In the autumn of 1998, the Discipline Scientists of the NASA Office of Space Science requested that the Astrophysics Working Group (AWG) look into the issue of detector availability from X-ray to infrared wavelengths. We have solicited comments from individuals involved in instrument development in order to make a preliminary assessment of the problem on the basis of ancedotal evidence. We conclude that the issue of detector availability is a chronic problem that needs to be addressed by NASA. Availability of detectors is not yet a critical problem, but this potential exists, and it will certainly occur without action by the Office of Space Science.
X-Ray Detectors
Most X-ray astronomy missions projected for the next decade rely on either CCD or cryogenic (microcalorimeter or STJ) technology. Several groups are actively engaged in development of cryogenic detectors and a substantial amount of funding is currently being invested in the technology not requiring substantial funding for its maintenance. Concerns with this inaccurate view are addressed here.
X-ray CCDs are currently the state-of-the-art detectors of choice for many applications. They have produced tremendous science returns from ASCA and are poised for launches on Chandra and XMM. At least half a dozen missions proposed in the last MIDEX round rely on X-ray CCD detectors for either spectroscopy or imaging spectroscopy, and CCDs are projected for use as grating readout devices on Constellation-X in the next decade. There is clearly a continuing role for these detectors, which provide a unique combination of high quantum efficiency, large format, and moderate spectral resolution.
The photon-counting X-ray CCDs needed for all these missions provide the most difficult challenge for detector manufacturers of any CCD type. They place very stringent requirements on several areas of device performance, including read noise (must be less than 4 electrons rms, and current state-of-the-art detectors achieve less than 2 electrons) and charge transfer efficiency (less than 0.001% of charge lost on each pixel transfer). Operation at energies up to 10-15 keV requires the use of either high resistivity bulk silicon (Chandra/ACIS) or high resistivity thick epitaxial silicon wafers (XMM/EPIC MOS), neither of which has commercial applications. Similarly, operations at low energies (below 0.5 keV) requires the use of special gate structures (thin gates or open gates) or backside illumination with special passivation. Neither of these technologies have commercial applications. X-ray astronomy must therefore rely on manufacturers who are willing to pursue specially designed CCDs with applications solely for X-ray purposes.
Over the years, a number of CCD manufacturers have built detectors that were evaluated for use in X-ray astronomy. Because of the extreme technical requirements and device complexity, these devices can only be manufactured at highly specialized fabrication plants with enormous capital investments and very high production quality standards. Over and over again, these sources have dried up as the companies decided to consolidate operations or to cut unproductive research projects. For example, the Chandra/ACIS CCDs were originally developed at Texas Instruments (as an offshoot of their HST/WFPC CCD development) until they closed their research development lab, shifted CCD development to Japan, and lost the technological capability of producing state-of-the-art X-ray detectors.
Another example of this type of problem is provided by the Penn State University experience with Loral. Ford Aerospace was building state-of-the-art optical CCDs in the mid-1980s. Ford Aerospace was purchased by Loral and continued to develop CCDs for use on missions including CRAF/Cassini and WF/PC 2. The Penn State group began working with this foundry on the design of a new CCD optimized for use as a low-energy X-ray spectrometer on its CUBIC instrument. Unfortunately, Loral decided to close down their research operations (where this work was being done) and consolidate CCD production to their fabrication plant. Penn State's devices were manufactured at the research plant in the midst of this consolidation. Because of decisions made at corporate headquarters, Loral research labs was unable to obtain good quality silicon wafers, and because of morale problems caused by the imminent closure of the plant, processing mistakes were made. Out of three lots producing hundreds of CCDs, only one CCD was good enough for use as an X-ray detector. This single device was the best CCD ever evaluated by Penn State, but with only one sample, they were unable to use it for a mission. Shortly after the Penn State CCD lots were finished, the Loral research lab was closed, and the Loral production lab has not been able to repeat the performance.
Only two CCD manufacturers in the world currently produce devices built on high resistivity silicon that can be used as photon-counting X-ray detectors: Lincoln Laboratory and EEV. (A third group was set up by MPE in Germany to build pn-CCD detectors for XMM/EPIC and ABRIXAS, but it is rumored that this plant has been or is about to be closed down.) Lincoln Lab CCDs are excellent, state-of-the-art devices. Unfortunately, to date only MIT has had access to these detectors. In spite of an enormous financial investment by the Chandra project, Lincoln Lab was not very successful in producing devices that perform at low energies — only two backside illuminated CCDs built for Chandra were of flight quality, and these have significant and poorly understood problems in terms of charge transfer efficiency and energy resolution. EEV is the only manufacturer producing X-ray CCDs on a commercial basis and is the only viable competitor to Lincoln Laboratory in this area. They have an excellent track record in development of new devices for X-ray use (financed by XMM), including several technologies that extend low energy quantum efficiency (using open gates or backside illumination).
