Nine-stage photomultiplier, side-on type having S-4 response. Wavelength of maximum response is 4000 +/- 500 angstroms. This type makes use of cesium-antimony dynodes and a cesium-antimony, opaque photocathode. Window material is Corning No. 0080 lime glass or equivalent. Has a non-hygroscopic base. 11pin Sub Magnal base.