The dashed red lines are the fitted function to find the mosaicity of the samples.
• Figure 3: X-ray topography. (a) PWO; (b) BGO.
• Figure 4: Experimental setup, top view. Photon spectra are detected with a NaI detector. The detector is shielded by a 100 mm thick lead wall with a 70 mm opening for the photons. The 44$^\circ$-ionization chamber, filled with air at standard pressure, is employed to detect crystal alignment. Downstream the crystal target, the beam is deflected horizontally by the 44$^\circ$ bending magnet BM1 and vertically by a 7.2$^\circ$ bending magnet BM2. Showers are produced by the electrons that leaved the nominal direction due to scattering or energy loss in the target (in red). Just in front of the beam dump, the beam spot can be monitored with a ZnS luminescent screen which is viewed by a CCD camera.
• Figure 5: This figure presents experimental data collected at MAMI, depicting the ratios of radiation spectra obtained for each crystal. The spectra were produced by directing the beam along specific crystallographic directions, and the results are compared with the spectrum obtained when the crystal was randomly oriented to simulate an amorphous case. Enhancement peaks are denoted by colored dots, with green dashed vertical lines indicating the reference energies at 50 and 100 MeV. The amorphous spectrum line, normalized to 1, is represented by the red dashed line.