Development of ZnTe Powder Phosphor for Protein Crystallographic X-ray Detectors

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Department of Health and Human Services
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Phase II
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DESCRIPTION (provided by applicant): We propose to continue development of a new, high-performance X-ray phosphor, based on zinc telluride (ZnTe), doped with oxygen. Currently, our phosphor is at least 3-fold brighter than present technology and further im provements are expected in Phase II. This increase in brightness will solve a major problem in the design of these detector systems. Simply stated: ZnTe:O will have a major impact on protein crystallography. Our project will be most valuable to this major field, which is extremely important to NIH. Every synchrotron beamline, assigned to structural molecular biology (there are ~20 of these), has a detector, for which NIH has already paid ~ 1million. Our phosphor will improve the performance of each of these substantially. PhosphorTech Corporation (PTC) is highly experienced in this area. In 2002 we developed ZnSe:Cu,Ce, a zinc selenide (ZnSe)-based phosphor, in collaboration with Bruker-AXS Inc., for use as an X-ray screen phosphor in their chemical crystall ography detector system Apex. ZnSe:Cu,Ce has proven to be an outstanding phosphor for this application, being about 3x brighter than Gd2O2S:Tb the standard phosphor for CCD crystallographic detectors. ZnSe:Cu,Ce exhibits a time response (afterglow) con siderably faster than Gd2O2S:Tb, which is another positive feature. Bruker AXS now dominates the chemical crystallography market in the USA and Europe, with over 85% market share. This is in part due to the remarkable success of our phosphor, and the stron g acceptance of this phosphor by Bruker customers. PTC supplies this phosphor to Bruker through commercial sales and thus the contribution of PTC to chemical crystallography has been profound. We propose in this project to make an equally profound contribu tion to another scientific discipline structural molecular biology. However, detectors used for protein crystallography cannot use ZnSe:Cu,Ce phosphor because it contains selenium (Se). Protein crystallographers today solve their structures by multiple en ergy anomalous dispersion (MAD) phasing methods1,2. About 80% of these studies make use of Se, chemically replacing sulfur in the amino acid methionine, as the atom providing this anomalous scattering signal. Since Se in the ZnSe:Cu,Ce phosphor will manif est the same discontinuous change in its X-ray absorption spectrum as the Se in the protein crystal sample, at precisely the same X-ray energy (12,658eV), it is inappropriate for the detector used in these measurements to contain Se. ZnTe has very similar chemical and physical properties as ZnSe. The two zinc-blend structures have virtually the same lattice constant of 6.1037E, and similar band gaps (2.3eV for ZnTe; 2.7eV for ZnSe). But the absorption edges of tellurium (4,341eV, 31,814eV) are far away from the selenium edge. Thus, in addition to higher efficiency, there is a strong motivation to produce ZnTe phosphor powders that can be used to make large screens for imaging the diffraction of protein crystals. We have completely met our Phase I goals. We f abricated a ZnTe-based phosphor, ZnTe:O, that is as bright (and could potentially be brighter) than our ZnSe phosphor. We have also established fabrication procedures that seal the ZnTe:O phosphor from the outside environment, thus preventing its oxidative degradation that was the original problem for this project. The light emission of our ALD- coated ZnTe:O phosphor is now time-independent and compatible with commercial screen processes. We therefore propose, during Phase II of this project, to develop th e means to synthesize larger quantities of high quality ZnTe:O phosphor powder, and optimize the large screens necessary for protein crystallography CCD detectors. ZnTe:O phosphor converting films will be most useful for protein crystallography detectors u sed at synchrotron beamlines, enhancing their value to NIH and to the molecular biology community as a whole. In addition, the improved performance of this

* information listed above is at the time of submission.

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