Cryogenic Detector Development at LLNL: Ultraviolet, X-ray, Gamma-ray and Biomolecule Spectroscopy

We are developing low-temperature detectors for optical, ultraviolet, X-ray, and gamma-ray spectroscopy, and for biomolecular mass spectrometry. We present here a survey of our recent work in developing these detectors and some of the first results in applying these detectors for materials analysis and biomolecular mass spectrometry. We have measured thin-film Nb/Al/Al2O3/Al/Nb superconducting tunnel junction (STJ) X-ray detectors in the 0.2 to 1 keV band with a range of different junction sizes and aluminum film thicknesses. In one case, we have achieved the statistical limit to the energy resolution in this band. We have measured the performance of these STJ detectors as a function of count rate and demonstrated a resolution of 13 eV FWHM at 277 eV with an output count rate of 20,600 cts/s. Using X rays from a synchrotron light source to study composite materials, we have demonstrated that we can resolve the L lines of transition metals from the nearby K lines of light elements. We describe the first use of a low-temperature X-ray detector to measure X-ray fluorescence from the dilute metal component in a protein. In a time-of-flight mass spectrometry experiment using Nb/Al2O3/Nb STJ detectors operating at 1.3 K, we have demonstrated that masses as large as 750 kDa can be detected in a time-of-flight mass spectrometer. This extends the mass range for this type of mass spectrometer by an order of magnitude. We also show that the energy resolving capability of the STJ can be used to measure the charge of the molecule, thus removing ambiguities in the measured mass spectrum. We describe the development of X-ray and gamma-ray spectrometers using small crystals of Ta with STJ and superconducting transition-edge sensors (TES). We present a new model for TES microcalorimeter that includes both the temperature and current dependence of the TES.