91̽»¨

Abstract:

Measuring radon emanation from detector materials is a key method for controlling radon, a significant background in rare-event physics experiments. Methods for measuring radon emanation are well-established but have predominantly focused on the²²²Rn isotope, the dominant radon isotope for these backgrounds. However, measurements of²²⁰Rn (thoron), the second most abundant radon isotope, remain relatively unexplored.²²⁰Rn emanation measurements are challenging because the²²⁰Rn must be transferred from the emanation chamber to the active detector within its short 55 s half-life. In this study, a direct in-chamber approach for measuring²²⁰Rn emanation is presented in which the sample is placed directly within the active detector chamber, thereby minimising losses during transfer. The method was demonstrated with a DURRIDGE RAD8 electrostatic radon detector, which measured²²⁰Rn emanation from low-activity thoriated rods with an activity of 76 ± 20 mBq. Compared with a conventional flowthrough²²⁰Rn emanation setup, the in-chamber method increased sensitivity by a factor of 3. Using helium as the carrier gas provided a further sensitivity increase, giving an overall sensitivity gain of ∼ 5. These results indicate that in-chamber²²⁰Rn emanation measurements provide an effective tool for low-background experiments and have the potential to accelerate radon studies by exploiting the shorter half-life of²²⁰Rn.

Abstract:

We report results of a search for nuclear recoils induced by weakly interacting massive particle (WIMP) dark matter using the LUX-ZEPLIN (LZ) two-phase xenon time projection chamber. This analysis uses a total exposure of $4.2Â\pm 0.1$ tonne-years from 280 live days of LZ operation, of which $3.3Â\pm 0.1$ tonne-years and 220 live days are new. A technique to actively tag background electronic recoils from ${}^{214}\mathrm{Pb}$ $\mathrm{β}$ decays is featured for the first time. Enhanced electron-ion recombination is observed in two-neutrino double electron capture decays of ${}^{124}\mathrm{Xe}$ , representing a noteworthy new background. After removal of artificial signal-like events injected into the dataset to mitigate analyzer bias, we find no evidence for an excess over expected backgrounds. World-leading constraints are placed on spin-independent (SI) and spin-dependent WIMP-nucleon cross sections for masses $â‰\yen 9\mathrm{GeV}/c^2$ . The strongest SI exclusion set is $2.2×{10}^{−48}{\mathrm{cm}}^2$ at the 90% confidence level and the best SI median sensitivity achieved is $5.1×{10}^{−48}{\mathrm{cm}}^2$ , both for a mass of $40\mathrm{GeV}/c^2$ . Published by the American Physical Society 2025