Speaker
Description
Near-threshold narrow resonances in light nuclei are crucial for nuclear structure studies: they are expected to provide information on the onset of clusterization phenomena, and they also play a key role in nucleosynthesis reactions in stars. A famous example is the Hoyle state in $^{12}$C. In this context, the $\gamma$ decay from near-threshold states, with branches of the order of $10^{-3}$–$10^{-6}$ with respect to particle emission, is one of the most powerful probes of their wave function.
In this contribution we will investigate, at first, the possible existence of a narrow resonance in $^{11}$B, lying just above the proton-decay threshold, which was originally suggested to explain the observation of an unexpectedly large proton emission after the $\beta^-$ decay of $^{11}$Be [1]. An explorative experiment performed in 2021 with GALILEO+TRACE at Laboratori Nazionali di Legnaro reported an upper limit of $1.12\cdot10^{-3}$ for the $\gamma$-ray branch from this possible resonance (with limited statistical confidence) [2], slightly above theoretical predictions from the Shell Model Embedded in the Continuum (SMEC) [3,4]. More recently, in October 2025, the same $^6\text{Li}(^6\text{Li},\text{p}\gamma)$ fusion-evaporation reaction was performed using a significantly improved setup consisting in the AGATA spectrometer coupled to the upgraded highly-segmented silicon charged-particle detector TRACE [5] and the silicon CD detector SAURON. Owing to the high statistics collected, a sensitivity of $\leq10^{-4}$ on the $\gamma$-decay branching ratio is expected. The data analysis is currently ongoing, and preliminary results will be presented.
As a second case, we will briefly discuss a similar investigation carried out for $^{14}$C, which was populated through the $^9\text{Be}(^6\text{Li},\text{p}\gamma)$ fusion-evaporation reaction at Argonne National Laboratory with the GRETINA+ORRUBA setup [6]. One of the aims of the experiment was the estimate of the $\gamma$-decay branch from the $2_2^+$ state located just above the neutron separation energy. An upper limit of $4.0 \cdot 10^{-5}$ was obtained for this branch, providing an additional constraint to the theoretical interpretation of its decay properties.
[1] Y. Ayyad, B. Olaizola, W. Mittig et al., Phys. Rev. Lett. 123, 082501 (2019).
[2] S. Bottoni, G. Corbari, S. Leoni et al., Phys. Lett. B 855, 138851 (2024).
[3] J. Okołowicz, M. Płoszajczak and W. Nazarewicz, Phys. Rev. Lett. 124, 042502 (2020).
[4] J. Okołowicz, M. Płoszajczak and W. Nazarewicz, J. Phys. G.: Nucl. Part. Phys. 49, 10LT01 (2022).
[5] S. Capra, D. Mengoni, J.A. Dueñas et al., Nucl. Instr. Meth. A 935, 178-184 (2019).
[6] G. Corbari, M. Ciemała, S. Bottoni et al., submitted to Phys. Lett. B.