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Focusing Optics for Hard X-rays
The impact of focusing optics on hard X-ray astronomy will be enormous. We outline many science goals that are within the grasp of the balloon instrument, but cannot guess what unexpected new discoveries this technology will enable. As a possible analogy, we point to the unexpected discoveries of the first X-ray focusing telescope, the Einstein observatory. This technology will make truly revolutionary discoveries about black holes, nucleosynthesis and astrophysical particle acceleration. InFOCmS will be the first look into this new and exciting world. In a single 12-hour balloon observation, InFOCmS will be ~10 times more sensitive than HEXTE (2 x 105 s) with 30 times better spatial resolution and will have the same sensitivity as IBIS on INTEGRAL (106 s) with 12 times better spatial resolution.
Fig. 1. InFOCmS sensitivity plot.
Building a full scale instrument will develop the critical process engineering techniques necessary to for a spaceflight telescope. This technology has reached the stage were we must demonstrate the mass production techniques to make thousands of foils. By flying this technology in a space-like environment, we will be identifying the key technology hurdles and testing the limits of our understanding to pave the way for the ASTRO-G and Constellation-X missions.
State of the Art
Accomplishments of the InFOCmS Team:
First end to end multilayer mirror test.
Designed and built sparse readout focal plane ASIC.
Fabricated and tested 2.5 X 2.5 cm CZT pixel focal plane.
Interaction depth analysis tools developed for background rejection.
All major balloon subsystems designed.
All new critical subsystems built and tested.
We are ready to build the flight system.
The replicated foil mirrors from the GSFC/Nagoya collaboration are the most advanced technology for high throughput, very low mass grazing incidence X-ray optics. They have been flown on BBXRT, ASCA and will fly on ASTRO E. These are the lightest/cm2 X-ray mirrors in existence with an Al foil substrate 10 times lighter than electroformed Ni shells. We have already demonstrated that this technology can be successfully coupled with mutilayering to extend the energy range of our mirror technology to > 40 keV (Yamashita et al. 1998). The use of high contrast Pt/C layer pairs allows mirrors to be fabricated with ~10 times fewer layers and 2 to 3 times larger reflection angles than other pairs such as W/Si. This is the only technology currently at the state of maturity required to make a full scale mirror.
The pixel CZT and CdTe hybrids currently under test at GSFC are the largest-area position-sensitive single detector systems that have been fabricated with this material. Our group has developed procedures to screen CZT material on the wafer scale which allowed us to procure 6 pieces of very high uniformity material. The CdTe material had contacts applied at GSFC and proprietary procedures not available to the manufacturer were used to increase the interpixel resistance. The 2.5 X 2.5 cm active area of these detectors completely covers the FWHM FOV of the mirror. They have both the uniformity and the spatial and energy resolution necessary to act as a highly efficient focal plane for the X-ray mirror.
We have developed data analysis procedures to determine the depth of interaction that can reduce the volume dependent component of background by > 10. This is very important for CZT which is not mechanically stable for 2.5 X 2.5 cm pieces < 2 mm thick. We have also fabricated and hybridized 0.5-mm thick detectors of the more durable CdTe material. Thin detectors not only reduce the background, but they reduce the charge spreading and can significantly improve the average energy resolution in a pixel detector. These techniques are also optimized to simultaneously extract the best estimate of the position and energy for events where the signal is shared between pixels. We were the first group to measure the CZT background at balloon altitudes with a full active shield and demonstrated that this thick active shielding can reduce background at our energies by a factor of > 50. The extremely low background (~1.2 count/keV in 12 hours for 100 cm2) is crucial to achieve the full sensitivity of this instrument concept for very long observations.
It is currently possible to fly balloon flights of 10 to 20 days, but NASA is committed to developing a superpressure balloon for flights of ~100 days. We are planning 1 to 2 day flights during the technology development stage, but we are designing our instrument and gondola to be easily upgraded to the longer balloon capability. This will allow significantly expanded science goals.
Build On Previous Work
Our proposal has significant new technology. The X-ray group is committed to developing advanced foil replication and improved mirror housings and alignment procedures that will enhance the imaging resolution of our mirrors to 0.5-arcmin FWHM. We expect enhanced resolution to be available for the high energy mirrors. We will also be pursuing approaches to substantially lower the cost of the mirror. The housing fabrication costs are dominated by the electron discharge machining of the alignment bars that position the individual foils. There are 26 groves machined to a tolerance of 25 Ám for each of 2040 foils in the first completed housing. If effective means of replicating the alignment bars can be developed the cost of the mirror housings could be greatly reduced.
We are planning a second run of our new sparse readout ASIC with improved shielding that will have significantly improved gain and energy resolution. We will be developing an alternative technique of interaction depth determination using the top side contact pulse height. This technique can be used separately or in conjunction with our current technique to improve the depth determination. We will be examining new sources of material, including more work with CdTe and horizontal Bridgman CZT material from IMARAD in Israel. We plan to continue our work on developing new detector contacts and processing procedures.
Our group has a long and successful experience in ballooning. Our previous instrument was a pointed high resolution gamma-ray spectrometer (GRIS) that successfully flew 9 times in 9 years and produced 3 Nature Letters. The gondola design is compatible with current balloon technology and provides the pointing stability and accuracy required (< 0.1 arcmin). It accommodates the 8-m focal length telescope without deployables.