First Measurement of a Weak r-Process Reaction on a Radioactive Nucleus.

Area of Science:

• Nuclear Astrophysics
• Nuclear Physics
• Cosmochemistry

Background:

• Understanding the origin of heavy elements requires precise knowledge of nuclear reaction rates.
• The rapid neutron-capture process (r-process) is responsible for synthesizing elements heavier than iron.
• Experimental data for weak r-process reactions, especially those involving unstable isotopes, are scarce.

Purpose of the Study:

• To experimentally determine the cross-section for the ^{94}Sr(α,n)^{97}Zr and ^{86}Kr(α,n)^{89}Sr reactions.
• To provide the first measurement of a weak r-process reaction cross section using a radioactive ion beam.
• To compare experimental results with theoretical predictions and assess their impact on nucleosynthesis models.

Main Methods:

• Utilized a novel solid helium target system with silicon thin films.
• Employed a radioactive ion beam for the ^{94}Sr(α,n)^{97}Zr reaction measurement.
• Measured reaction yields at specific center-of-mass energies within the Gamow window for 5 GK.
• Identified reaction products via prompt gamma-ray detection in coincidence with heavy ions from a recoil mass spectrometer.

Main Results:

• Obtained the first cross-section measurements for ^{94}Sr(α,n)^{97}Zr and ^{86}Kr(α,n)^{89}Sr.
• The measured cross sections for both reactions are significantly smaller than theoretical predictions.
• The reaction rate for ^{94}Sr(α,n)^{97}Zr is reduced by an order of magnitude below 5 GK.

Conclusions:

• The experimental findings challenge existing nucleosynthesis models for the r-process.
• The reduced reaction rate for ^{94}Sr(α,n)^{97}Zr is expected to alter the predicted abundance of ruthenium.
• This work highlights the importance of experimental data for unstable isotopes in understanding cosmic element abundances.