A high spectral resolution observation of the diffuse X-ray background in the 60 -1000 eV energy range has been made using an array of thirty-six 1 mm 2 microcalorimeters flown on a sounding rocket. Detector energy resolution ranged from 5-12 eV FWHM, and a composite spectrum of 1 steradian of the background centered at l = 90°, b = +60° was obtained with a net resolution of ~ 9 eV. The target area includes bright 1/4 keV regions, but avoids Loop I and the North Polar Spur. Lines of C VI, O VII, and O VIII are clearly detected with intensities of 5.4 ± 2.3, 4.8 ± 0.8, and 1.6 ± 0.4 photons cm -2 s -1 sr -1 , respectively. The oxygen lines alone account for a majority of the diffuse background observed in the ROSAT R4 band that is not due to resolved extragalactic discrete sources. We also have a positive detection of the Fe-M line complex near 70 eV at an intensity consistent with previous upper limits that indicate substantial gas phase depletion of iron. We include a detailed description of the instrument and its detectors.

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We have characterized the intrinsic 1/f noise of ion-implanted silicon thermistors in the 0.05 -0.5 K temperature range.This noise can have a significant effect on detector performance and needs to be taken into account in the design optimization of infrared bolometers and x-ray microcalorimeters. The noise can be reasonably well fit as AR/R fluctuations whose spectral density varies as 1/fand increases steeply with lower doping density and lower temperatures. The observed 1/f noise can be approximated as a resistance fluctuation: (iR2)/ R2 = O.O34(7 I 1K)2453(T /015)_(5.2+O.9Io(To)) 1(3.5 x 1018 cmVhef).