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Search for nuclear two-photon decay with GRIFFIN
Natzke, Connor
Natzke, Connor
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2024
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The nuclear equation of state requires rigorous benchmarking of experimentally observable properties, such as the electric polarizability of nuclear matter and its difference for an excited nuclear state. This difference can be extracted through measurement of the second order electromagnetic process of nuclear two-photon emission. Nuclear two-photon, or double-gamma (2$\gamma$), decay is the simultaneous emission of two gamma-rays from an excited nuclear state. Searching for this decay mode in systems where the first excited and ground states have $0^+$ spin/parity, where single photon emission is forbidden, provides significant experimental advantages. The first excited state of $^{90}$Zr satisfies these conditions, and has been previously observed to undergo two-photon emission. A proof-of-concept measurement of the 1760 keV two-photon transition in $^{90}$Zr, using a $^{90}$Sr source, has been performed using the GRIFFIN spectrometer at TRIUMF-ISAC, and is the first measurement of this transition using a general-purpose HPGe array. The techniques developed in this measurement informed the first direct search for the 691 keV two-photon transition in $^{72}$Ge, where a statistically-limited branching ratio upper limit of $1.14(7)\times10^{-3}$ was set. These results are consistent with recent observations of two-photon decay in $^{72}$Ge using bare ions and provide new insights into using general-purpose arrays to measure nuclear two-photon decay.
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