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

TIGERISS is an Ultra-Heavy Galactic Cosmic Ray (UHGCR) detector that was recently awarded under the NASA Astrophysics Pioneers program. TIGERISS will be capable of measuring the abundances (relative to ₂₆Fe) of every element from ₅B to ₈₂Pb with E > 350 MeV/nucleon. Its observations of UHGCR elements will cover those synthesized in s-process and r-process nucleosynthesis to 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 is an evolution of the TIGER and SuperTIGER long-duration-balloon instruments and incorporates detector improvements out of the previously proposed Heavy-Nuclei Explorer SMEX, including silicon strip detectors in place of scintiallator radiators and scintillating fibers. TIGERISS’s capability to definitively identify UHGCR nuclei with resolution σ_Z < 0.25 has been demonstrated in component accelerator tests at CERN. The geometry factor for TIGERISS has been estimated to range from 1 to 1.6 m² sr depending on the ISS attachment point. With these improvements, TIGERISS will observe statistics comparable to the current SuperTIGER data set and expand over a larger charge region within 1 year of operating time. TIGERISS initial results will be cleaner, not requiring corrections for atmospheric interactions and scintillator saturation effects, and give the first single element resolution measurements of higher charges. This will allow us to test and eliminate 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. More recent SuperTIGER 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.