Discovery of ^{72}Rb: A Nuclear Sandbank Beyond the Proton Drip Line

Area of Science:

• Nuclear Physics
• Astrophysics
• Nuclear Chemistry

Background:

• The proton drip line defines the boundary of nuclear stability for proton-rich nuclei.
• Understanding the properties of nuclei near the proton drip line is crucial for nucleosynthesis models.

Purpose of the Study:

• To report the first observation of ^{72}Rb and ^{77}Zr isotopes.
• To investigate the properties of ^{72}Rb, including its half-life and decay modes.
• To explore the implications of these findings for the location of the proton drip line and nucleosynthesis.

Main Methods:

• Experimental observation of new isotopes using nuclear reaction techniques.
• Measurement of the half-life of ^{72}Rb.
• Comparison of experimental results with theoretical models (finite-range droplet model, shell model).

Main Results:

• First observation of ^{72}Rb (14 events) and ^{77}Zr (1 event).
• Measured half-life of ^{72}Rb as 103(22) ns.
• Derived an upper limit for the ground-state half-life of ^{73}Rb as 81 ns.
• ^{72}Rb half-life is consistent with theoretical predictions for proton decay.
• ^{72}Kr identified as a waiting point in x-ray burst nucleosynthesis.

Conclusions:

• The existence of relatively long-lived ^{72}Rb indicates a diffuse proton drip line.
• These findings challenge the sharp boundary of the proton drip line and suggest the existence of 'sandbanks' beyond it.
• ^{72}Kr's role as a waiting point has significant implications for understanding the rapid proton capture (rp) process in x-ray bursts.