Dark Matter Search Backgrounds from Primordial Radionuclide Chain Disequilibrium

Dark matter direct-detection searches for weakly interacting massive particles (WIMPs) are commonly limited in sensitivity by neutron and gamma backgrounds from the decay of radioactive isotopes. Several common radioisotopes in detector construction materials are found in long decay chains, notably those headed by 238U, 235U, and 232Th. Gamma radioassay using Ge detectors identifies decay rates of a few of the radioisotopes in each chain, and typically assumes that the chain is in secular equilibrium. If the chains are out of equilibrium, detector background rates can be elevated significantly above expectation. In this work we quantify the increase in neutron and gamma production rates from an excess of various sub-chains of the 238U decay chain. We find that the 226Ra sub-chain generates x10 higher neutron flux per decay than the 238U early sub-chain and 210Pb sub-chain, in materials with high (alpha,n) neutron yields. Typical gamma screening results limit potential 238U early sub-chain activity to x20-60 higher than 226Ra sub-chain activity. Monte Carlo simulation is used to quantify the contribution of the sub-chains of 238U to low-energy nuclear recoil (NR) and electron recoil (ER) backgrounds in simplified one tonne liquid Ar and liquid Xe detectors. NR and ER rates generated by 238U sub-chains in the Ar and Xe detectors are found after comparable fiducial and multiple-scatter cuts. The Xe detector is found to have x12 higher signal-to-background for 100 GeV WIMPs over neutrons than the Ar detector. ER backgrounds in both detectors are found to increase weakly for excesses of 238U early sub-chain and 210Pb sub-chain relative to 226Ra sub-chain. Experiments in which backgrounds are NR-dominated are sensitive to undetected excesses of 238U early sub-chain and 210Pb sub-chain concentrations. Experiments with ER-dominated backgrounds are relatively insensitive to these excesses.