While the situation regarding X-ray CCDs may not be desperate today, since there are two (or three) sources of these devices at the present time, past experience suggests that this situation could change at any time. Lincoln Labs is not subject to corporate takeovers, but it is difficult to predict their long-term commitment to continued X-ray CCD development and fabrication. EEV has provided assurances on several occasions that they have a long-term commitment to CCD fabrication, and in particular, to X-ray CCD development, but in the absence of program funding to keep both their interest and their expertise alive, how long will EEV's commitment survive? As the end of the XMM program approaches, the company is not pursuing any other major photon counting X-ray CCD program. It would be prudent to continue funding X-ray CCD development at a level sufficient to retain EEV's interest in this technology, lest we find ourselves at the mercy of a sole vendor for Constellation-X and other future missions.
Infrared Detectors
The recent explosive interest in high spatial resolution and high dynamic range imaging and interferometry puts new requirements on IR detectors. Scientific goals such as extra-solar planet detection, imaging of stellar disks and dust rings, detection of zodiacal light around other stars require interferometers and chronographs that operate in the near IR with virtually noiseless detectors. Existing or new photon counting detectors are ideal for these applications if their sensitivity can be extended into the near IR with high quantum efficiency. GaAs photocathodes now have greater than 40% quantum efficiency to 0.9 microns and some work has been done on developing such materials as InGaAs, which can extend the sensitive range to nearly 2 microns. Support for IR photocathode development and its integration into existing or new photon-counting detectors could be crucial for such NASA missions as TPF and NGST. Any discussion of IR detectors is complicated by the fact that coverage of the spectrum from the near-IR to 200 microns requires five different types of detectors.
Detectors used for IR astronomy have typically resulted from useful collaborations between small divisions of aerospace/defense companies and university researchers. The former wanted these detectors for high-background applications; the latter were tasked with making them suitable for low-background applications, astronomy in particular. The large companies fabricate the detectors and the NASA-supported researchers provide the electronics and the critical testing. With the end of the Cold War, consolidation of defense contractors began apace, and many of the corporations began to realize that their detector fabrication operations were not central to their "mission."
The result has been a rapid decrease in vendors capable of producing viable detectors, less motivation for considering the needs of the astronomical community, and a pervasive concern among astronomy users that the situation will only get worse in the future. This concern stems partly from the recent trend for large companies to take over detector houses (Boeing absorbed Rockwell, and Raytheon absorbed Santa Barbara Research Corporation [SBRC]), and partly from the need for the very best multiplexers for future projects. The best foundries (producers of the multiplexers/electronic readouts, also known as "muxes") are either closed or being sold. Even the very good SBRC has sometimes resisted progress to the most sensitive devices. The large aerospace companies do not have small, unique customers such as space astronomy in mind when they consider about productivity and profit.
SBRC is a highly competent vendor, but strategic decision-making is now done by their parent company Raytheon. This has led to numerous difficulties in procuring suitable detectors for astronomical applications. Some examples are the following:
These examples, which can be regarded as typical experiences, underscore two points: (1) for sensitive devices, research astronomers, the end users of these devices, are more attentive to the subtle effects which need to be overcome for space missions, and (2) technology transfer is in practice difficult. From our point of view, the specific problem with SBRC is not a question of competence, but that Raytheon now makes the strategic decisions, which are likely to be disadvantageous to astronomy. SBRC has neither the facilities nor the appropriate personnel to do the testing required when pushing the envelope, which SIRTF/IRAC does, and which NGST will do to an even greater degree.
Low/Medium-Energy Gamma-Ray Detectors
In the energy band above ~100 keV, where focussing optics are no longer practical, large areas (>0.5 m2) of pixelated detectors are required to achieve interesting sensitivities. Although coded aperture instruments may have useful sensitivity near the lower end of this energy range, it is generally conceded that the Compton-scatter telescope technique provides the best combination of wide field of view, sensitivity and angular/energy resolution. However, both techniques measure their performance in terms of stopping power of the detector material, pixelization or position resolution, and energy resolution. Detector options include scintillators, germanium, silicon, CdZnTe and liquid Ar/Xe. New developments in solid state detectors have potential for enabling much more capable gamma-ray instruments in the future. The realization of this potential will not occur without the support of the scientific communities for there are few commercial applications for these costly detectors.
