Argon Target Research Articles

Atomic responses to general dark matter-electron interactions

In the leading paradigm of modern cosmology, about 80% of our Universe's matter content is in the form of hypothetical, as yet undetected particles. These do not emit or absorb radiation at any observable wavelengths, and therefore constitute the so-called Dark Matter (DM) component of the Universe. Detecting the particles forming the Milky Way DM component is one of the main challenges for astroparticle physics and basic science in general. One promising way to achieve this goal is to search for rare DM-electron interactions in low-background deep underground detectors. Key to the interpretation of this search is the response of detectors' materials to elementary DM-electron interactions defined in terms of electron wave functions' overlap integrals. In this work, we compute the response of atomic argon and xenon targets used in operating DM search experiments to general, so far unexplored DM-electron interactions. We find that the rate at which atoms can be ionized via DM-electron scattering can in general be expressed in terms of four independent atomic responses, three of which we identify here for the first time. We find our new atomic responses to be numerically important in a variety of cases, which we identify and investigate thoroughly using effective theory methods. We then use our atomic responses to set 90% confidence level (C.L.) exclusion limits on the strength of a wide range of DM-electron interactions from the null result of DM search experiments using argon and xenon targets.

GLAD Magnetron Sputtered Ultra-Thin Copper Oxide Films for Gas-Sensing Application

Copper oxide (CuO) ultra-thin films were obtained using magnetron sputtering technology with glancing angle deposition technique (GLAD) in a reactive mode by sputtering copper target in pure argon. The substrate tilt angle varied from 45 to 85° and 0°, and the sample rotation at a speed of 20 rpm was stabilized by the GLAD manipulator. After deposition, the films were annealed at 400 °C/4 h in air. The CuO ultra-thin film structure, morphology, and optical properties were assessed by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray reflectivity (XRR), and optical spectroscopy. The thickness of the films was measured post-process using a profilometer. The obtained copper oxide structures were also investigated as gas-sensitive materials after exposure to acetone in the sub-ppm range. After deposition, gas-sensing measurements were performed at 300, 350, and 400 °C and 50% relative humidity (RH) level. We found that the sensitivity of the device is related to the thickness of CuO thin films, whereas the best results are obtained with an 8 nm thick sample.

Effective field theory interactions for liquid argon target in DarkSide-50 experiment

We reanalize data collected with the DarkSide-50 experiment and recently used to set limits on the spin-independent interaction rate of weakly interacting massive particles (WIMPs) on argon nuclei with an effective field theory framework. The dataset corresponds to a total (16660 $\pm$ 270) kg d exposure using a target of low-radioactivity argon extracted from underground sources. We obtain upper limits on the effective couplings of the 12 leading operators in the nonrelativistic systematic expansion. For each effective coupling we set constraints on WIMP-nucleon cross sections, setting upper limits between $2.4 \times 10^{-45} \, \mathrm{cm}^2$ and $2.3 \times 10^{-42} \, \mathrm{cm}^2$ (8.9 $\times 10^{-45} \, \mathrm{cm}^2$ and 6.0 $\times 10^{-42} \, \mathrm{cm}^2$) for WIMPs of mass of 100 $\mathrm{GeV/c^2}$ (1000 $\mathrm{GeV/c^2}$) at 90\% confidence level.

Design and construction of a new detector to measure ultra-low radioactive-isotope contamination of argon

Large liquid argon detectors offer one of the best avenues for the detection of galactic weakly interacting massive particles (WIMPs) via their scattering on atomic nuclei. The liquid argon target allows exquisite discrimination between nuclear and electron recoil signals via pulse-shape discrimination of the scintillation signals. Atmospheric argon (AAr), however, has a naturally occurring radioactive isotope, 39Ar, a β emitter of cosmogenic origin. For large detectors, the atmospheric 39Ar activity poses pile-up concerns. The use of argon extracted from underground wells, deprived of 39Ar, is key to the physics potential of these experiments. The DarkSide-20k dark matter search experiment will operate a dual-phase time projection chamber with 50 tonnes of radio-pure underground argon (UAr), that was shown to be depleted of 39Ar with respect to AAr by a factor larger than 1400. Assessing the 39Ar content of the UAr during extraction is crucial for the success of DarkSide-20k, as well as for future experiments of the Global Argon Dark Matter Collaboration (GADMC). This will be carried out by the DArT in ArDM experiment, a small chamber made with extremely radio-pure materials that will be placed at the centre of the ArDM detector, in the Canfranc Underground Laboratory (LSC) in Spain. The ArDM LAr volume acts as an active veto for background radioactivity, mostly γ-rays from the ArDM detector materials and the surrounding rock. This article describes the DArT in ArDM project, including the chamber design and construction, and reviews the background required to achieve the expected performance of the detector.

