The San Francisco State University
Cryogenic Electronics Group

Research Program

Atlas Scientific Corporation, the Advanced Detector Group at LLNL, and the Cryogenic Electronics Group at San Francisco State University (SFSU/CEG) are working together to develop advanced X-ray spectrometers for use at synchrotron facilities and in scanning electron microprobes. Our systems will incorporate high-efficiency, high-rate, high-resolution superconducting tunnel junction X-ray detectors cooled by user-friendly, liquid-cryogen free, cost-effective cryostats.

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 instruments will be exceptionally effective probes of the chemistry of dilute metals. There are several major applications for this technology.

First of all, trace concentrations of first-row (3d) 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. For example, our incomplete knowledge about the role of the manganese cluster in the photosystem II (PS-II) enzyme limits our understanding of the mechanism of photosynthetic oxygen evolution, which is one of the most important reactions on earth. As another example, the potential of a future hydrogen economy would be greatly increased if we could analyze the mechanisms of biogenic hydrogen production by hydrogenase, a Ni-Fe-containing enzyme.  Yet another example is that unraveling the process of nitrogen fixation by nitrogenase, a Fe-Mo-containing enzyme, could be a key factor in increasing food production sufficiently to meet the needs of the increasing number of people on this planet.

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.

Two other applications of our advanced detectors are especially relevant to the semiconductor device industry. For example, XAS systems based upon our detectors can address the evolution of dopant characteristics upon processing. XAS measurements on nitrogen dopants have been used successfully to explain the observed band gap shifts in GaInNAs upon annealing. STJs have also been essential to understand the difficulties of nitrogen-doping for wide band-gap semiconductor ZnO. Furthermore, when a defect is detected during manufacturing of an integrated circuit, powerful electron microscopes are used to produce an image of the defect and to take an X-ray spectrum to identify what materials make up the defect. In order to probe very small defects it is necessary to use very low electron beam energies. At a beam energy of 1 keV, the low-energy X-rays generated in a defect could be only a few 10’s of eV apart, so that existing X-ray detectors could not distinguish one element from another. Our proposed STJ detectors offer both the required high resolution (30 eV for 2 keV X-rays) and also a high enough count rate (~ 3000 counts per second) to be useful in a scanning electron microprobe system. 

Our improved detector is based upon development of a Ta-Al-Al2O3-Al-Ta superconductor-insulator-superconductor (SIS) superconducting tunnel junction (STJ) sensor. Use of tantalum rather than niobium in the STJ improves both the absorption efficiency and the energy resolution. No tantalum-based STJ detectors are commercially available at this time. 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 existing cryogenic facilities at Atlas Scientific. Pulse testing will be done using custom STJ readout electronics, X-ray detector test cryostats, and the invaluable expertise of the Advanced Detector Group at Lawrence Livermore National Laboratory.

Our improved cryostat is based upon development of a custom 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. Atlas Scientific is an R&D company with considerable experience building and operating sorption coolers and pulse tube coolers. Atlas Scientific will 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. This cryostat will be very cost effective compared to commercial adiabatic demagnetization refrigerator / pulse tube systems.

Educational Mission

The SFSU Physics and Astronomy Department offers a terminal Master's degree requiring extensive coursework with a thesis as a popular option. Completion of a thesis related to this STJ project would be recognized as significant professional experience and thus would circumvent the "Catch-22" situation faced by young people who cannot figure out how to get that first job without having had a previous job. The SFSU/CEG consistently involves students not only from SFSU but also from nearby colleges having limited or nonexistent research opportunities. Because SFSU and its neighboring institutions have multicultural student populations, recruiting efforts attract talented individuals from underrepresented groups. We are well aware of the profound professional loneliness that is felt by students from underrepresented groups, and we understand how difficult it is to persist in a profession that may not be valued by one’s peers. These problems can be overridden by the scientific interest of our research projects; by the excitement of interacting with scientists from both corporate and national laboratories; and by the camaraderie among a group of people working toward a common goal.

This research program to develop a new generation of STJ detectors will enable students to learn highly marketable microfabrication techniques by working in the SFSU Thin Film Lab, the U.C. Berkeley Microlab, and the Stanford Nanofabrication Facility. R&D activities conducted with Atlas Scientific will introduce students to the entrepreneurial creativity of a small high-tech company. Device testing at LLNL will expose students to research in a state-of-the-art facility. Students will develop a sense of membership in the scientific community that will inspire them throughout their careers.

Updated 05 February 2010