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Future Work

It is a sobering thought that many of the developments described here are tailored for the observatories which are already planned, but which will probably not be launched until the end of the century. One alternative CCD technology receiving some attention, and which may fly on XMM is the pn CCD [Strüder et al]. This device offers potential advantages such as improved tolerance to ionising radiation damage, and enhanced time response. The device is being developed in a research environment which facilitates the design of devices to be optimized for particular applications, in contrast with the above programs which seek to adapt existing CCD designs available from semiconductor manufacturers.

For XMM, for example, a number of advantages accrue for the pn-CCD design:

• The device has been designed with large pixel sizes to better match the telescope angular response, and each row of pixels is read out to an individual amplifier signal chain. Each pixel requires far fewer transfers to be read out to the amplifier, so that data access is far faster
• The larger pixels are less subject to inter-pixel splitting
• The transfer channel design can be tailored to minimize interaction with traps, and the faster readout rate may improve the charge transfer performance in the presence of traps

One may speculate that future developments may include the fabrication of devices with very small pixels to allow the possibility of observing polarization effects: At energies higher than ~20 keV, the anisotropy of charge packets caused by the polarization dependent preferential ejection direction of the primary photoelectron may be discernable (albeit with low efficiency) with pixels as small as 5 microns.

The development of intelligent pixel devices (ie. devices employing random access and on-chip processing) by the particle physics community may benefit X-ray astronomy. Silicon detector technology may allow more integration of signal detection and processing circuits on chip, to allow multiple addressing and reading of individual pixels, breaking the dependence on the store and serial readout nature of CCDs, and allowing higher point source count rates to be accommodated.

In a similar vein, multi-layer devices and different semi-conductor materials (eg. InSb) might be used to produce novel detectors. In this context it is clear that thin film technologies which allow the production of quantum well devices, Superconducting Tunnel Junctions and bolometers are likely to produce the next generation of X-ray detectors. This can be viewed as a further evolution along the path from hand-crafted gas counters produced on the lab bench, through the adaptation of existing CCD technology which many groups have attempted in collaboration with manufacturers, to a custom built device which is feasible only with the resources of large national laboratories. A possible change to this evolutionary scheme may be wrought in the CCD field by the recent availability of PC-based CAD software to design CCD photo-lith masks which may then be executed by a silicon foundry.