Photonic-Crystal Scintillators: Molding the Flow of Light to Enhance X-Ray and $\gamma$-Ray Detection

Scintillators are central for detection of $\gamma$-ray, x-ray, and high energy particles in various applications, all seeking higher scintillation yield and rate. However, these are limited by the intrinsic isotropy of spontaneous emission of the scintillation light and its inefficient outcoupling. We propose a new design methodology for scintillators that exploits the Purcell effect to enhance their light emission. As examples, we show 1D photonic crystals from scintillator materials that achieve directional emission and fivefold enhancement in the number of detectable photons per excitation.

Figure 1

Photonic crystal scintillator: illustration and main results. (a) Illustration of the scintillation process. An incident energetic photon (x ray or $\gamma$ ray) is converted by the scintillator material (green) into an energetic electron (e.g., photoelectric electron, blue), which excites a large number of electron-hole pairs. The electron-hole pairs thermalize until reaching luminescence centers and radiatively recombine while emitting light only into the possible photonic modes of the structure (yellow). The PhC is designed so that most of these modes are coupled out and detected by a photodetector (e.g., silicon photomultiplier, gray). By alternating wavelength-size layers of a scintillator material (green) and another dielectric material (pink), the photonic modes can be shaped, and the emission process can be controlled. (b) The number of detectable photons over time, normalized to the total number of detectable photons for a bulk scintillator with the same scintillation volume. We present the results of a LYSO:Ce/air PhC with the emission coupled out to air (blue curves), and of a ${\mathrm{Gd}}_2{\mathrm{O}}_2\mathrm{S}:\mathrm{Tb}/{\mathrm{SiO}}_2$ PhC with the emission coupled out to ${\mathrm{SiO}}_2$ (red curves). The PhC structure enables more detectable photons with a faster emission rate. (c) The coincidence time resolution (CTR) that correlates two detectors, measuring the arrival times of the first (detected) photon in each detector. The CTR determines the resolution for PET scans and other time-of-flight applications. The PhC structure enhances the CTR by a factor of 2.4.

Figure 2

The photonic-crystal scintillator emission features. (a) The emission rate enhancement for each in-plane momentum $k_x$ and wavelength, calculated for an infinite LYSO\air PhC with period $D$. The geometrical features of the structure are optimized to fit the emitter spectral distribution (dashed blue). (b) The total emission rate for the PhC (turquoise) and the outcoupled part (blue) as a function of emission angle, normalized by the bulk emission rate. In the PhC structure, the emission is created below the critical angle ${\theta }_c$ (orange), and thus the efficiency is enhanced. (c) The total emission rate for the bulk (light green) and the outcoupled part (green) as a function of emission angle. The emission into angles beyond the critical angle (orange) is undetectable. The PhC effective emission rate enhancement is the ratio between the blue area in (b) and the green area in (c). The luminescence centers are assumed homogeneous and with random dipole orientation. The analytical expressions for these plots are derived in the Supplemental Material [33], Sec. S.6.

Figure 3

Control and robustness of the photonic-crystal scintillators. (a) Enhancing scintillation by controlling the dipole orientation of the luminescence centers. The plot shows the efficiency $\eta$, and effective emission rate ${\mathrm{\Gamma }}_{\text{eff}}$ normalized by ${\tau }_{d,0}$ as a function of the dipole orientation relative to the $z$ axis for the structure from Fig. 2. When the dipoles are oriented in the in-plane direction (angle $\pi /2$) the effective emission rate is enhanced, leading to an overall enhancement factor of above 8. (b) The overall enhancement of $\eta {\mathrm{\Gamma }}_{\text{eff}}{\tau }_{d,0}$ as a function of the standard deviation (STD) of each LYSO layer width, for a different number of layers. As the randomness increases, Anderson localization reduces the transmission of light, and the overall response decreases.