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Instrument Design to Measure the Optical Properties of Reflectance and Transmittance

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M Shin

M Shin1*, J Star-Lack2, M Janecek3, D Shedlock2, E Abel2, R Fahrig1 (1) Stanford University, Stanford, CA, (2) Varian Medical Systems, Palo Alto, CA, (3) Lawrence Berkeley National Laboratory, Berkeley, CA

TH-A-141-8 Thursday 8:00AM - 9:55AM Room: 141

Purpose: To estimate performance of crystaline scintillator-based pixelated detectors depending on the optical properties of the crystal surface (cut, etched, polished) and the septum material between each crystal. We propose a new device design to measure the optical reflectance and transmittance properties of candidate crystal-septum structures.

Methods: Using a measurement device including a laser and an arc of photodiode detectors, the reflectance of two sandwich samples that each mimic a pixel was measured: CdWO₄-glue-ESR-glue-CdWO₄ and CdWO₄-glue-(Al-sputtered-ESR)-glue-CdWO₄. Reflectance was normalized to that of ESR (a multi-layer optical film, highly specular reflector). To provide the required range of incident light angles at the interface of interest, a BGO hemisphere with a high index of refraction (n=2.2) was glued (Meltmount, n=1.7) to the top surface of the sandwich.

Results: The sandwich structure demonstrated constant reflectivity over all laser angles with reflectances of 95% and 60% for ESR and Al-sputtered-ESR sandwiches respectively. This is because the highest incident angle achieved at the crystal-glue interface was only 25.8 degrees due to the large difference in refractive indices between air (n=1) and CdWO₄ (n=2.2), which is below the critical angle for total internal reflection. With the BGO hemisphere added, there are two boundaries where total internal reflection can occur, and three levels of reflectivity were detected with the steps corresponding to the critical angles of 45 degrees and 54 degrees. Normalization is required to remove the influence of the Meltmount and BGO crystal from the measured reflectivities.

Conclusion: The optical reflectance due to surface conditions and septum materials can be accurately measured via the addition of a high-refractive-index hemisphere to a sandwich structure that has equivalent characteristics to the finished pixel matrix. Reflectance and transmittance are both important to develop an efficient pixelated detector, therefore the instrument has been modified and being assembled to measure them.

Funding Support, Disclosures, and Conflict of Interest: This work is supported by National Institutes of Health (NIH R01 CA138426) and the Richard M. Lucas Foundation. There is no conflict of interest to disclose.

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