Speaker
Description
The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic decay process whereby a nucleus in an excited state emits two gamma rays simultaneously. Compared to first-order decay pathways, such as single photon emission or internal electron conversion, the two-photon decay rate is very small. Ideal cases for this search are $0^+ \rightarrow 0^+$ transition where single photon emission is prohibited. However, the only cases where the 2γ decay from a first-excited 0+ state was successfully observed using γ-ray spectroscopy are $^{16}$O, $^{40}$Ca and $^{90}$Zr [1, 2], where the high energy of the transitions is favorable for the 2γ branch. More recently, also the competitive 2γ decay was observed from the long-lived 11/2$^-$ isomer in $^{137}$Ba [3].
For lower decay energies the 2γ branch becomes prohibitively small to be observed in γ-ray spectroscopy (<10$^{-6}$). We have therefore combined the isochronous mode of the Experimental Storage Ring (ESR) at GSI with Schottky resonant cavities. This newly developed Schottky plus Isochronous Mass Spectrometry (S+IMS) allows to study exotic decays of short-lived nuclear states. The obtained mass resolving power enables experiments on nuclear isomers with excitation energies down to ∼100 keV and half-lives as short as a few ms. The first measurement of the partial half-life for the isolated 2γ decay of the 0$^+$ isomer in $^{72}$Ge turned out to be surprisingly short [4]. Recent results for the 2γ decay of the 0$^+$ isomers in $^{98}$Zr and $^{98}$Mo will be presented as well as first steps to measure the weak 2γ branch in $^{72}$Ge by direct γ-ray spectroscopy.
[1] J. Schirmer J. Schirmer, D. Habs, R. Kroth, N. Kwong, D. Schwalm, and M. Zirnbauer, {Double gamma decay in 40Ca and 90Zr, Phys. Rev. Lett. 53, 1897–1900 (1984).
[2] J. Kramp, D. Habs, R. Kroth, M. Music, J. Schirmer, D. Schwalm, and C. Broude, Nuclear two-photon decay in 0+ → 0+ transitions, Nuclear Physics A 474, 412–450 (1987).
[3] C. Walz, H. Scheit, N. Pietralla, T. Aumann, R. Lefol, and V. Yu. Ponomarev, Observation of the competitive double-gamma nuclear decay, Nature 526, 406-409, (2015).
[4] D. Freire-Fernández, W. Korten, Y. Litvinov et al., Measurement of the Isolated Nuclear Two-Photon Decay in 72Ge, Phys. Rev. Lett. 133, 022502 (2024) and https://arxiv.org/pdf/2312.11313.
The main results are based on the experiments E143 and G22-00018, which were performed at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt (Germany) in the context of FAIR Phase-0. This work was supported by the Slovenian Research and Innovation Agency under Grants No. I0-E005 and No. P1-0102, by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC-AdG NECTAR, grant agreement No 884715; ERC-CoG ASTRUm, grant agreement No 68284), by the State of Hesse (Germany) within the Research Cluster ELEMENTS (Project ID 500/10.006), by the STFC (UK), by the NSF grant PHY-2110365, by the BMBF under grant NuSTAR.DA 05P19RDFN1, by the JSPS KAKENHI Grant Number T23KK0055. Work at ANL is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under contract No. DE-AC02-06CH11357.