Backgrounds in the DEAP-3600 Dark Matter Experiment

The DEAP-3600 experiment, located at SNOLAB, is searching for dark matter with a single phase liquid argon (LAr) target. For a background-free exposure of 3000 kg·yr, the projected sensitivity to the spin-independent WIMP-nucleon cross section at 100 GeV/c2 WIMP mass is 10−46 cm2.The experimental signature of dark matter interactions is keV-scale argon recoils producing 128 nm LAr scintillation photons which are wavelength shifted and observed by 255 PMTs. To reach the large background-free exposure, a combination of careful material selection, passive shielding, active vetoes, fiducialization and pulse shape discrimination (PSD) is used. The main concept of the background rejection in DEAP-3600 is the powerful PSD, employing the large difference between fast and slow components of LAr scintillation light. The designed background level of DEAP-3600 is less than 0.6 events in a 3000 kg·yr exposure. The experiment was filled in November 2016 and is currently taking dark matter search data.

Energy response and position reconstruction in the DEAP-3600 dark matter experiment

DEAP-3600 is a dark matter WIMP (Weakly Interacting Massive Particles) search experiment, which aims to detect nuclear recoils from WIMP scattering in a liquid argon target. At WIMP masses of 100 GeV, DEAP-3600 has a projected sensitivity of 10−46 cm2 for the spin-independent elastic scattering cross section of WIMPs. The beta emissions from the intrinsic 39 Ar present in the natural Ar target, as well as external calibration sources, can be used to tune the energy calibration and position reconstruction in the energy region of interest (ROI) for WIMP signals. These proceedings will present the techniques used to determine the energy response and position reconstruction in DEAP-3600. Using these techniques, the light yield was found to be LY = (math) (fit syst.) ± 0.22(SPE syst.) PE/keVee with a fiducial mass for WIMP detection of 2 223 ± 74 kg for the first data set.

GAS-PUFF TARGET FOR LASER-PRODUCING PLASMA SXR SOURCE

Mass density spatial distributions of selected gas-puff targets have been determined by radiography, using quasi-monochromatic radiation at a wavelength of 13 nm. Results of spatial distributions of mass densities at various initial reservoir pressures for Nitrogen, Argon and Krypton targets are presented. Changes in the mass density spatial distribution due to a nozzle wear are reported.

AC electrical resonances in nanocomposites with partly oxidized FeCoZr grains embedded in CaF2 ceramic matrix – effects of annealing

This paper presents the frequency and temperature dependences of the phase shift angle, the active part of the admittance, the capacitance, and the dielectric loss factor tanδ for as-deposited and annealed (FeCoZr)x(CaF2)(100-x) nanocomposite films prepared via ion-beam sputtering of a complicated target in a mixed argon–oxygen gas atmosphere. For films with a metallic phase content of x = 62.7 at.%, after 15 min of annealing at a temperature of 398 K, a strong ‘negative capacitance’ effect occurred, indicating an inductive-like contribution to the admittance. This effect was accompanied by two types of resonances: a voltage resonance and two current resonances. It was determined that description of AC parameters of (FeCoZr)x(CaF2)(100-x) films can be made by using compound equivalent circuit, consisting of RLC series circuit and two conventional circuits differing in values of their RLC elements, which means that the material can be used in microelectronics instead of such circuits.The joint analysis of the σ(f), θ(f), Cp(f), and tanδ(f) curves allowed the resonances to be explained on the basis of the previously developed alternating current (AC)/direct current (DC) hopping model with three different characteristic times of electron hopping.

