The CUBIC experiment is designed to study the spectrum of the diffuse X-ray background with unprecedented sensitivity and spectral resolution between 0.2 and 10.0 keV for selected regions of the sky. These include the Galactic center, the North Polar Spur, the Gemini-Monoceros and Eridanus enhancements, the Lupus Loop, and a variety of high and low galactic latitude directions sampling the diffuse soft X-ray background. We will also be able to observe Sco X-1, the Crab nebula, and some of the other bright X-ray sources, but our main emphasis will be on the diffuse X-ray background and nearby supernova remnants with large angular scales that cannot be observed with this kind of spectral resolution by any other instrument currently scheduled for flight.
CUBIC is based on a pair of CCD (Charge Coupled Device) detectors,
and draws on our experience
with developing CCD X-ray detectors and cameras for use in the AXAF CCD Imaging
Spectrometer ( ACIS ) experiment
and several sounding rocket CCD cameras.
The X-ray CCDs used in CUBIC were made by EEV and consist of
a 
element array of 27 
m square photosensitive
pixels.
X-rays are absorbed in the photosensitive depletion region of a CCD
pixel, generating a cloud of electrons whose number is proportional to
the energy of the incident X-ray.
These electrons are stored in a MOS capacitor within the pixel until
the device is read out, at which time they are transferred to the
output amplifier.
The low read noise
(
rms)
and efficient charge collection and charge transfer
of this chip provide very sensitive X-ray detection with moderate
nondispersive spectral resolution as low as 40 eV over the
range 0.2-10 keV.
The low noise performance of this device has been verified by measuring the
width of X-ray calibration lines ranging from 0.3 keV to 5.9 keV. The
results, shown in Figures 1 and 2, are consistent with
resolution determined by counting statistics for a device with Fano factor
0.116.
This allows the CCD camera to function as an imaging spectrometer, with moderate-resolution nondispersive spectroscopy capable of separating the major line groups produced by hot plasmas in the energy range 100-1500 eV.
In the following sections we discuss the scientific objectives of this experiment, and show how CUBIC is uniquely qualified to carry these out.
