First direct timing of an excited 0+ state in tin-118 shows signs of three coexisting nuclear shapes
This paper reports the first direct measurement of the lifetime of an excited 0+ state called 0+3 in the nucleus tin-118 (118Sn). The team found a short lifetime of 74(13) picoseconds. From that lifetime they extracted an unusually large electric monopole (E0) transition strength between the two excited 0+ states, ρ2(E0; 0+3→0+2) = 150(30) milliunits (m.u.). That enhanced E0 strength is a clear experimental sign that different shapes of the same nucleus coexist and mix in 118Sn.
To get this result the authors used thermal-neutron capture at the Institut Laue-Langevin (ILL) in Grenoble. They irradiated a powder target enriched in 117Sn and recorded gamma rays with a combination of high-purity germanium (HPGe) detectors for good energy resolution and fast-timing lanthanum bromide (LaBr3) detectors for precise timing. They collected about 1.3×108 gamma events over roughly 14 days and extracted the 0+3 lifetime using the generalized centroid-difference method. The analysis used triple-gamma coincidences to reduce background and careful timing calibrations with known sources.
Why this matters: nuclei can take different shapes (roughly, different patterns of how protons and neutrons arrange themselves). When two states with different shapes mix, the E0 transition between their 0+ band heads is enhanced. Previous work had already found strong E0 signals in neighboring tin isotopes (for example 116Sn and 120Sn). But lifetime information for the 0+3 state in 118Sn was lacking. By measuring the lifetime and deriving the E0 strength, the authors provide direct evidence that multiple shapes coexist and mix in 118Sn, strengthening a pattern seen across several tin isotopes.
The paper also reports theoretical calculations that support the interpretation. The authors used a modern quantum-number-projected generator coordinate method (GCM) built on a relativistic energy density functional, a many-body framework that mixes mean-field states with different quadrupole shapes. Those calculations naturally produce several low-lying 0+ states and suggest three distinct shapes in 116, 118 and 120Sn. The experimental E0 strength and the calculated wave functions together make a consistent picture of multiple shape coexistence.