Electromagnetic backgrounds and potassium-42 activity in the DEAP-3600 dark matter detector

The DEAP-3600 experiment is searching for WIMP dark matter with a 3.3 tonne single phase liquid argon (LAr) target, located 2.1 km underground at SNOLAB. The experimental signature of dark matter interactions is keV-scale $^{40}$Ar nuclear recoils (NR) producing 128 nm LAr scintillation photons observed by PMTs. The largest backgrounds in DEAP-3600 are electronic recoils (ER) induced by $\beta$ and $\gamma$-rays originating from internal and external radioactivity in the detector material. A background model of the ER interactions in DEAP-3600 was developed and is described in this work. The model is based on several components which are expected from radioisotopes in the LAr, from ex-situ material assay measurements, and from dedicated independent in-situ analyses. This prior information is used in a Bayesian fit of the ER components to a 247.2 d dataset to model the radioactivity in the surrounding detector materials. While excellent discrimination between ERs and NRs is reached with pulse shape discrimination, utilizing the large difference between fast and slow components of LAr scintillation light, detailed knowledge of the ER background and activity of detector components, sets valuable constraints on other key types of backgrounds in the detector: neutrons and alphas. In addition, the activity of $^{42}$Ar in LAr in DEAP-3600 is determined by measuring the daughter decay of $^{42}$K. This cosmogenically activated trace isotope is a relevant background at higher energies for other rare event searches using atmospheric argon e.g. DarkSide-20k, GERDA or LEGEND. The specific activity of $^{42}$Ar in the atmosphere is found to be $40.4 \pm 5.9$ $\mu$Bq/kg of argon.

Adhesion and dynamic impact wear of nanocomposite TiC-based coatings prepared by DCMS and HiPIMS

Nanocomposite nc-TiC/a-C:H coatings exhibit a unique combination of mechanical properties such as high hardness, and low friction and wear. These physical and mechanical properties make those coatings attractive for application in industry. However, the properties of the whole coating/substrate system such as adhesion of the coating to substrate and its response on repeated impact loading known such as dynamic impact wear are also important for industrial applications. Thus, this paper is focused on the adhesion and the dynamic impact wear of nc-TiC/a-C:H coatings prepared by the hybrid PVD-PECVD process. Two series of nc-TiC/a-C:H coatings with a different amount of carbon were deposited onto commonly used industrial cemented tungsten carbide substrates using DC magnetron sputtering (DCMS) and the high power impulse magnetron sputtering (HiPIMS) of a titanium target in argon and acetylene mixture atmosphere. Both series of coatings were analysed using a scratch test and dynamic impact tester with an impact load of 600 N. The HiPIMS prepared coatings exhibited lower thickness and lower thicknesses of the Ti adhesive interlayers between the substrates and coatings than the DCMS prepared coatings. Thus, the adhesion and the impact wear of both series were discussed separately. These properties were discussed with respect to the coating microstructure, phase composition and mechanical properties such as the hardness H, the effective elastic modulus E, and the H/E and H3/E2 ratios. The scratch adhesion of coatings depended on the H3/E2 ratio and coating microstructure, hardness and surface roughness. The impact wear of the nc-TiC/a-C:H coatings depended on the H/E ratio and coating microstructure.

Assessing a growth anomaly in ion-beam sputtered non-stoichiometric NiOx

Nonstoichiometric NiOx thin films were grown on single crystal substrates of c-plane (0001) sapphires by ion-beam sputter-deposition (IBSD) of a Ni metal target in a mixed argon and oxygen atmosphere. Structural characterization was carried out by X-ray diffraction and scanning electron microscopy. All samples grew (111)-oriented out-of-plane and with a defined in-plane orientation relationship relative to the crystalline substrate. The chemical bonding information of the films was examined by X-ray photoelectron spectroscopy showing that the composition x could be varied by adjusting the oxygen-to-argon ratio in the IBSD process. However, a growth anomaly was detected for a certain range of synthesis parameters, standing out due to an enhanced growth rate, nickel excess, and unusually elongated surface structures. With joint solid-state and plasma diagnostic tools, the underlying processes on the atomic scale were studied. An increased proportion of atomic oxygen species in the intermediate range of the oxygen-to-argon gas flux ratio was identified to be responsible for an enhanced generation of NiOx species. Optical emission spectroscopy was found to be a tool especially well-suited, since the in situ examination of various locations of interest (plasma, ion beam, and vicinity of the target) is feasible nonintrusively.

