Immersed Radiation Detectors

This paper is concerned with the gain in performance that may be obtained by placing a detector element in optical contact with a lens having a high index of refraction. Three different kinds of “immersed” detectors are considered. The first is a highly impractical variety in which the detector element is spherical, and is placed inside a spherical shell of high index material. The second is a practical type in which the plane detector element is placed in optical contact with the plane surface of a hemisphere. The third involves a plane detector element in optical contact with the plane surface of an aplanatic hyperhemisphere. The evaluation is based on the detectivities usually denoted by D* and D**, and two mutually exclusive cases are considered: First, where the detectivity is determined by the internal noise of the detector, and second, where the detectivity is limited by the radiation noise of the ambient radiation. The immersion of the lens may change the effective area A of the detector (as seen from the outside), it may change the solid angle Ω from which the detector will accept radiation, or it may change both. The conclusions of the paper are summarized in two tables. Only the conclusions with respect to D** may be summarized simply: For detectors limited by internal noise, immersing the lens increases D** by the factor n, where n is the smallest index of refraction among the index of the lens, the index of the detector element, and the index of the cement (if any) used to fasten the detector to the lens; for detectors limited by radiation noise, immersion does not change D**. The results are interpreted in terms of the optical invariant AΩn2.