Last June, the Harvard-Smithsonian Center for Astrophysics (CfA) Director, Irwin Shapiro, announced the formation of the Center for X-ray Technology (CXT), with Stephen Murray as its first director. The goal of the CXT is to provide a focus for new technology development in the areas of X-ray optics and detectors. The Center is located at the CfA, but welcomes participation from other institutions. The CfA has a long history of excellence and leadership in X-ray sensors and optics, leading to major hardware roles in the Einstein, ROSAT, and Chandra observatories. A robust basic research group working in detector development and X-ray optics will be a source of ideas and proposals for new flight opportunities. Support of basic research is essential for establishing the core capabilities needed for future X-ray astronomy missions such as light weight, large area, arc second telescopes along with giga-pixel high performance sensors.
Principal areas that the CXT will address include:
(a) Investigate and develop new X-ray sensor technologies
(b) Extend the performance of existing X-ray sensor technologies
(c) Develop novel approaches for the design and fabrication of X-ray optics
(d) Develop techniques for X-ray interferometry (spectral and spatial)
(e) Develop multi-layer technology for X-ray optics
Creating a new center within the CfA requires building upon already existing capabilities and resources with a plan to seek out new resources as the CXT becomes established. Initially the CXT draws upon the expertise and facilities within the CfA, mainly in HEA, including the HRC laboratory at Porter Exchange, and the independent research time of CfA scientists. The CXT calls upon scientists with interests in X-ray technology to work on a few initial projects using internal support. For promising projects, CXT scientists will be encouraged to pursue outside funding (e.g., NASA, SR&T) for continued development. In parallel, the CXT will seek substantial increases in the Federal allocation to support scientists, engineers and students, as well as an increase research funds for equipment and supplies. Efforts will also be made to obtain private philanthropic support for the CXT.
For FY 2004, the Center has obtained funding through Internal Research and Development (IR&D) and Research Equipment Funds (REF) to initiate two studies. The first project is to examine the potential of active pixel type devices for X-ray sensing. This type of detector will be needed to meet the demands of future X-ray missions where data rates will be too high for traditional CCD's, and the focal plane size will be very large. There are at least two approaches that are available for study. One is the CMOS type of imaging array where each pixel contains the electrical circuits that detect charge deposited in the silicon and convert that signal into a voltage that can be read out through a multiplexed output amplifier. CMOS devices typically have higher readout noise than the best X-ray CCDs. Whether this is an intrinsic property of the device, or simply because lower noise has not been pursued, needs to be investigated. The higher readout noise can in part be compensated through multiple read outs of the same pixel (the charge collection is non-destructive). Another type of active pixel device is a Si PIN diode that is connected to a multiplexer readout (i.e., a CMOS device). The advantage of the hybrid Si-PIN is that the sensitive area is 100% of the array. If the PIN diode is thick then the energy range of the detector is extended. Diodes with 150 microns already exist which would work up to about 50 keV with good efficiency. Readout noise is the same as for the direct CMOS detector. Using FY 2004 REF we plan to obtain at least one test device (probably the Si-PIN type) for evaluation. This will not be optimized for X-ray use, but will provide a baseline for future development. We will make use of an existing X-ray CCD test facility in the HRC laboratory for working with this device.
The second study involves X-ray optics. A NASA Vision Mission (Gen-X) as well as the revised X-ray Astronomy Program Working Group report (2000) highlight the need for very large (50-150 square meter) and 0.1-1.0 arcsecond resolution X-ray telescopes. In order to achieve sub-arcsecond image quality with lightweight mirrors (required to achieve the large collecting area), it is very likely that some sort of active control will be needed. Regardless of the actuator mechanism, it will be necessary to have the capability to monitor active X-ray optics in the laboratory. One way to do this is to use an optical interferometer to measure the small controlled changes in an X-ray optic as forces are applied. Using FY 2004 REF we will obtain such an interferometer (from ZYGO) and set up an X-ray optics test facility (at Porter Exchange). To apply forces to the thin mirror shells of the type used in making X-ray optics, we need to explore various technologies such as direct deposition of piezo-electric materials on X-ray foils and how to control them. Alternatively, applying electric heaters on foils and varying temperatures might also be a means of controlling a mirror's figure. Some initial studies and testing in these areas will help identify new directions for X-ray optics. Along with the effort to actively control mirror figure, we will also need to examine feedback processes that will measure the change in telescope performance and properly command the active control system. The algorithms, sensors, and computer control systems needed to "close the loop" are important research areas for the CXT.
These two initial areas of study are only examples of the type of research that we anticipate for the CXT. As new sources of funds are developed, we hope to reach a steady state that will support 10-15 scientists, engineers, post-docs and students working full time on the technological challenges of advancing the state-of-the-art in X-ray optics and sensors. An initiative in X-ray interferometry is one such example. Reaching the microarcsecond regime would allow direct imaging of the regions close to the event horizon of a super-massive black hole, testing the predictions of general relativity in the strong gravitational field that can only be found in these environments. NASA has already included such a mission in its SEU Roadmap - MAXIM, and identified X-ray interferometry as a key technology.
We envision a several year process of acquiring the resources and people to implement the vision of this center and invite interested parties to contact us with their comments, suggestions and most importantly their offers of help.
Stephen S. Murray