Observation of the competitive double-gamma nuclear decay

Area of Science:

• Nuclear Physics
• Quantum Electrodynamics

Background:

• Double-gamma (γγ)-decay is a fundamental second-order quantum electrodynamics process involving simultaneous emission of two gamma quanta.
• Previously, γγ-decay was only observed for specific nuclear transitions (J(π) = 0(+) → 0(+)) where single-gamma decay is forbidden.
• Single-gamma (γ)-decay is the primary experimental challenge for observing γγ-decay.

Purpose of the Study:

• To report the observation of γγ-decay from an excited nuclear state with J(π) = 11/2(-).
• To investigate γγ-decay competing directly with allowed single γ-decay.
• To determine the branching ratio and multipolarities of the γγ-decay process.

Main Methods:

• Experimental observation of γγ-decay in the 11/2(-) isomer of Barium-137 ((137)Ba).
• Measurement of angular correlation and energy spectra of emitted gamma rays.
• Calculation of transition matrix elements using the quasiparticle-phonon model.

Main Results:

• The γγ-decay of the 11/2(-) isomer of (137)Ba was observed competing with allowed γ-decay to the 3/2(+) ground state.
• The branching ratio for this competitive γγ-decay was determined to be (2.05 ± 0.37) × 10(-6).
• Analysis of angular correlations and energy spectra allowed determination of contributing multipolarities.

Conclusions:

• The observed γγ-decay provides a new pathway to probe nuclear structure.
• This decay mode offers access to previously unexplored nuclear properties, including generalized nuclear electric polarizabilities and magnetic susceptibilities.
• The quasiparticle-phonon model successfully reproduces the experimental measurements.