Photoluminescence response of acrylic (PMMA) and polytetrafluoroethylene (PTFE) to ultraviolet light

Some publications indicate that poly(methyl methacrylate) (PMMA) and polytetrafluoroethylene (PTFE) exhibit low levels of photoluminesence (fluorescence and/or phosphorescence) when irradiated with photons in the ultraviolet (UV) to visible range. PMMA (also known as acrylic) and PTFE are commonly used to contain the liquid argon (LAr) or xenon (LXe) target material in rare-event search experiments. LAr and LXe scintillate in the vacuum UV region, and the PMMA and PTFE can be directly illuminated by these photons. Photoluminescence from support materials could cause unexpected signals in these detectors. We investigate photoluminesence in the 400 nm to 550 nm region in response to excitation with UV light between 130 and 250 nm at levels relevant to rare-event search experiments. Measurements are done at room temperature and the signal intensity is time-integrated over several minutes. We tested PMMA and PTFE samples from the batches used in the DEAP-3600 and LUX experiments and observed no photoluminescence signal. We put limits on the efficiency of the plastics to shift UV photons to a wavelengths region of 400 nm–550 nm at 0.05%–0.35% relative to the wavelength shifting efficiency of tetraphenyl-butadiene.

Database support of detector operation and data analysis in the DEAP-3600 Dark Matter experiment

The DEAP-3600 detector searches for dark matter interactions on a 3.3 tonne liquid argon target. Over nearly a decade, from start of detector construction through the end of the data analysis phase, well over 200 scientists will have contributed to the project. The DEAP-3600 detector will amass in excess of 900 TB of data representing more than 10^{10} particle interactions, a few of which could be from dark matter. At the same time, metadata exceeding 80 GB will be generated. This metadata is crucial for organizing and interpreting the dark matter search data and contains both structured and unstructured information. The scale of the data collected, the important role of metadata in interpreting it, the number of people involved, and the long lifetime of the project necessitate an industrialized approach to metadata management. We describe how the CouchDB and the PostgreSQL database systems were integrated into the DEAP detector operation and analysis workflows. This integration provides unified, distributed access to both structured (PostgreSQL) and unstructured (CouchDB) metadata at runtime of the data analysis software. It also supports operational and reporting requirements.

Characteristics of magnetron sputtering of hot titanium target in argon

In this work, the discharge characteristics of 130 mm diameter planar balanced magnetrons equipped with cold and hot titanium targets were studied. It is found that these devices operating in argon have significantly different I-V characteristics measured in the current density range of 10–100 mA/cm2. At different argon pressures, a hot target magnetron provided a given current at a higher voltage, and its I-V characteristics had maxima. The observed characteristics can be caused by gas heating and its rarefaction near the target, as well as by thermionic emission of the heated target.

Enhancement of discharge and deposition rate in dual-pulse pulsed magnetron sputtering: Effect of ignition pulse width

To increase deposition rate during pulsed magnetron sputtering (PMS), a novel dual-pulse pulsed magnetron sputtering (DP-PMS) was proposed. The discharge mode of DP-PMS firstly produced a pulsed high ignition voltage with short duration followed by a subsequent pulsed low work voltage with long duration. CrN coatings were deposited by DP-PMS and PMS. The effect of ignition pulse width of DP-PMS on the discharge of Cr target in argon and the deposition rate of CrN coatings was explored. The target current of DP-PMS was featured by an initial peak, which was different from the triangular waveform in PMS. The DP-PMS with a longer ignition pulse produced a higher substrate current and a larger number of Cr0 at unit target power. There existed a proper width of initial ignition pulse (e.g. 15 us) producing the maximum deposition rate at unit target power, which was nearly four times higher than that of PMS.

First measurement of the Ar(e,e′)X cross section at Jefferson Laboratory

The success of the ambitious programs of both long- and short-baseline neutrino-oscillation experiments employing liquid-argon time-projection chambers will greatly rely on the precision with which the weak response of the argon nucleus can be estimated. In the E12-14-012 experiment at Jefferson Lab Hall A, we have studied the properties of the argon nucleus by scattering a high-quality electron beam off a high-pressure gaseous argon target. Here, we present the measured $^{40}$Ar$(e,e^{\prime})$ double differential cross section at incident electron energy $E=2.222$~GeV and scattering angle $\theta = 15.541^\circ$. The data cover a broad range of energy transfers, where quasielastic scattering and delta production are the dominant reaction mechanisms. The result for argon is compared to our previously reported cross sections for titanium and carbon, obtained in the same kinematical setup.

