Abstract: The performance of single photon microcalorimeters for high-resolution X-ray spectroscopy has improved to the point that it can be limited by our ability to calibrate these detectors. X-ray fluorescent lines have been the usual standard for calibrations, but they have intrinsic widths much broader than the current detector resolution. Since microcalorimeters respond to total energy deposited as heat, we have investigated the idea that five hundred 3 eV photons from an ultraviolet laser delivered in a pulse much shorter than the thermal integration time of detectors should look the same as a single 1500 eV X-ray photon.
We have illuminated devices that have 290 μm square gold absorbers, superconducting transition-edge sensor (TES) thermometers, and about 1 eV FWHM resolution with ∼ 100 ns pulses from an ultraviolet laser. This produces combs of lines with 3 eV spacing and negligible intrinsic width that can be distinguished to at least 1700 eV. The accuracy of the line energies is limited only by our knowledge of the laser wavelength. Simultaneous illumination with oxygen and aluminum K fluorescent lines shows that the response to an X-ray photon is indistinguishable from the response to a burst of UV photons with the same total energy to better than 0.4 eV at 1500 eV. This performance is more than adequate for our current sounding rocket instrument. We discuss the path forward to possibly demonstrating the 0.1 eV at 10 keV standard that would be desirable for instruments on major space missions.