This Letter details a measurement of the ionization yield (Q y ) of 6.7 keV 40 Ar atoms stopping in a liquid argon detector. The Q y of 3.6-6.3 detected e − /keV, for applied electric fields in the range 240-2130 V/cm, is encouraging for the use of this detector medium to search for the signals from hypothetical dark matter particle interactions and from coherent elastic neutrino-nucleus scattering. A significant dependence of Q y on the applied electric field is observed and explained in the context of ion recombination. PACS numbers: 95.35.+d, 25.30.Pt, 34.50.Fa, 29.40.Mc Liquid-phase argon has long been used as a target medium for particle detection via scintillation and charge collection. Recently there has been considerable interest in direct detection of both hypothetical dark matter particles [1] and coherent elastic neutrino-nucleus scattering (CENNS) [2,3]. These as-yet unobserved neutral particle interactions are expected to result in a recoiling argon atom O(keV), generally referred to in the literature as a nuclear recoil. This prompts the question of the available signal produced by such recoils in a liquid argon detector. This quantity must be directly measured due to the difference in signals from nuclear recoils as opposed to electron recoils (e.g. Compton electrons and β-particles). In this Letter we report the first measurement of the ionization yield (Q y ) (detected electrons per unit energy) resulting from nuclear recoils in liquid argon, measured at 6.7 keV. This is also the lowest-energy measurement of nuclear recoils in liquid argon.These results are of interest not only for particle detection, but for theoretical studies of condensed media as well. Models of the production of ions and excited atoms from low-energy recoils in liquid argon exist, but are not fully understood in the few-keV energy range [4]. To study the influence of the electric field on recombination, and thus Q y , data were obtained at applied electric field values of 240, 640, 1600, 2130 V/cm.The scintillation efficiency of nuclear recoils in liquid argon has been measured from 10-250 keV at zero electric drift field using the kinematically constrained scatter of 2.8 MeV neutrons [5] and from 11-50 keV at electric drift fields from 0-1000 V/cm using the kinematically constrained scatter of 0.60 and 1.17 MeV neutrons [6]. No measurements of nuclear recoils in liquid argon exist below 10 keV.Liquid argon dual-phase detectors have been shown to be sensitive to single electrons generated in the bulk [7]. This enhances the detection capability of the ionization channel over the scintillation channel at very low energies. A lowenergy threshold and calibration are critical in both dark matter searches and CENNS discovery. Both interactions exhibit a recoil energy spectrum that rises rapidly with decreasing energy [4,8,9]. Our results suggest that dark matter searches using only the ionization channel in liquid argon (as has been done in liquid xenon [10]) could probe an interesting new parameter space. The observation and...

Unexplained periodic fluctuations in the decay rates of 32 Si and 226 Ra have been reported by groups at Brookhaven National Laboratory (32 Si), and at the Physikalisch-Technische-Bundesandstalt in Germany (226 Ra). We show from an analysis of the raw data in these experiments that the observed fluctuations are strongly correlated in time, not only with each other, but also with the distance between the Earth and the Sun. Some implications of these results are also discussed, including the suggestion that discrepancies in published half-life determinations for these and other nuclides may be attributable in part to differences in solar activity during the course of the various experiments, or to seasonal variations in fundamental constants. Following the discovery of radioactivity by Becquerel in 1896 [1] an intense effort was mounted to ascertain whether the decay rates of nuclides could be affected by external influences including temperature, pressure , chemical composition, concentration, and magnetic fields. By 1930, Rutherford, Chadwick, and Ellis [2, p. 167] concluded that "The rate of transformation of an element has been found to be a constant under all conditions." (For decays resulting from K-capture, or for beta-decays in strong ambient electromagnetic fields, the situation is slightly more complicated, since these decays are influenced by the electron wave functions which can be affected by external pressure or fields [3, 4, 5].) For 32 Si and 226 Ra, which decay by beta-and alpha-emission, respectively, fluctuations in the counting rates (in the absence of strong external electromagnetic fields) should thus be uncorrelated with any external time-dependent signal, as well as with each other. In what follows we show that neither of these expectations is realized in data we have analyzed for 32 Si and 226 Ra, thus suggesting that these decays are in fact being modulated by an external influence. Between 1982 and 1986, Alburger, et al. [6] measured the half-life of 32 Si at Brookhaven National Laboratory (BNL) via a direct measurement of the counting rate as a function of time. If N (t) denotes the number of surviving atoms starting from an initial population N 0 at t = 0, then the familiar exponential decay law, N (t) = N 0 e −λt , leads to ˙ N ≡ dN/dt = −λN 0 e −λt where λ = ln(2)/T 1/2. A plot of ln ˙ N (t) as a function of time is then a straight line whose slope is λ, which then gives the half-life T 1/2. At the time this experiment was initiated, the 32 Si half-life was believed to be in the range of 60 T 1/2 700 yr, and hence a multi-year counting experiment was needed to obtain a mea-sureable slope. As in other counting experiments, the counting rate for 32 Si was continually monitored in the same detector against a long-lived comparison standard, which in the BNL experiment was 36 Cl (T 1/2 =301,000 yr). Since the fractional change in the 36 Cl counting rate over the four year duration of the experiment was only O(10 −5), which was considerably smaller than the overall uncertainty of the final...

We describe the first demonstration of a sub-keV electron recoil energy threshold in a dual-phase liquid argon time projection chamber. This is an important step in an effort to develop a detector capable of identifying the ionization signal resulting from nuclear recoils with energies of order a few keV and below. We obtained this result by observing the peaks in the energy spectrum at 2.82 keV and 0.27 keV, following the K-and L-shell electron capture decay of 37 Ar, respectively. The 37 Ar source preparation is described in detail, since it enables calibration that may also prove useful in dark matter direct detection experiments. An internally placed 55 Fe x-ray source simultaneously provided another calibration point at 5.9 keV. We discuss the ionization yield and electron recombination in liquid argon at those three calibration energies.