Technical Approach

The design of sensitive hard X-ray instruments has traditionally been very challenging. Faint sources combined with high background from cosmic-ray secondaries and cosmic diffuse emission have resulted in very heavy instruments with low signal to background ratios and serious systematic problems. Poor energy resolution has limited the value of spectroscopy, creating its own systematic errors. We are now at the fortunate confluence of three new technologies (multicoated foil mirrors, cadmium zinc telluride detectors, and long duration ballooning) that allow us to propose an instrument with focusing optics to produce direct arcmin images at ~100 mCrab sensitivities and superior energy resolution (~ few keV). This configuration achieves extremely low background (~2.5 counts/keV in a 12 hours) that for most known hard X-ray sources is negligible. Our collaboration combines the efforts of several groups, each with proven expertise in one of these critical technologies. We have carefully assembled the right team to deliver an instrument that can demonstrate the potential of these groundbreaking new technologies.

By applying thin alternating layers of different index materials to the surface of a mirror, one can create an artificial crystal that will enormously enhance the reflectivity at high energies. This technology has been used to make normal incidence UV mirrors (Smith 1990). When applied to grazing incidence X-ray mirrors, it can provide focusing optics for energies up to ~100 keV. Focusing optics can produce the same enormous improvements in sensitivities at these energies that they have in the soft X-ray band. Since all the X-rays from an 2-arcmin size spot in the sky are focused onto a 4-mm spot on the detector, the background both from cosmic diffuse and detector volume dependent components are significantly reduced. Besides the obvious advantages of direct imaging for making pictures, it also facilitates the detection of point sources and elimination of systematic errors in many ways. By averaging over adjacent pixels, an accurate determination of the background can be achieved without offset pointings. Arcmin imaging eliminates all source confusion problems at these energies. The foil mirror technology developed by the GSFC and Nagoya groups is the only flight proven way to produce mirrors with the combination high thoughput and excellent smoothness (< 3 Å) required to make a efficient hard X-ray mirror.

New technology cadmium zinc telluride (CZT) detectors are the natural choice for the focal plane elements. They are room temperature, solid state detectors with excellent energy resolution. The addition of Zn to the standard CdTe detector crystal reduces the room temperature leakage current, especially in large area detectors, that limited the usefulness of CdTe in the past. CZT with a pixellated electrode can achieve < 2 keV of noise. Pixellated CZT detectors with custom ASIC electronics have been fabricated by our collaborators at the University of Arizona for medical applications (Barber et al. 1997). With a 380-mm pitch, these position sensitive detectors provide more than adequate quantization for the 4-mm telescope point spread function. The high Z of CZT ensures that the photoelectric cross-section dominates over our entire energy range and that the energy deposit is localized to maintain the fine imaging properties. The high stopping power allows us to use thin detectors (2-mm) to minimize the volume dependent background. Anticoincidence timing is performed on the un-pixellated front-contact signal. The interaction depth analysis can be used to tailor the thickness of the detector at each energy to minimize background for optimum sensitivity as discussed in our first proposal. Since CZT produces high resolution spectra near room temperature, there are no consumables (such as LN2 for cooling Ge detectors) to interfere with long duration balloon or satellite applications.

The very long focal lengths required by this approach can be readily accommodated by a balloon instrument. Only on a balloon gondola can we launch an 8-m focal length telescope as a rigid tube. Constellation-X and ASTRO-G will use deployable tubes to accommodate long focal lengths. Deployable structures seem promising for space flight, but we need an approach to demonstrate and develop imaging technology now. Balloon gondolas have been built that provide the necessary pointing accuracy and stability. We propose to model our pointing on a system developed in Germany for a far infrared telescope (Drapatz et al. 1980). It achieved 5-arcsec pointing stability and accuracy. The balloon program currently supports flights of 10 to 20 days in Antarctica and NASA is committed to developing a new superpressure balloon capable of 100-day flights. Flights of this duration with InFOCmS could support major scientific breakthroughs.