The Trans-Iron Galactic Element Recorder for the International Space Station (TIGERISS)*

TIGERISS is an Ultra-Heavy Galactic Cosmic Ray (UHGCR) detector awarded under the second round of the NASA Astrophysics Pioneers program. It will measure the abundances of every element from ₅B to ₈₂Pb relative to ₂₆Fe with kinetic energies over 350 MeV/nucleon. TIGERISS is an evolution of the TIGER and SuperTIGER long-duration balloon instruments incorporating detector improvements from our previous Heavy-Nuclei Explorer SMEX, including silicon strip detectors in place of both scintiallator radiators and scintillating fibers. The performance of these new detector components was demonstrated at CERN/SPS beam tests and will provide TIGERISS with high fidelity charge assignment with σ_Z <0.25. All available ISS external attachment accommodations are currently under study, including those for the JAXA JEM-EF, the ESA Columbus Laboratory, and the NASA ELC. The TIGERISS geometry factor depends on attachment location (~1 to 1.6 m² sr), but in one year the baseline instrument would obtain statistics comparable to the current SuperTIGER data set and expand measurements to higher and lower atomic numbers. TIGERISS measurements will be cleaner than SuperTIGER’s as they will not require corrections for atmospheric interactions and scintillator saturation effects. They will also give the first single-element resolution measurements of elements above ₅₆Ba that with extended observations would test models for cosmic-ray origins and acceleration. The SuperTIGER UHGCR measurements through ₄₀Zr have supported a model of CR origins in OB Associations with preferential acceleration of refractory elements more likely found in dust grains than volatiles superposed on Z dependence from grain sputtering injection in SN shocks, but more recent results from ₄₁Nb to ₅₆Ba are inconsistent with that model and require a difference in the acceleration mechanism and/or an additional r-process source. These UHGCR observations will cover elements produced in s-process and r-process neutron capture nucleosynthesis, adding to the multi-messenger effort to determine the relative contributions of supernovae (SN) and Neutron Star Merger (NSM) events to r-process nucleosynthesis.