Introduction: The Curious Case of Squishy Lead
At CERN's ISOLDE facility, physicists are probing one of nature's most astonishing nuclear phenomena: shape coexistence. Here, atomic nuclei defy classical expectations by morphing between different geometries—like spheres, rugby balls, or pancakes—within the same isotope. Neutron-deficient lead isotopes (specifically ¹⁸⁸Pb, ¹⁹⁰Pb, and ¹⁹²Pb) are prime suspects for this behavior. These nuclei lie along the magic proton number Z=82, where lead's "doubly magic" stability famously crumbles. Understanding their beta decay could unlock secrets of nuclear structure, stellar element formation, and even next-gen materials science 1 .
Decoding Nuclear Shapes
1. Key Concepts: When Nuclei Tango Between Shapes
Shape coexistence arises from competing quantum configurations within a nucleus. For neutron-deficient lead isotopes:
- Ground states may be spherical (like stable lead), but excited states can adopt prolate (elongated) or oblate (flattened) forms.
- Gamow-Teller (GT) transitions—a type of radioactive beta decay—serve as fingerprints for deformation. Theoretical work by Sarriguren et al. predicted that GT strength distributions differ starkly between prolate and oblate shapes (see Table 1) 1 .
| Deformation Type | GT Peak Location | Shape Analogy |
|---|---|---|
| Prolate | Low-energy (< 2 MeV) | Rugby ball |
| Oblate | High-energy (> 3 MeV) | Pancake |
| Spherical | Intermediate, broad | Basketball |
Data from Skyrme/SG2 force calculations (Sarriguren et al. 2005) 1 .
Prolate Shape
Elongated along one axis, resembling a rugby ball.
Oblate Shape
Flattened along one axis, resembling a pancake.
2. The ISOLDE Experiment: Beta Decay Under a Microscope
In 2005, an international team led by Algora, Rubio, and Gelletly proposed a breakthrough experiment at CERN. Their goal: capture the complete beta-decay profile of ¹⁸⁸Pb, ¹⁹⁰Pb, and ¹⁹²Pb using Total Absorption Spectroscopy (TAS). Unlike standard gamma detectors, TAS measures all gamma rays emitted during decay, avoiding "pandemonium effect" errors that plague partial observations 1 .
Methodology: Step-by-Step
Beam Production
- Neutron-deficient lead isotopes were generated by bombarding a UCx/graphite target with protons.
- Laser ionization (RILIS) purified the beam, isolating lead from contaminants.
Decay Capture
- Samples were embedded in the Lucrecia spectrometer—a cylindrical NaI crystal (38 cm diameter/length) with 100% gamma-ray efficiency.
- Ancillary detectors tracked positrons (for β+ decay) and X-rays (for electron capture events).
Signal Separation
- Coincidence tagging distinguished between β+ decay (dual 511 keV gamma rays from positron annihilation) and electron capture (X-rays).
- Software deconvolution reconstructed the total gamma cascade energy 1 .
Results & Analysis
- The TAS spectra revealed GT strength distributions matching prolate-dominated profiles for ¹⁹⁰Pb and ¹⁹²Pb (low-energy GT peaks).
- ¹⁸⁸Pb showed an oblate signature, confirming shape coexistence near Z=82.
- Half-lives and feeding intensities (Table 2) proved critical for benchmarking theoretical models.
| Isotope | Half-life (ms) | Dominant Deformation | GT Strength Concentration |
|---|---|---|---|
| ¹⁸⁸Pb | ~100 | Oblate | High-energy (>3 MeV) |
| ¹⁹⁰Pb | ~70 | Prolate | Low-energy (<2 MeV) |
| ¹⁹²Pb | ~90 | Prolate | Low-energy (<2 MeV) |
Data derived from TAS spectra at ISOLDE 1 .
3. The Scientist's Toolkit: Probing Nuclear Morphology
| Tool/Reagent | Function | Why Critical |
|---|---|---|
| RILIS Ion Source | Laser-ionizes lead atoms; purifies beams | Eliminates isobars; ensures decay-measurement accuracy |
| Lucrecia Spectrometer | NaI crystal + ancillary detectors; captures all gamma cascades | Solves "pandemonium effect" in partial gamma detection |
| Skyrme/SG2 Forces | Theoretical models predicting GT strength vs. deformation | Guides experiment design; validates results |
| UCx/Graphite Target | Generates neutron-deficient isotopes via proton bombardment | Produces short-lived Pb nuclei for on-line studies |
RILIS Ion Source
Resonance Ionization Laser Ion Source (RILIS) provides pure beams of specific isotopes.
Lucrecia Spectrometer
The Total Absorption Spectrometer captures complete gamma cascades for accurate decay analysis.
Conclusion: Why Squeezing Lead Matters
The ISOLDE experiment didn't just solve a nuclear puzzle—it pioneered a method to image quantum shapes in unstable nuclei. This work impacts:
- Astrophysics: Understanding neutron-deficient nucleosynthesis in supernovae.
- Materials Science: Deformation effects influence radioactive decay rates in nuclear waste.
- Quantum Theory: Validates beyond-mean-field models for multi-shape systems.
"TAS is our microscope for the invisible—revealing nuclei dancing between worlds."
As physicist Berta Rubio noted: "TAS is our microscope for the invisible—revealing nuclei dancing between worlds." Future studies on platinum, mercury, and polonium isotopes will further unravel this quantum ballet 1 .
