The TIGERISS Ultra-Heavy Galactic Cosmic Ray Detector

The Trans-Iron Galactic Element Recorder for the International Space Station (TIGERISS) is an Ultra-Heavy Galactic Cosmic Ray (UHGCR) detector under the NASA Astrophysics Pioneers program with a planned 2026 launch. TIGERISS will measure the abundances of individual elements from ₅B to ₈₂Pb with superior charge resolution and detector linearity. It will also measure the energy spectra of the more abundant cosmic-ray elements through ₂₈Ni from its trigger threshold ~100 – 400 MeV/nucleon to the saturation of the aerogel Cherenkov detector ~10 GeV/nucleon. TIGERISS observations of UHGCR elements will cover those synthesized in stellar fusion and in s-process and r-process neutron capture nucleosynthesis, which will add to the multi-messenger effort to answer the question of whether these r-process elements are synthesized in supernovae (SN) or Neutron Star Merger (NSM) events. TIGERISS improves on the predecessor TIGER and SuperTIGER long-duration-balloon instruments by using silicon strip detectors (SSDs) in place of scintillator detectors and using silicon photomultipliers (SiPMs) instead of photomultiplier tubes (PMTs) to read out the acrylic and silica aerogel Cherenkov-light-radiator detectors. The superior resolution of the SSDs over the scintillator detectors has been demonstrated in component accelerator tests at CERN, allowing TIGERISS to identify UHGCR nuclei with resolution σ_Z < 0.25. The payload ISS accommodation location assignment is pending the completion of the formulation phase and successful completion of the attendant gate review, and European Space Agency (ESA) Columbus Laboratory external payload Starboard Overhead X-Direction (SOX) and Japan Aerospace Exploration Agency (JAXA) Japanese Experiment Module (JEM) “Kibo” Exposed Facility Unit 7 (EFU7) locations have been studied. The geometry factor for SOX is 1.28 m² sr and that for JEM-EFU7 is 1.19 m² sr for the standard payload envelope and 1.83 m² sr if a waiver allows for the payload to be 0.2 m wider. Predicted TIGERISS statistics for 1 year of observations for UHGCRs equal or surpass those measured in the first 55-day SuperTIGER flight in their overlapping dynamic range through ₅₆Ba without the need to correct for atmospheric interactions, while extending single-element resolution measurement through ₈₂Pb. TIGERISS measurements will address questions of the model of cosmic-ray origins with a major contribution from OB Associations supported by UHGCR measurements through ₄₀Zr raised by more recent SuperTIGER inconsistent results from ₄₁Nb to ₅₆Ba that hint at an additional r-process source.