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. Consequently, the decay constant of radioactive nuclei is modulated in time. These 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 axion-gluon coupling [1]. Then, we built a rather simple set-up with 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). With a few months of data and a sensitivity of about 10-5 we observe no evidence for a periodic modulation [2]. We just started the second phase of the experiment, with a Cerium Bromide crystal as detector and a stronger Americium-241 source, to achieve a sensitivity of  1 part per million on the modulation amplitude in a wide range of oscillation periods, from 1 microsecond to 1 year. In the end, we will reach sensitive constraints on the axion decay constant across a large axion mass range between 10^-9 and 10^-22 eV with a 3-year data acquisition time.

Also for the Potassium-40 electron-capture decay (half-life: 1.25 billion years) we had develop the theoretical framework to determine the relation of the time modulation amplitude of the decay constant to the axion-gluon coupling [3]. We just started 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 explored the possibility to produce scalars in a nuclear reaction at the Gran Sasso Bellottti Ion Beam Facility and to detect them in one of the already existing large underground detectors [4].

[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

[2] C. Broggini, G. Di Carlo, L. Di Luzio, D. Piatti, C. Toni, RadioAxion results on the search for axion dark matter under Gran Sasso, accepted for publication on Phys. Rev D, arXiv:2602.18392v2

[3] J. Alda, C. Broggini, G. Di Carlo, L. Di Luzio, D. Piatti, S. Rigolin, C. Toni, Weak nuclear decays deep underground as a probe of axion dark matter, Phys. Rev. D 111 (2025) 035022, arXiv:2412.20932

[4] C. Broggini, G. Di Carlo, L. Di Luzio, D. Piatti, C. Toni, Probing a light scalar boson with a few MeV proton beam deep underground, Phys. Rev.D 112 (2025) 9, 095037, arXiv:2509.03486v2