Below 1 keV the soft X-ray diffuse background (SXRB) is believed to be largely Galactic in origin. Thermal emission from hot gas is the likely source of this background flux (McCammon & Sanders 1990), and its spectrum should be dominated by emission lines from highly ionized species (see Figure 3).
The emission below 0.3 keV is dominated by hot gas in a low density region
within 
pc of the Sun referred to as the Local Bubble, while the
emission between 0.3 and 1.0 keV is of more uncertain origin.
Four all-sky
surveys of the SXRB have now been completed: the UW sounding rocket
survey (McCammon et al. 1983), the SAS-3 survey (Marshall
& Clark 1984), the HEAO-1 A-2 LED (Garmire et al. 1991) and the ROSAT \
PSPC survey (Snowden et al. 1995).
All of these all-sky surveys used proportional counters with broad-band
filters to define soft X-ray bandpasses.
Although we now have a good picture of the spatial distribution of the SXRB with an angular resolution of a fraction of a degree, the poor energy resolution available from proportional counters has prevented direct observation of the spectral lines believed to dominate this emission. Schnopper et al. (1982) and Rocchia et al. (1984) used Si(Li) detectors to observe the SXRB on several sounding rocket flights. These data were limited by the energy resolution of the FET, typically about 150 - 160 eV (FWHM). This is still insufficient to resolve any spectral lines. Furthermore, a low energy cutoff of about 0.4 keV must be applied to these data because of amplifier noise, which prevents observation of the million degree component of the Local Bubble. Thus, only crude temperature estimates based on fits of collisional equilibrium spectral models to data with fairly poor energy resolution have been available to date.
With the tremendous advances in CCD capabilities in recent years, we can now significantly improve on these data. Typical modern scientific CCDs have read noise floors of 3--4 electrons (rms), corresponding to a resolution of about 30 eV -- 40 eV (FWHM) at low energies. This resolution is sufficient to allow separation of the strong line groups of oxygen, neon, and iron expected in spectra of the SXRB.
CUBIC takes advantage of the latest technological developments
in this area.
The detectors are state-of-the art CCDs built by EEV, Ltd. of Chelmsford,
England. EEV is in the forefront of scientific CCD development,
and is one of the few commercial suppliers building CCDs designed
specifically for soft X-ray imaging spectroscopy. EEV has provided
the flight detector for the JET-X program and is developing the
flight detectors for the EPIC instrument on XMM. The CUBIC detectors
are direct descendents of those program (in fact, they
employ the current EPIC architecture with JET-X gate structures).
This device, EEV's model CCD-12-30-4-202,
uses a thin gate structure to obtain reasonable quantum efficiency
at energies well below the cutoff of detectors such as the Lincoln
Labs CCID7s used on the ASCA SIS instrument.
CUBIC employs two 768 x 1024 CCDs with mechanical
collimation.
The CCDs are operated in the frame-store mode, resulting in active
areas of 768 x 418 pixels (94 rows abutting the serial register are
covered up by our 
Fe calibration sources).
With 
pixels, each CCD has a total area of 
.
These detectors will be able to acquire high-quality spectra of the SXRB
with a moderate solid angle of 31.5 square degrees below 1 keV and
128 square degrees above 3 keV.
The resulting effective area is shown in Figure 4.
Figure 4: The CUBIC effective area-solid angle product.
