A Practical Multiple Reflection Technique for Improving the Quantum Efficiency of PhotomultiplierTubes J. B. Oke and Rudolph E. Schild A technique is described by which multiple reflection techniques can be used to increase the quantum efficiency of some end-on photomultiplier tubes in the red and near ir. The method can be used in practice for astronomical and other applications where field lens imaging on the cathode is required and where small cathodes are desirable. Tests of a group of unselected production model S-20 and S-1 photomulti- plier tubes show quantum efficiencygains as high as factors of 3.8 and 1.8, respectively, at practical oper- ating wavelengths. Introduction During the last few years it has been demonstrated that the quantum efficiencies of certain photocathodes can be substantially improved by means of multiple re- flection techniques.' - However, the experimental techniques which have been used are not practical for astronomical applications. In astronomical photo- electric photometry and spectrophotometry of stars, it is necessary to allow starlight to pass through a diaphragm in the focal plane of the telescope. Because the star image appears to change size and wander erratically in the telescope due to small telescope motions, tracking errors, and effective variations of the refractive index of the atmosphere (seeing), the focal plane diaphragm must be much larger than the star image itself. To maintain signal uniformity as the star image wanders within the focal plane diaphragm, a field lens is em- ployed to image the telescope objective on the cathode of the photomultiplier tube. Unless this imaging is done moderately well, quantitative measurements are not possible. Thus, in any practical multiple reflection method, this image quality must be maintained. A further requirement is usually present. Since astro- nomical sources are faint, it is important to minimize the photomultiplier dark current. Thus, it is necessary to minimize the effective photocathode area, and this re- quirement places severe limitations on any multiple re- flection technique. Optical Configurations In Fig. 1 (a) the usual configuration for multiple bounce experiments is illustrated; it clearly violates both re- The authors are with Mount Wilson and Palomar Observa- tories, Carnegie Institution of Washington, California Institute of Technology, Pasadena, California 91109. Received 25 October 1967. quirements outlined above. In particular, proper field lens imaging of the telescope objective is lost and cath- ode area must be increased. In Fig. 1(b) a simple con- figuration is shown that overcomes both of these difficulties. A partial hemisphere is attached to the tube face by means of a suitable oil or grease with an in- dex of refraction near 1.5, so that the combined partial hemisphere and tube window form a true hemisphere centered on the cathode. The hemisphere is aluminized except for a slot, illustrated in Fig. 1(c), through which the beam enters. In the experiments described here, the hemisphere was made of fused quartz, and silicon grease was used between the hemisphere and the tube window. It is clear from the figure that by rotating the photomultiplier tube with its attached hemisphere rela- tive to the incoming light beam and about the hemi- sphere's center, the angle of incidence of light on the photocathode 0 can be varied from 00 (normal inci- dence) to approximately 600. In previous multiple bounce experiments with prisms, the angle of incidence on the photocathode could only be varied from 40 to 750. In the present configuration, smaller angles can be employed for experimental purposes, although the smaller angles are of little practical interest. A cold box was built which allowedthe tube to be operated at inci- dence angles from 00 to 600 and which allowed the tube to be refrigerated to dry ice temperatures. Such refrig- eration is a standard practice in astronomical applica- tions to ensure stability of operating conditions and to minimize the dark current. At small angles of incidence, reflection is relatively un- important, and the absorption path for light incident upon the photocathode is proportional to see . Reflec- tion from both surfaces of the photocathode becomes in- creasingly important with increasing angle of incidence. The reflected light proceeds to the aluminized surface of the hemisphere and is reimaged on the same spot on the photocathode. Any radiation reflected a second time April 1968/ Vol. 7, No. 4 / APPLIED OPTICS 617