Theory-Based High-QE Photocathode Development
While substantial advancements have been made in realizing tube based large area detectors such as the Large Area Picosecond Photo-Detector (LAPPD), there is widespread agreement in the cathode community that a higher QE bialkali cathode is possible, and that the present understanding is largely recipe-driven without the adequate control or understanding to achieve the previously observed high yield of narrowly clustered QE & apos;s. There has been progress in understanding some of the factors, resulting in commercial tubes with QE & apos;s of ~32-40%. Developing and implementing higher QE photocathodes would significantly improve sensitivity of the tube- based detectors. Thus, the goal of this SBIR is to develop high quantum efficiency photocathodes, based on a robust understanding of the underlying physics and chemistry. The challenges associated with the deposition of bialkali-antimony photocathode, which is a ternary compound, will be addressed using two main strategies, 1) co-evaporation of constituents using independent sources, and 2) evaporation of a pre- synthesized ternary compound. In either case deposition methods including thermal evaporation and e-beam technique will be explored. Studies of the photocathode behavior using such techniques as XRD will be performed. The purpose of the Phase I is to demonstrate feasibility of our approach, where photocathodes will be deposited on small B33 substrates. The Phase II research will extend the Phase I study to further enhance the photocathode QE through a robust understanding of the correspondence between theory and its growth. Commercial Applications and Other Benefits: The ability to consistently make photocathodes with QE of 40-50% at high yield will substantially lower the cost per detected photon in large water Cherenkov counters for short- and long-baseline neutrino experiments, Positron-Emission Tomography, and other applications limited by photon counting. The improved QE also significantly ameliorates the difference in QE between vacuum- based and silicon technologies, while retaining the big advantage in noise, gain, and bandwidth. In addition, a robust theoretical understanding of the relationship between cathode growth procedures and electronic band- structure, scattering, and morphology will allow improvements in cathodes for other regions of the spectrum, including in the UV (liquid-Argon, liquid-Xe) and the IR (defense and astronomy).
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