The overall TIGERISS mission goal is to increase understanding of how the galaxy produces and distributes the elements

This has been a timely topic since the joint detection of GW170817 in gravitational waves/GRB 170817A in gamma rays, and the subsequent kilonova observed at optical wavelengths re-energized the debate about where and how nuclei are created across the galaxy.

Galactic cosmic rays (GCRs) provide clues on this issue, as GCRs are one of our few direct samples of matter from outside the solar system. Their relative rarity at the highest charges, however, means that most absolute ultra-heavy galactic cosmic ray (UHGCR) individual elemental abundances above nuclear charge Z = 56 are difficult to measure and remain unknown.

Origins of heavy elements

TIGERISS aims to employ silicon detector technology from a vantage point on the ISS to capture novel data on these rare UHGCRs to illuminate the grand cycle of matter in the galaxy. TIGERISS is the first instrument capable of measuring UHGCR abundances with single-element resolution spanning the periodic table from boron to lead. 

Predicted TIGERISS abundances after 1 month, 1 year, and 3 years, compared to measurements by SuperTIGER over its 55-day balloon flight.
Good silicon resolution from a CERN beam test demonstrating the resolution we need to identify individual elements through Pb

The TIGERISS science goals reach beyond the context of particle acceleration. TIGERISS will constrain sites of r-process nucleosynthesis, a topic widely debated since 2017, when multi- messenger observations led to direct evidence of r-process material created in a binary neutron star merger. In addition, TIGERISS observations will inform models of stellar feedback in the circulation of matter in the galaxy. 

The important scientific questions that motivate extensive, high-precision measurement of UHGCR abundances by TIGERISS are: 

  1. What astrophysical sources synthesize r-process nuclei and how much does each source contribute to the galactic r-process budget? 
  2. What are the limits of cosmic-ray nucleosynthesis and acceleration in OB associations (stellar associations), and of their contribution of metal and energy transport in the galaxy? 
Left: Model of massive star nucleosynthesis; Right: Model of binary neutron start merger nucleosynthesis

More specifically, TIGERISS science goals include: 

  • Conducting novel UHGCR measurements and improvements over existing data.
  • Illuminating the OB Association Model of galactic cosmic ray (GCR) origin.
  • Illuminating whether binary neutron star mergers contribute a new component to GCRs above Z = 40.
  • Illuminating the current grain injection/acceleration model.