Characterization of thin optical filters for high purity Cherenkov light readout from scintillating crystals

TL;DR

This work addresses separating Cherenkov light from scintillation in a dual-readout crystal calorimeter by evaluating thin optical filters placed in front of SiPMs. It combines detailed crystal characterization (transmission spectra, scintillation time constants, and light yield) with systematic filter testing (interference vs absorptive long-pass filters) and a convolution-based model to predict scintillation-light suppression: $ S/S_0 = \frac{\int EM(\lambda)\, Tc(\lambda)\, T_f(\lambda)\, d\lambda}{\int EM(\lambda)\, Tc(\lambda)\, d\lambda} $. Experimental validation using a LYSO:Ce crystal confirms the model within ~20%, revealing that angular dependence of interference filters degrades performance for broad emission directions. The study identifies 100 μm absorptive long-pass filters with cutoff around 590 nm (e.g., Kodak-24/25) as highly effective for PWO, while Kodak-29 provides strong suppression for BGO/BSO with Cherenkov discrimination; the results hold practical significance for designing high-purity Cherenkov readout in future e^{+}e^{-} collider calorimeters. Overall, the work offers a path to achieving Cherenkov purity and the target photon statistics required for dual-readout calorimetry through informed filter selection and crystal characterization.

Abstract

A hybrid dual-readout calorimeter concept, comprising both electromagnetic and hadronic sections, has recently been proposed to meet the performance requirements of experiments at future e$^{+}$e$^{-}$ colliders. The front compartment consists of a homogeneous electromagnetic calorimeter made of high-density crystals, each coupled to a pair of Silicon Photomultipliers (SiPMs) providing the simultaneous readout of scintillation and Cherenkov light. To efficiently detect Cherenkov photons in the presence of dominant scintillation signals, an optical filter is placed in front of one of the two SiPMs to suppress photons in the wavelength region corresponding to that of scintillation emission. % In this study, PWO, BGO, and BSO crystals with different dimensions were tested to measure their scintillation light yield and decay time, as well as their transmission and emission spectra. A set of $\sim 100~\rm μm$-thick optical filters was also characterized by measuring their transmittance curves. The experimental results were used to model and estimate the expected filter performance in attenuating scintillation light for the various crystals. % The performance of each filter was experimentally validated by measuring the crystal light output with and without the filter using a $^{22}$Na radioactive source and a LYSO:Ce crystal, confirming the accuracy of the calculations. % The results show that interference filters are unsuitable for this application because their transmittance strongly depends on the photon incidence angle. Conversely, two absorptive long-pass filters with cutoff wavelengths around 590~nm were found to block more than 99\% of the scintillation light from PWO crystals, satisfying the calorimeter specifications.

Figures (12)

• Figure 1: Transmittance curves are shown for BGO, BSO and PWO crystals of different length and their respective absorption coefficients are calculated using equation \ref{['eq:mu_intr']}. The emission spectra of BGO, BSO and PWO are also reported with dashed lines respectively on the left, central and right panel.
• Figure 2: Scintillation decay time profile of $10\times10$ mm$^2$ for BGO, BSO and PWO crystals (from left to right). The red curves represent the result of a fit using eq. \ref{['eq:eff_decay_time']}. The residuals of the distribution with respect to the red curve are reported in the bottom panels.
• Figure 3: Left: Light output of PWO, BGO and BSO crystals with $1\times 1~\rm cm^2$ section and different lengths measured with a PMT (brand). Right: light output of 5 cm long BGO crystals with different sections.
• Figure 4: Light output of BGO crystals readout with a $4\times 4$ mm$^2$ SiPM measured with a $^{22}$Na source and compared with prediction from Geant4 ray-tracing simulation. Results are shown for $10\times10$ mm$^2$ section crystals of different lengths (left panel) and for 5 cm long crystals of different sections (right panel).
• Figure 5: Picture of the filters tested. From left to right: Everix-Int-560, Everix-Int-650, Eve-Abs-580, Hoya-U330, Hoya-O56, Kodak-8, Kodak-22, Kodak-24, Kodak-25, Kodak-29. Side views of the Kodak-24 (absorptive) and the Everix-Int-560 (interference) filter being warped.