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1. Separating $${^{39}\hbox {Ar}}$$ from $${^{40}\hbox {Ar}}$$ by cryogenic distillation with Aria for dark-matter searches

   https://doi.org/10.1140/epjc/s10052-021-09121-9  

   Agnes, P. ; Albergo, S. ; Albuquerque, I. F. M. ; Alexander, T. ; Alici, A. ; Alton, A. K. ; Amaudruz, P. ; Arba, M. ; Arpaia, P. ; Arcelli, S. ; et al ( April 2021 , The European Physical Journal C)

   Aria is a plant hosting a$${350}\,\hbox {m}$$$350\text{m}$cryogenic isotopic distillation column, the tallest ever built, which is being installed in a mine shaft at Carbosulcis S.p.A., Nuraxi-Figus (SU), Italy. Aria is one of the pillars of the argon dark-matter search experimental program, lead by the Global Argon Dark Matter Collaboration. It was designed to reduce the isotopic abundance of$${^{39}\hbox {Ar}}$$${}^{39}\text{Ar}$in argon extracted from underground sources, called Underground Argon (UAr), which is used for dark-matter searches. Indeed,$${^{39}\hbox {Ar}}$$${}^{39}\text{Ar}$is a$$\beta $$$\beta$-emitter of cosmogenic origin, whose activity poses background and pile-up concerns in the detectors. In this paper, we discuss the requirements, design, construction, tests, and projected performance of the plant for the isotopic cryogenic distillation of argon. We also present the successful results of the isotopic cryogenic distillation of nitrogen with a prototype plant.

    

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2. Probing the Rock Mass Fraction and Transport Efficiency inside Uranus Using 40 Ar Measurements

   https://doi.org/10.3847/PSJ/ad3b93  

   Nimmo, Francis ; Lunine, Jonathan ; Zahnle, Kevin ; Stixrude, Lars ( May 2024 , The Planetary Science Journal)

   The bulk of Uranus consists of a rock–ice core, but the relative proportions of rock and ice are unknown. Radioactive decay of potassium in the silicates produces⁴⁰Ar. If transport of argon from the core to the gaseous envelope is efficient, a measurement of⁴⁰Ar in the envelope will provide a direct constraint on the rock mass present (assuming a chondritic rock composition). The expected⁴⁰Ar concentrations in this case would be readily detectable by a mass spectrometer carried by a future atmospheric probe. For a given envelope concentration there is a trade-off between the rock mass present and the transport efficiency; this degeneracy could be overcome by making independent determinations of the rock mass (e.g., by gravity and seismology). Primordial⁴⁰Ar is a potential confounding factor, especially if Ar/H₂is significantly enhanced above solar or if degassing of radiogenic⁴⁰Ar were inefficient. Unfortunately, the primordial⁴⁰Ar/³⁶Ar ratio is very uncertain; better constraints on this ratio through measurement or theory would be very helpful. Pollution of the envelope by silicates is another confounding factor but can be overcome by a measurement of the alkali metals in the envelope.

    

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3. A study of events with photoelectric emission in the DarkSide-50 liquid argon Time Projection Chamber

   The DarkSide-50 Collaboration) Agnes, P. ; Albuquerque, I. F. ; Alexander, T. ; Alton, A. K. ; Ave, M. ; Back, H. O. ; Batignani, G. ; Biery, K. ; Bocci, V. ; Bonivento, W. M. ; et al ( July 2021 , ArXivorg)

   null (Ed.)

   Finding unambiguous evidence of dark matter interactions in a particle detector is a main objective of physics research. The liquid argon time projection chamber technique for the detection of Weakly Interacting Massive Particles (WIMP) allows sensitivities down to the so-called neutrino floor for high and low WIMP masses. Based on the successful operation of the DarkSide-50 detector, a new and more sensitive experiment, DarkSide-20k, was designed and is now under construction. A thorough understanding of the DarkSide-50 detector response to events classified as dark matter as well as all other interactions is essential for an optimal design of the new experiment. In this paper, we report on a particular set of events, for which scintillation-ionization signals are observed in association with signals from single or few isolated electrons. We identified and provided an interpretation for two event types in which electrons are produced via photoelectric effect on the cathode electrode and in the bulk liquid. Events with photoelectric emissions are observed in association with most interactions with large energy depositions in the detector. From the measured rate of these events, we determine the photo-ionization probability, or photoelectric quantum efficiency, of tetraphenyl butadiene (TPB) at wavelengths around 128 nm. 

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4. Argon constraints on the early growth of felsic continental crust

   https://doi.org/10.1126/sciadv.aaz6234  

   Guo, Meng ; Korenaga, Jun ( May 2020 , Science Advances)

   null (Ed.)

   The continental crust is a major geochemical reservoir, the evolution of which has shaped the surface environment of Earth. In this study, we present a new model of coupled crust-mantle-atmosphere evolution to constrain the growth of continental crust with atmospheric 40 Ar/ 36 Ar. Our model is the first to combine argon degassing with the thermal evolution of Earth in a self-consistent manner and to incorporate the effect of crustal recycling and reworking using the distributions of crustal formation and surface ages. Our results suggest that the history of argon degassing favors rapid crustal growth during the early Earth. The mass of continental crust, highly enriched in potassium, is estimated to have already reached >80% of the present-day level during the early Archean. The presence of such potassium-rich, likely felsic, crust has important implications for tectonics, surface environment, and the regime of mantle convection in the early Earth. 

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5. Calibration of the liquid argon ionization response to low energy electronic and nuclear recoils with DarkSide-50

   The DarkSide collaboration) Agnes, P. ; Albuquerque, I. F. ; Alexander, T. ; Alton, A. K. ; Ave, M. ; Back, H. O. ; Batignani, G. ; Biery, K. ; Bocci, V. ; Bonivento, W. M. ; et al ( July 2021 , ArXivorg)

   null (Ed.)

   DarkSide-50 has demonstrated the high potential of dual-phase liquid argon time projection chambers in exploring interactions of WIMPs in the GeV/c2 mass range. The technique, based on the detection of the ionization signal amplified via electroluminescence in the gas phase, allows to explore recoil energies down to the sub-keV range. We report here on the DarkSide-50 measurement of the ionization yield of electronic recoils down to about 180 eVer, exploiting 37Ar and 39Ar decays, and extrapolated to a few ionization electrons with the Thomas-Imel box model. Moreover, we present the determination of the ionization response to nuclear recoils down to ∼ 500 eVnr , the lowest ever achieved in liquid argon, using in situ neutron calibration sources and external datasets from neutron beam experiments. 

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