Is DAMA Bathing in a Sea of Radioactive Argon?

TL;DR

This paper hypothesizes that argon impurities in DAMA/LIBRA's nitrogen purge gas could cause a modulating 2.8 keV background, potentially explaining the long-standing DAMA/LIBRA puzzle.

Contribution

It introduces a novel hypothesis linking argon impurities in the purge gas to the observed modulation signal in DAMA/LIBRA.

Findings

Argon impurities can produce a 2.8 keV background via decay of 37Ar.

Radon in the purge gas may modulate background signals seasonally.

Testing methods include gas assay and laboratory studies of detector housings.

Abstract

A hypothesis is proposed to explain the long-standing DAMA/LIBRA puzzle. Introduced into the DAMA/LIBRA shielding is a purge gas of nominally high-purity nitrogen, which under this hypothesis contains argon impurities. Argon is introduced into the nitrogen purge gas either through leaks in the purge gas plumbing, or through commercially-supplied bottled nitrogen, diffuses through materials in the detector housings, and then comes in direct contact with the DAMA/LIBRA detectors. These argon impurities can then lead to a modulating 2.8 keV background under two scenarios. Scenario 1): These impurities include the isotope 37Ar, which decays by electron capture, emitting a 2.8 keV x-ray. These decays appear as single-site, monoenergetic events in DAMA/LIBRA, and produce an annual modulation due to the variation of neutron flux in the atmosphere and at the Earth's surface, which in turn leads to a seasonal variation in 37Ar production from the reactions $^{40}\textrm{Ca}(\textrm{n,}\alpha)^{37}\textrm{Ar}$ and $^{36}\textrm{Ar}(\textrm{n,}\gamma)^{37}\textrm{Ar}$. Scenario 2): Radon is also in the DAMA/LIBRA purge gas, modulating seasonally at a rate below the current DAMA/LIBRA limits. When radon or its short-lived daughters decay, the resulting beta, gamma, and bremsstrahlung radiation cause stable 40Ar to be ionized within the copper housings surrounding the NaI(Tl) detectors, resulting in characteristic 2.8 keV x-rays. Modulating backgrounds might also result from radon-induced neutron or gamma-ray flux from the surrounding cavern, leading to a small modulating background enhanced at low energy by the presence of 40Ar within the copper housings. These two scenarios are straightforward to test through assay of the purge gas as well as Monte Carlo and laboratory study of the DAMA/LIBRA copper housings when excited by ionizing radiation.