Photomultiplier, at a wild guess - the technology was perfected by '37. No semiconductors needed. https://en.wikipedia.org/wiki/Photomultiplier#First_photomulti

Photomultiplier, at a wild guess - the technology was perfected by '37. No semiconductors needed. https://en.wikipedia.org/wiki/Photomultiplier#First_photomultiplier.2C_single-stage_.28early_1934.29 https://en.wikipedia.org/wiki/Photomultiplier#First_photomul...