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Measurements have been made of the efficiency in detecting gamma rays of a 0.3-mm, a 3-mm, and a 5-mm silicon detector covered with different absorbers. Calibrated sources covering the range from 279 keV to 2.75 MeV were used. /The need for the absorbers in order to obtain meaningful results and their contribution to the response of the detectors at electron biases from 50 to 200 keV are discussed in detail. It will be shmm that the results are virtually independent of the atomic number of the absorber. In addition, the role of the absorber in increasing the efficiency with increasing photon energy for low bias settings is demonstrated for the 0.3-mm crystal. Qualitative explanations are given for the shapes of all curves of efficiency versus energy at each bias. 10. Work Unit No. Use the agency-wide code (for example, 923-50-10-06-72), which uniquely identifies the work unit under which the work was authorized. Non-NASA performing organizations will leave this blank.11. Insert the number of the contract or grant under which the report was prepared. 17. Key Words. Insert terms or short phrases selected by the author that identify the -principal subjects covered in the report, and that are sufficiently specific and precise to be used for cataloging• . 18. Distribution.Statement. Enter one of the authorized statements used to denote releasability to the public or a limitation on dissemination for reasons other than security of defense information. Authorized statements are "Unclassified-Unl imited, " "U. S. Government and Contractors only, " "U. S. Government Agencies only, " and "NASA and NASA Contractors only. "19. Security Classification (of report). NOTE: Reports carrying a security classification will require additional markings giving security and downgrading information as specified by the Security Requirements Checkl ist and the 000 Industrial Security Manual (000 5220. 22-M).20. Security Classification (of this page). NOTE: Because this page may be used in preparing announcements, bibliographies, and data banks, it should be unclassified if possible. If a classification is required, indicate separately the classification of the title and the abstract by following these items with either "(U)" for unclassified, or "(C)" or "(S)" as appl icable for classified items.

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