High-temperature ²⁰⁵Tl decay clarifies ²⁰⁵Pb dating in early Solar System

Area of Science:

• Nuclear Astrophysics
• Cosmochemistry
• Stellar Nucleosynthesis

Background:

• Long-lived radioactive nuclei in meteorites offer insights into the Sun's formation and nucleosynthesis.
• Lead-205 (Pb-205) is synthesized via slow neutron captures (s-process) in asymptotic giant branch (AGB) stars, but its abundance predictions are limited by uncertain decay rates.
• Accurate decay rates for Pb-205 and Thallium-205 (Tl-205) are crucial for understanding early Solar System chronologies.

Purpose of the Study:

• To experimentally constrain the decay rates of Pb-205 and Tl-205 at stellar temperatures.
• To determine the isolation time of solar material within its parent molecular cloud.
• To validate the Pb-205/Tl-205 decay system as a chronometer for the early Solar System.

Main Methods:

• Measured the bound-state beta-minus decay of fully ionized Tl-205 (Tl-205^81+) for the first time.
• Utilized AGB stellar models with experimentally constrained decay rates to calculate Pb-205 yields.
• Integrated galactic chemical evolution (GCE) models with calculated yields and meteorite isotopic ratios (Pb-205/Pb-204) to determine isolation times.

Main Results:

• The measured half-life of Tl-205^81+ was found to be 4.7 times longer than theoretical estimates, with a 10% experimental uncertainty.
• New Pb-205 yields were calculated using experimentally validated decay rates.
• Positive isolation times were determined, consistent with other short-lived radioactive nuclei in the early Solar System.

Conclusions:

• The study resolves a major nuclear-physics limitation in predicting Pb-205 abundances.
• The findings support the Sun's birth in a long-lived, giant molecular cloud.
• The Pb-205/Tl-205 decay system is confirmed as a valuable chronometer for the early Solar System.