Abstract. Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft X-ray irradiance. These measurements are carried out by a multichannel photometer system. The spectral range between 2 and 20 nm is covered by three channels with bandpasses of 2-7 nm, 6 -19 nm, and 17 -20 nm respectively. Absolute sensitivities were measured prefiight using the Synchrotron Ultraviolet Radiation

The stability of the quantum efficiency of inversion layer, phosphorus-diffused (n on p), and boron-diffused (p on n) photodiodes has been investigated. Unsatisfactory silicon-silicon dioxide interfaces, latent recombination centers in the diffused layers, and moisture absorption by the device were identified as possible causes of instability. Diodes were fabricated using processes in which these sources of instability were carefully controlled. The resulting diodes were subjected to various accelerated aging tests, and the external quantum efficiency of the diodes was monitored during the tests. Diodes made by older procedures, in which some important parameters affecting stability were not controlled, were included in the study for comparison. The major result of this work is the demonstration that n on p photodiodes are inherently more stable than p on n types in the ultraviolet and blue spectral regions, but that stable p on n devices can also be produced with sufficient care.

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Silicon n-on-p photodiodes with 100 % internal quantum efficiency have been studied in the 160 nm to 254 nm spectral range. Preliminary values have been determined for the quantum yield of silicon at these wavelengths. Using these values, a trap detector is presented for absolute flux measurement in this region. The stability under intense 193 nm irradiation, a property of importance in lithography and in photorefractive keratectomy, has been measured, and the diodes tested were found to be several orders of magnitude more stable than p-on-n diodes tested by other investigators at this wavelength. Spatial nonuniformities of the n-on-p diodes were found to be less than 1 % at wavelengths of 254 nm and 161 nm.