RadioAxion results on the search for axion dark matter under Gran Sasso

Abstract

We report first results from RadioAxion, an underground experiment searching for axion dark matter through periodic modulations of radioisotope decays. We monitor the $α$ decay of $^{241}\mathrm{Am}$ via its $59.5$ keV $γ$ line using a NaI detector installed at the Gran Sasso Laboratory, where cosmic-ray-induced systematics are strongly suppressed. We present the measured spectra and the corresponding time-series analysis. No evidence for a periodic modulation is observed. From these data we derive constraints on the axion decay constant in the axion mass range from $10^{-21}$ to $10^{-9}$ eV.

Figures (4)

• Figure 1: Schematic (not to scale) of the RadioAxion setup installed at the Gran Sasso Laboratory. A $3"\times 3"$ NaI crystal views the $^{241}$Am source mounted in front of the detector; the scintillation light is read out by a PMT coupled to an ORTEC digiBASE unit. The detector assembly is enclosed in a polyethylene housing and surrounded by passive shielding consisting of $5$ cm oxygen-free high-conductivity copper and $15$ cm lead. Events above threshold are recorded in list mode and sent to the acquisition PC, while timing is stabilized by injecting a 1 Hz reference derived from a rubidium frequency standard (Rb) into the digiBASE.
• Figure 2: $\gamma$-spectrum (counts per second per keV) of the $^{241}$Am source (upper curve) compared to the background (lower curve). The dominant contribution arises from the $\gamma$ at 59.5 keV. The peaks at lower energies are due to the X-rays from $^{237}$Np atomic transitions. The features beyond the $59.5\,\mathrm{keV}$ line arise from: the coincidence of the $59\,\mathrm{keV}$$\gamma$ with $^{237}$Np X-rays (yielding a peak around $75\,\mathrm{keV}$), the $^{237}$Np $\gamma$ lines at $99$ and $103\,\mathrm{keV}$, and the sum of two $59.5\,\mathrm{keV}$$\gamma$ rays.
• Figure 3: Left panel: Differential (green) ad integral (black) distributions of the normalized Fourier amplitudes for short periods ($<1\,\mathrm{s}$). The frequency range is from 1 Hz to 0.5 MHz. The two red horizontal lines are the 68$\%$ and 95$\%$ Confidence Levels. Differential amplitudes are in arbitrary units, integral amplitudes are in percent. The yellow curve shows the Rayleigh distribution expected for the modulus of the complex Fourier amplitude in the absence of an oscillatory signal, since the sine and cosine projections (with random phase) are Gaussian-distributed. Right panel: Same as the left panel, but for long periods ($>1\,\mathrm{s}$), corresponding to frequencies in the range $6.6\times10^{-7}\,\mathrm{Hz}$--$0.5\,\mathrm{Hz}$.
• Figure 4: Constraints on the axion decay constant as a function of the axion mass. The exclusion limits derived in this work are shown in the yellow region, together with the projected sensitivity of a 3-year data taking (orange area). The result is displayed as a continuous band for readability, although the analysis is performed at a discrete set of frequencies, as described in Sect. \ref{['sec:data']}. Limits from other laboratory experiments and astrophysics/cosmology are shown as well for comparison (see text for details). Figure adapted from AxionLimits.