The Cryogenic Electronics Group at San Francisco State University (SFSU/CEG) proposes to develop a prototype X-ray absorption spectrometer for use at synchrotron facilities that will incorporate a high-efficiency, high-rate, high-resolution superconducting tunnel junction (STJ) X-ray detector cooled by a user-friendly, liquid-cryogen free, cost-effective cryostat. The research will be done in collaboration with Atlas Scientific Corporation, an R&D company with considerable experience building custom cryostats; and with the Cryogenic Detector Group at Lawrence Livermore National Laboratory (LLNL), which is the foremost user of STJ detectors in a variety of biological applications.

Synchrotron-based X-ray absorption spectroscopy (XAS) samples the electronic structure of specific elements with natural-linewidth-limited resolution by scanning the energy of a monochromatic synchrotron beam across an absorption edge of interest. Fine structure in the absorption features provides information about the chemical state of the element, including its oxidation and spin state, the bond symmetry, and ligand field strength. For dilute samples, the background can be greatly reduced, and the sensitivity therefore greatly enhanced, if the associated fluorescence is used as a measure of the absorption, provided an X-ray detector is used that can separate the weak fluorescence line of interest from the X-ray background.

Our scientific motivation is to produce an instrument that will be an exceptionally effective probe of the chemistry of dilute metals. There are three principal applications of interest. First of all, trace concentrations of transition elements play a critical role in the catalytic mechanisms of many enzymes, but the oxidation state of the metal ion is as yet unresolved in many important metalloprotein molecules. Examples include the nickel site in the ACDS protein responsible for incorporating carbon into cell material, the manganese-cluster in the oxygen-evolving complex of photosystem II, and the iron-molybdenum-cofactor of the nitrogenase protein responsible for nitrogen fixation. The second major area of interest is measurement of relative concentrations of trivalent and tetravalent cerium ions in scintillator crystals so that crystal growth parameters can be adjusted to favor the trivalent (luminescent) state. Improvement of efficiency of cerium-doped scintillators is important for PET and SPECT applications. The third major area of interest is to identify the oxidation state of metallic environmental contaminants, such as chromium, because the oxidation state of the ion determines its solubility and therefore its bioavailability and toxicity.

Our improved detector is based upon development of tantalum-based STJ sensors. Use of tantalum rather than niobium in an STJ significantly improves both the absorption efficiency and the energy resolution. The SFSU/CEG, which has a microfabrication facility dedicated to the production of superconducting thin film devices, will design and build single-pixel tantalum-based STJs. DC testing of the devices will be done using cryogenic facilities at Atlas Scientific. Pulse testing will be done using the custom STJ readout electronics, the X-ray detector test cryostats, and the invaluable expertise of the Cryogenic Detector Group at LLNL.

Our improved cryostat is based upon development of a 300 mK ³He sorption refrigerator coupled to a commercial pulse-tube cooler. The system will be a compact, nonmagnetic, low-vibration, low-maintenance cryostat that uses no liquid cryogens and that will be easy to operate. The SFSU/CEG will work with Atlas Scientific to design a custom sorption cooler with a sample stage that accepts an STJ chip and a cold finger extension that will interface with test ports at synchrotron radiation facilities.