Muon-Induced Neutrons Do Not Explain the DAMA Data

We present an accurate model of the muon-induced background in the DAMA/LIBRA experiment. Our work challenges proposed mechanisms which seek to explain the observed DAMA signal modulation with muon-induced backgrounds. Muon generation and transport are performed using the $\mathrm{MUSIC}/\mathrm{MUSUN}$ code, and subsequent interactions in the vicinity of the DAMA detector cavern are simulated with Geant4. We estimate the total muon-induced neutron flux in the detector cavern to be ${\mathrm{\Phi }}_n^{\nu }=1.0\times {10}^{-9}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. We predict $3.49\times {10}^{-5}\mathrm{counts}/\mathrm{day}/\mathrm{kg}/\mathrm{keV}$, which accounts for less than 0.3% of the DAMA signal modulation amplitude.

Figure 1

The distribution of ${\mathrm{\Phi }}_n^{\mu }$ as a function of neutron energy for neutrons entering the cavern in which DAMA/LIBRA is situated (solid lines) and entering the DAMA/LIBRA detector modules after all shielding is traversed (dashed line). The distributions excluding and including backscattered neutrons are shown in black and red, respectively.

Figure 2

The energy spectrum for all crystals with energy depositions, with resolution, and nuclear recoil quenching factors applied. The equivalent of 20 y of muon-induced data are presented.

Figure 3

The distribution of the hit multiplicity in events with $E_{\mathrm{Dep}}\ge 2\mathrm{keV}$ per crystal. At least one crystal has a total energy absorption in the range 2–6 keV. The equivalent of 20 y of muon-induced data is presented.

Figure 4

The distribution of the total energy deposition in single hit events. The blue dashed line and the black solid line show the sum of all energy depositions before and after energy resolution is considered, respectively. The stacked colored bars indicate the relative fraction of all events (before energy resolution is considered) attributed to events in which only neutrons deposit energy (yellow) and other events (blue). The equivalent of 20 y of muon-induced data is presented.