The ACROMASS project for the study of the charged components of atmospheric cosmic radiation

In the second half of the 20th century, experimental research into cosmic rays in the Earth's atmosphere—particularly the measurement of muon fluxes—experienced significant growth. This led to a wide range of measurements conducted under diverse conditions. However, the literature still lacks a comprehensive and systematic study of the energy and angular spectra of atmospheric muons as functions of geographic location, altitude, and solar cycle phase. Consequently, the study of atmospheric muons remains relevant, especially in two key areas of physics. The first is neutrino physics. Accurate determination of neutrino oscillation parameters through the study of atmospheric neutrinos requires detailed knowledge of their production spectra. This can be achieved through simulation tools that are calibrated using precise measurements of atmospheric muon spectra. The second area is muon radiography, a non-invasive imaging technique that uses atmospheric muons to generate radiographic images of large volumes of material. This technique depends on reliable simulations of muon fluxes and their interactions with matter, including absorption effects. Between the late 1990s and early 2000s, the Florence Section of the National Institute for Nuclear Physics (INFN) and the Department of Physics at the University of Florence designed and built the ADAMO detector—a powerful magnetic spectrometer originally intended as a test system for the PAMELA satellite experiment. Independently of PAMELA, ADAMO was used in 2004 to carry out a preliminary measurement of the all-particle cosmic ray spectrum at ground level, across different zenith angles, and within a momentum range from 100 MeV/$c$ to 130 GeV/$c$. Approved by INFN in 2024, the ACROMASS project involves the development of a compact, transportable detector based on the ADAMO magnetic spectrometer, augmented by two auxiliary particle identification systems. Its primary objective is to study atmospheric cosmic rays at various locations and altitudes. Building on the experience gained from the PAMELA and ADAMO projects, ACROMASS aims initially to perform precise measurements of muon spectra at altitudes between 3,500 and 4,000 m a.s.l. These measurements will help calibrate simulations used to estimate atmospheric neutrino spectra, thereby reducing uncertainties in the determination of oscillation parameters and improving estimates of the atmospheric neutrino background—crucial for identifying neutrinos originating beyond the Earth. Construction of the detector is currently underway and is expected to be completed between 2025 and 2026. A fully operational prototype will undergo testing with particle beams by the end of 2025.