Microstructure, optical and electrical properties of thin films of gallium-phosphorus-titanium alloys synthesized by asymmetric bipolar pulsed direct current magnetron sputtering

Thin films of gallium-phosphorus-titanium (Ga-P-Ti) alloys were prepared on glass substrates at 573 K by an asymmetric bipolar pulsed direct current sputtering technique using an argon atmosphere and targets made from gallium phosphide (GaP) powder and metallic titanium (Ti), at the surface ratios of 8:1, 5:1, 2:1 and 1:1 GaP to Ti (GaP:Ti) on the sputtered area. Examination by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscope indicated that the as-deposited films from the sputtering targets having GaP:Ti ratios of 8:1, 5:1, and 2:1 were polycrystalline with the cubic zinc-blende crystal structure having GaP as the host material, i.e., Ti-doped GaP. Elemental compositions of the film obtained from the target at a GaP:Ti ratio of 5:1 closely resembled the theoretically predicted intermediate band compound Ga4P3Ti. It was projected that the Ga4P3Ti compound could be fabricated by co-sputtering of GaP and Ti from a single target having the surface area ratio GaP:Ti of 3.5:1. Optical transmission and reflection spectra, temperature dependence of electrical resistivity, and light response of the electrical resistivity showed semiconductor-like behavior for the films obtained from the targets with the GaP:Ti of 8:1 and 5:1, and were metal-like for those deposited from the other targets. Optical band gaps determined from the transmission spectrum of the semiconducting films by Tauc's expression for indirect transition were 1.2–1.5 eV. The results of the study could provide an alternative route for fabricating the intermediate band material based on the Ga-P-Ti system.

MICROSTRUCTURE AND BLUE PHOTOLUMINESCENCE OF HYDROGENATED SILICON CARBONITRIDE THIN FILMS

Hydrogenated silicon carbonitride (SiCN:H) thin films were deposited by sputtering of silicon carbide target in hydrogen-doped argon and nitrogen atmospheres. The properties of the SiCN:H films were analyzed by scanning electron microscopy with energy dispersive spectrometer, atomic force microscope, Fourier transform infrared spectroscopy, X-ray diffraction and fluorescence spectrophotometer. No distinct crystal was formed in the SiCN:H films as-deposited and annealed at 600∘C and 800∘C. The SiCN:H films were mainly composed of Si–N, Si–C, Si–O, C–C, C–N, C[Formula: see text]N, N–Hn bonds and SiCxNy network structure. The strong blue photoluminescence observed from the SiCN:H film annealed at 600∘C was attributed to SiCxNy network structure.

Design and construction of the DEAP-3600 dark matter detector

The Dark matter Experiment using Argon Pulse-shape discrimination (DEAP) has been designed for a direct detection search for particle dark matter using a single-phase liquid argon target. The projected cross section sensitivity for DEAP-3600 to the spin-independent scattering of Weakly Interacting Massive Particles (WIMPs) on nucleons is 10−46cm2 for a 100 GeV/c2 WIMP mass with a fiducial exposure of 3 tonne-years. This paper describes the physical properties and construction of the DEAP-3600 detector.

Backgrounds and pulse shape discrimination in the ArDM liquid argon TPC

The ArDM experiment completed a single-phase commissioning run (ArDM Run I) with an active liquid argon target of nearly one tonne in mass. The analysis of the data and comparison to predictions from full detector simulations allowed extraction of the detector properties and an assessment of the low background conditions. The 39Ar specific activity from the employed atmospheric argon is measured to be (0.95±0.05) Bq/kg. The cosmic muon flux at the Canfranc underground site was determined to be in the range (2–3.5)× 10−3m−2s−1. The statistical rejection power for electronic recoil events using the pulse shape discrimination method was estimated using a 252Cf neutron calibration source. Electronic and nuclear recoil band profiles were found to be well described by Gaussian distributions. Employing such a model we derive values for the electronic recoil statistical rejection power of more than 108 in the tonne-scale liquid argon target for events with more than 50 detected photons at a 50% acceptance for nuclear recoils. The 222Rn emanation rate of the ArDM cryostat at room temperature was found to be (65.6±0.4) μHz/l. These results represent an important physics milestone for the next run in the double-phase mode and in the context of foreseen developments towards the use of depleted argon targets.