44Ti

Titanium-44 (44Ti) is a radioactive isotope of titanium with mass number 44. It has 22 protons and 22 neutrons. 44Ti decays by electron capture to scandium-44 (44Sc) with a half-life of about 59 years, and the subsequent decay of 44Sc leads to stable calcium-44 (44Ca). The decay of 44Ti produces characteristic gamma rays with energies near 67.9 keV and 78.4 keV, which are used to observe and study the isotope in astrophysical environments.

Production and occurrence: 44Ti is primarily produced in explosive nucleosynthesis during core-collapse supernovae and related energetic events, where high-temperature, alpha-rich conditions enable its synthesis. Its yield depends on the specifics of the explosion, including temperature, density, and mixing. Type II or other core-collapse supernovae are considered the main astrophysical sources, while 44Ti production in other stellar events is less certain.

Astrophysical significance: Because of its multi-decade half-life, 44Ti serves as a diagnostic of recent supernova activity within the Galaxy. The most enduring detections have come from the young remnant Cassiopeia A, where gamma-ray observations have identified the 68 and 78 keV lines. Measurements of 44Ti flux constrain supernova explosion models, nucleosynthesis pathways, and the distribution of freshly synthesized material in remnants. 44Ti yields are typically in the range of 10^-4 solar masses per event, though estimates vary with model assumptions.

In meteoritics, the decay of extinct 44Ti to 44Ca in early solar system materials provides evidence about the timing and intensity of nearby nucleosynthetic events shortly before or during Solar System formation. This makes 44Ti a useful chronometric and nucleosynthetic tracer in both astrophysics and cosmochemistry.