Prediction of Underground Argon Content for Dark Matter Experiments

In this paper, we demonstrate the use of physical models to evaluate the production of $^{39}$Ar and $^{40}$Ar underground. Considering both cosmogenic $^{39}$Ar production and radiogenic $^{40}$Ar production in situ and from external sources, we can derive the ratio of $^{39}$Ar to $^{40}$Ar in underground sources. We show for the first time that the $^{39}$Ar production underground is dominated by stopping negative muon capture on $^{39}$K and ($alpha,n)$ induced subsequent $^{39}$K(n,p)$^{39}$Ar reactions. The production of $^{39}$Ar is shown as a function of depth. We demonstrate that argon depleted in $^{39}$Ar can be obtained only if the depth of the underground resources is greater than 500 m.w.e. below the surface. Stopping negative muon capture on $^{39}$K dominates over radiogenic production at depths of less than 2000 m.w.e., and that production by muon-induced neutrons is subdominant at any depth. The depletion factor depends strongly on both radioactivity level and potassium content in the rock. We measure the radioactivity concentration and potassium concentration in the rock for a potential site of an underground argon source in South Dakota. Depending on the probability of $^{39}$Ar and $^{40}$Ar produced underground being dissolved in the water, the upper limit of the concentration of $^{39}$Ar in the underground water at this site is estimated to be in a range of a factor of 1.6 to 155 less than the $^{39}$Ar concentration in the atmosphere. The calculation tools presented in this paper are also critical to the dating method with $^{39}$Ar.

Cosmogenic Production as a Background in Searching for Rare Physics Processes

We revisit calculations of the cosmogenic production rates for several long-lived isotopes that are potential sources of background in searching for rare physics processes such as the detection of dark matter and neutrinoless double-beta decay. Using updated cosmic-ray neutron flux measurements, we use TALYS 1.0 to investigate the cosmogenic activation of stable isotopes of several detector targets and find that the cosmogenic isotopes produced inside the target materials and cryostat can result in large backgrounds for dark matter searches and neutrinoless double-beta decay. We use previously published low-background HPGe data to constrain the production of $^{3}H$ on the surface and the upper limit is consistent with our calculation. We note that cosmogenic production of several isotopes in various targets can generate potential backgrounds for dark matter detection and neutrinoless double-beta decay with a massive detector, thus great care should be taken to limit and/or deal with the cosmogenic activation of the targets.

Neutron Inelastic Scattering Processes as Background for Double-Beta Decay Experiments

We investigate several Pb$(n,ngamma$) and Ge$(n,ngamma$) reactions. We measure $gamma$-ray production from Pb$(n,ngamma$) reactions that can be a significant background for double-beta decay experiments which use lead as a massive inner shield. Particularly worrisome for Ge-based double-beta decay experiments are the 2041-keV and 3062-keV $gamma$ rays produced via Pb$(n,ngamma$). The former is very close to the ^{76}Ge double-beta decay endpoint energy and the latter has a double escape peak energy near the endpoint. Excitation $gamma$-ray lines from Ge$(n,ngamma$) reactions are also observed. We consider the contribution of such backgrounds and their impact on the sensitivity of next-generation searches for neutrinoless double-beta decay using enriched germanium detectors.