PROTOTYPE OF A DETECTOR FOR TIME RESOLVED PROTEIN CRYSTALLOGRAPHY

Nguyen-huu Xuong ¹ Philip Datte ¹, Jacques Millaud ² and Eric Beuville ²

1 Department of Physics, Biology, Chemistry and Biochemistry, University of California at San Diego, CA 92093-0359 - USA, nxuong@ucsd.edu
2 Lawrence Berkeley National Laboratory, Engineering Division,Berkeley CA 94720 - USA

Keywords: Pixel, Detector, CCD, column architecture

One exciting application of the Laue diffraction method is the possibility to study transient phenomena in crystals using time-resolved experiments. In our laboratory we are designing a 2D photon counting digital pixel array detector (DPAD) that can take up to 8 successive pictures with a very short switching time (a few microseconds) ⁽¹⁾ The optimum exposure time for each picture would be about 10 ms, therefor this detector will allow us to follow the changes in protein structures at a period of 80 ms or longer by taking 8 snap shots of its diffraction. This feature does not exist for CCD based or Imaging plate detectors.

The room temperature detector is a high resistivity N-type Si with a pixel pitch of (150x150) microns, and a thickness of 300 microns, and is bump bonded to an application specific integrated circuit (ASIC). The detector event driven readout is based on the column architecture and allows an independent pixel hit rate above 1 million photons/sec/pixel over the entire detector. The device provides energy discrimination and sparse data readout which yields minimal dead-time. This type of architecture allows a continuous (frame-less) data acquisition. For the targeted detector size of (1000x1000) pixels, average hit rates greater than 8 billion photons/sec for the complete detector appears achievable. The detector has almost infinite photon counting dynamic range and exhibits superior spatial resolution when compared to present crystallographic phosphor imaging plates or phosphor coupled CCD detectors.

This detector can also be useful in a standard monochromatic protein crystallography experiment because the detector has a very small dead time to read out an address. Therefor in a high X-ray flux beam it can reduce the data collection time by a factor of five ore more when compared with a CCD based detector.

An 8x8 pixel array X-ray detector prototype (<15 keV) has been built and tested. To characterize the analog portion of the readout and the digital characteristics of the detector, the pixel electronics contains only the analog portion of the circuit and is independent of the surrounding cells. The conversion of a photon hit into a pixel address is generated by conventional external electronics. The measured results are very encouraging. The analog electronics demonstrate the capability of processing charge pulses at a rate of 1x106 photon/s/pixel, with an energy resolution of 480 eV (FWHM at 5.9 keV) at room temperature. The detector displays uniform digital behavior and has a very low point-spread function. The full-width at 1/100 maximum is less than 1 pixel width (150 mm), which is less than ½ that of a CCD and 1/7 that of an imaging plate ⁽²⁾.

A 16x16 pixel array with most of the readout electronics included in the ASIC has been designed and built. The results of the testing will presented at this conference.

Work funded from the NIH-RR-10748 and from the LucilleP. Markey foundation.

1. E.Beuville et. al., IEEE, Transactions of Nuclear Science, 43 (3) 1996 pp. 1243 - 1247.
2. P.Datte et. al. , Nuclear Instruments and Methods in Physics Research, A391 (1997) 471 - 480.