CdZnTe is the newest and one of the most exciting new detector materials applicable to gamma ray instruments. It, along with Si, has the possibility of operation at or near room temperature, thus making implementation of a large detector system vastly simpler. It is configured as either a strip detector with parallel electrodes on each face, as a pixel detector with pad electrodes on one face, or as a single channel device up to ~1 cm on a side. The main areas of development for astrophysical applications are: 1) better quality material for larger volumes; 2) improved electrode design for position readout and hole-signal rejection; 3) greater electrode reliability for large arrays; 4) hole-trapping correction to improve energy resolution; 5) understanding and minimizing the activation background; and 6) evaluation of radiation damage effects and annealing to restore performance. CdZnTe detector development has been largely driven by the medical imaging community but there is still no reliable source for large quantities of high quality CdZnTe. Thick silicon strip detectors provide both good spectral and spatial resolution for a scattering detector in a Compton telescope but have limited stopping power for other applications.
Germanium detectors have been available and used in space missions for years to provide the best spectral resolution possible. However, these detectors were in the form of large co-axial detectors without positioning resolution appropriate for good imaging. Recent developments in contact technology have resulted in planar germanium strip detectors similar to the similar to the silicon strip detectors used in high-energy physics experiments. The thick (1 - 2 cm) planar germanium detectors with strips on both cathode and anode provide the potential for both high-resolution spectroscopy and good spatial resolution critical to Compton telescope designs.
There is currently no US source for these detectors. Due to the cost of these detectors and the requirement for cryogenic temperatures, there is no commercial market to drive the development. The technology has been developed in small demonstration programs at a few US research institutions and one French company. The nuclear physics community of the DOE has recently expressed interest in these germanium strip detectors for the Greta detector which will replace Gammasphere. Without the investment by the scientific communities, these technologies, which are critical to the next generation of gamma ray space instruments will not be available.
Ultraviolet/Optical Detectors
While the situation is in some ways better than in other wavelength regimes, there are still great difficulties in procuring suitable detectors for UV/optical applications.
Timely delivery of astronomical-quality CCD imaging devices remains a problem. In particular, the backside illumination required for optimal sensitivity generally lies far outside the routine processing capabilities of the mass manufacturers of commercial devices, leaving us with a small number (sometime even zero) of vendors who at any given time can realistically deliver even a few such imaging devices. This situation is especially acute for proposed survey instruments that need large focal plane mosaics of 10-20 or more large-format CCDs; most vendors simply cannot promise the delivery of such large numbers in less than two years or more. This puts any accelerated mission (especially Explorer-class missions, which are intended to launch within 40 months of selection) at grave risk.
There are many potential missions that require solar-blind photon-counting detectors. Delay-line detectors, the current detectors of choice, have limits on resolution, pixel size, count rate, dynamic range, and stability of its point spread function which either do not meet or barely meet the scientific requirements of these missions. In the recent past, the support environment has not been conducive to developing new detectors due mostly to the large investment in one or two existing technologies. Development of new UV detector technologies that have extended capabilities should be encouraged in tandem with support for existing technologies. We note that electron-bombarded CCDs are currently showing some promise as alternatives to delay-line detectors, but they are still under development.
Recommendations
The Astrophysics Working Group is deeply concerned about future availability of flight-quality astronomical detectors at all wavebands. The situation is not yet acute, though the problem is chronic and the situation is increasingly unstable. It may be that NASA will have to engage in long-term strategies that will assure availability when detectors are actually needed. For this reason, we believe that this is primarily a strategic issue that ought to be considered by the Space Science Advisory Committee (SScAC) rather than the AWG; the AWG will be happy to assist the SScAC if we can be of assistance. A temporary (band-aid) partial solution might for NASA to carry out a fairly complete inventory of already existing NASA detector assets from previous and current missions. For instance, projects like STIS (completed) or ACS (under development) should generate a fair number of spare imaging devices of performance comparable to the flight detectors. In addition, there should be a large number of rejected devices that nevertheless might well serve the needs of a less-demanding mission. These valuable assets should be preserved and a permanent inventory system established to document their characteristics for future missions, particularly Explorer-class missions. NASA might also consider small, low-cost follow-on procurements where possible, once a vendor is geared up to produce as space-qualified detectors suitable for astronomical applications. This might also be carried out in partnership with other agencies with similar if not identical concerns, such the Department of Defense, the National Reconnaissance Office, and the European Space Agency.
In any case, NASA needs to guard against allowing an existing hardware capability to die; trying to duplicate lost capabilities years later is not only a false economy, it sometimes proves to be virtually impossible to accomplish in a limited time and is potentially fatal to any mission requiring delivered state-of-the-art detectors on time scales shorter than several years.
Based on contributions from Michael D. Bicay (Caltech), David N. Burrows (Penn State Univ.), John Geary (CfA), F. Rick Harnden, Jr. (CfA), W. Neil Johnson (NRL), and Judy Pipher (Univ. of Rochester). Compiled by Bradley M. Peterson (Ohio State Univ.).