Results from the First Neutron Reflectometry Application of a Fast Photon-Counting Camera

A detector based on Timepix3 readout of image-intensified light from a scintillator ($\mathrm{ZnS}: \mathrm{Ag}: \mathrm{LiF})$ has been applied and tested in neutron reflectometry for the first time. The detector has been characterized at a monochromatic beamline with lambda $=4.2 \mathrm{AA}$ at HFIR reactor at ORNL. It was then installed on the Time-of-Flight (ToF) reflectometer ASTERIX at LANL and compared to a conventional ${ }^{3} \mathrm{He}$ reflectometry detector. The search for faster reflectometer detectors has been underway using multiple technologies around the world. Our new detector has made a successful entry into this field. This paper presents the measured reflectivity profiles for several standard samples - Ni, Ir and quartz. The detection efficiency and signal-to-background were nearly indistinguishable from ${ }^{3} \mathrm{He}$ detector. Reflectivity profiles for these samples are presented in comparison to ${ }^{3} \mathrm{He}$ demonstrating excellent performance of the detector. The rate that could be used before the onset of non-linearities was significantly greater with instantaneous rate of 100 kHz and above, presenting no issues already in the current build of the detector. Several improvements are already underway. These will further increase the detector rate performance by reducing the scintillator afterglow as well as alleviate the rate limitations of the image intensifier. The eventual upgrade to Timepix4 is promising to unlock yet another order of magnitude in rate capability. We further propose handling even more intense direct beams using a smaller lens aperture. This serves as a controlled adjustment of the detection efficiency in place of the necessarily wavelength-dependent beam attenuation typically used on for direct beams with current ${ }^{3} \mathrm{He}$ detectors. Further design and prototyping will be performed for the QIKR reflectometer at the Second Target Station at ORNL.