The RadioAxion experiment aims to detect axion dark matter by observing time-modulated changes in the decay constants of Americium-241 (alpha decay) and Potassium-40 (electron-capture decay). In order to study the decays independently of the comic ray flux, in particular of its time variation during the year (a few percent ), we installed the experimental set-up deep underground in the Gran Sasso Laboratory,  in a container placed in front of Hall B.

The axion-gluon interaction affects the mass of the pion, sigma and omega particles, which are the mediators of the strong nuclear force. In particular, it changes the nuclear binding energy and the mass difference between the proton and the neutron. These time modulations take place with a frequency proportional to the axion mass: as an example, a 1 neV mass corresponds to a frequency of about 1 MHz .

We started our investigation from the study of the decay of Americium-241 (half-life: 432.2 y). In particular, we had at first to determine the relation of the time modulation amplitude to the coupling between the axion and the gluon [1]. Then, we built a rather simple set-up with a core made from a 3’’ x 3’’ NaI crystal to detect the gamma and X rays emitted from the alpha decay of Americium-241 to Neptunium-237 (mostly a 59.5 keV gamma). We expect to achieve a sensitivity of a few parts per million on the modulation amplitude in a wide range of oscillation periods, from microseconds to a year, enabling sensitive constraints on the axion decay constant across a large axion mass range between 10^-9 and 10^-22 eV with a data acquisition time of 3 years.

For the Potassium-40 electron-capture decay (half-life: 1.25 billion years) we have to determine the relation of the time modulation amplitude to the coupling between the axion and the gluon and then, in early 2025, we will start the study of the decay under Gran Sasso with a large NaI crystal (4 liters) surrounded by an extended Potassium-40 source. The signature of the decay will be the 1461 keV gamma ray from the excited state of the Argon-40 daughter nucleus. The expected sensitivity is similar to the one previously given for the Americium-241 within the same mass range.

Finally, as possible future development of the project, we are exploring the possibility to produce axions in a nuclear reaction at the Gran Sasso Ion Beam Facility and to detect them in one of the already existing large underground detectors.

[1] C. Broggini, G. Di Carlo, L. Di Luzio, C. Toni, Alpha radioactivity deep-underground as a probe of axion dark matter, Phys. Lett. B 855 (2024) 138836. arXiv:2404.18993, doi:10.1016/j.physletb.2024.138836