Oct. 1, 2019:
DM Radio Cubic Meter is a full-scale QCD axion search that will probe QCD axion dark matter over 1.5 orders of magnitude of mass. It is a Consortium of SLAC, Stanford, UC Berkeley, LBNL, MIT, UNC Chapel Hill, and Princeton university.
The Irwin group has a significant program to design, build, and field advanced detectors for X-ray spectroscopy at the Stanford Synchrotron Radiation Lightsource (SSRL) and Linac Coherent Light Source (LCLS), located at SLAC National Laboratory. We currently operate a state-of-the-art transition edge sensor (TES) array at SSRL beamline 10-1 where we focus on exploring core-level spectroscopy through collaborations with chemists and material scientists. The TES array has already proven to be capable of measuring extremely dilute and damage sensitive chemical systems which cannot be measured anywhere else in the world, and we look forward to further pushing the limits of spectroscopy with a second spectrometer at SSRL beamline 13-3 which will focus on small-angle X-ray scattering from solid-state systems such as high-TC superconductors. The Irwin group is leading an effort to bring these detectors to LCLS-II, which will be the brightest free-electron laser in the world when it turns on in 2020.
Research in chemistry, biology, and materials science seeks to understand novel molecular systems. One of the experimental challenges in these fields is to understand the unique local electronic structure that gives rise to reactions, transformations, and bulk properties. Precise measurements of electron orbitals can provide a window into chemical bonds as they undergo reactions and catalysis. Core-level spectroscopy is a group of techniques that probe transitions between the core electron orbitals and the valence orbitals in order to yield information about the symmetry, spin state, and energy of electron orbitals. Conceptually, this information can be obtained by measuring the transmission of a monochromatic beam of X-rays through a sample. When core-to-valence transitions are excited, the number of photons transmitted changes drastically. When the X-ray absorption cross-section is measured as a function of incident energy, we call the measurement "X-ray absorption spectroscopy" (XAS). In practice, we often measure secondary photons (fluorescence yield XAS) or electrons (electron yield XAS) in a scattering geometry, where the detected signal is proportional to the absorption cross-section. If an energy-resolving detector is used resonant inelastic X-ray scattering (RIXS) is a technique which often yields more information than basic XAS. Unfortunately, if is often difficult to collect enough photons to make these techniques practical, especially because interesting samples are often dilute. These problems can be solved through advancements in detector technology.
Superconducting transition edge sensor (TES) technology has been used to build novel detectors with greatly increased sensitivity in the soft x-ray regime at intermediate energy resolution. The Irwin group commissioned a new soft x-ray superconducting TES spectrometer at SSRL in spring of 2016, with a scientific agenda driven by measurements of ultra-low concentration samples, radiation sensitive samples, and dilute samples with an overwhelming background. The use of a pixel-based spectrometer allows for a dramatically improved solid angle compared to traditional grating spectrometers.
Charles Titus, Michael Baker, Sang Jun Lee, et al., Journal of Chemical Physics 147