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...

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.

Transmission nuclear resonance fluorescence measurements were made on targets consisting of Pb and depleted U with total areal densities near 86 g/cm 2 . The 238 U content in the targets varied from 0 to 8.5% (atom fraction). The experiment demonstrates the capability of using transmission measurements as a non-destructive technique to identify and quantify the presence of an isotope in samples with thicknesses comparable to the average thickness of a nuclear fuel assembly. The experimental data also appear to demonstrate the process of notch refilling with a predictable intensity. Comparison of measured spectra to previous backscatter 238 U measurements indicates general agreement in observed excited states. Evidence of two new 238 U excited states and possibly a third state have also been observed.

The design of a neutron source capable of producing 24 and 70 keV neutron beams with narrow energy spread is presented. The source exploits near-threshold kinematics of the $^{7}$Li(p,n)$^{7}$Be reaction while taking advantage of the interference `notches' found in the scattering cross-sections of iron. The design was implemented and characterized at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. Alternative filters such as vanadium and manganese are also explored and the possibility of studying the response of different materials to low-energy nuclear recoils using the resultant neutron beams is discussed