Liquid Argon Scintillation Research Articles

Doping liquid argon with xenon in ProtoDUNE Single-Phase: effects on scintillation light

Doping of liquid argon TPCs (LArTPCs) with a small concentration of xenon is a technique for light-shifting and facilitates the detection of the liquid argon scintillation light. In this paper, we present the results of the first doping test ever performed in a kiloton-scale LArTPC. From February to May 2020, we carried out this special run in the single-phase DUNE Far Detector prototype (ProtoDUNE-SP) at CERN, featuring 720 t of total liquid argon mass with 410 t of fiducial mass. A 5.4 ppm nitrogen contamination was present during the xenon doping campaign. The goal of the run was to measure the light and charge response of the detector to the addition of xenon, up to a concentration of 18.8 ppm. The main purpose was to test the possibility for reduction of non-uniformities in light collection, caused by deployment of photon detectors only within the anode planes. Light collection was analysed as a function of the xenon concentration, by using the pre-existing photon detection system (PDS) of ProtoDUNE-SP and an additional smaller set-up installed specifically for this run. In this paper we first summarize our current understanding of the argon-xenon energy transfer process and the impact of the presence of nitrogen in argon with and without xenon dopant. We then describe the key elements of ProtoDUNE-SP and the injection method deployed. Two dedicated photon detectors were able to collect the light produced by xenon and the total light. The ratio of these components was measured to be about 0.65 as 18.8 ppm of xenon were injected. We performed studies of the collection efficiency as a function of the distance between tracks and light detectors, demonstrating enhanced uniformity of response for the anode-mounted PDS. We also show that xenon doping can substantially recover light losses due to contamination of the liquid argon by nitrogen.

A novel cryogenic VUV spectrofluorometer for the characterization of wavelength shifters

We present a novel cryogenic VUV spectrofluorometer designed to characterize wavelength shifters (WLS) crucial for experiments based on liquid argon (LAr) scintillation light detection. Wavelength shifters like 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) or polyethylene naphthalate (PEN) are used in these experiments to shift the VUV scintillation light to the visible region. Precise knowledge of the optical properties of the WLS at liquid argon's temperature (87 K) and LAr scintillation wavelength (128 nm) is necessary to model and understand the detector response. The cryogenic VUV spectrofluorometer was commissioned to measure the emission spectra and relative wavelength shifting efficiency (WLSE) of samples between 300 K to 87 K for VUV (120 nm to 190 nm) and UV (310 nm) excitation. New mitigation techniques for surface effects on cold WLS were established. As part of this work, the TPB-based wavelength shifting reflector (WLSR) featured in the neutrinoless double-beta decay experiment LEGEND-200 was characterized. The WLSE was observed to increase by (54 ± 5) % from room temperature (RT) to 87 K. PEN installed in LEGEND-200 was also characterized, and a first measurement of the relative WLSE and emission spectrum at RT and 87 K is presented. The WLSE of amorphous PEN was found to be enhanced by at least (37 ± 4) % for excitation with 128 nm and by (52 ± 3) % for UV excitation at 87 K compared to RT.

Liquid argon light collection and veto modeling in GERDA Phase II

The ability to detect liquid argon scintillation light from within a densely packed high-purity germanium detector array allowed the Gerda experiment to reach an exceptionally low background rate in the search for neutrinoless double beta decay of {}^{76}Ge. Proper modeling of the light propagation throughout the experimental setup, from any origin in the liquid argon volume to its eventual detection by the novel light read-out system, provides insight into the rejection capability and is a necessary ingredient to obtain robust background predictions. In this paper, we present a model of the Gerda liquid argon veto, as obtained by Monte Carlo simulations and constrained by calibration data, and highlight its application for background decomposition.

Application and development of liquid argon detector in rare event detection

Rare event detection is a frontier subject in particle physics and nuclear physics. In particular, dark matter detection, neutrino-free double beta decay and neutrino-nucleon coherent elastic scattering are being planned and implemented gradually. Rare event detection requires not only the detectors to possess excellent performances but also extremely low environmental background, so the selection of detectors and related materials is an important issue in rare event detection. Liquid argon has become an important scintillator material for scintillator detectors because of its low cost, good scintillation performance and large volume. Liquid argon was first studied in the 1940s as a sensitive material for ionizing radiation detectors. The first measurements of high-energy beta particles were obtained by using a liquid argon ionization chamber in 1953. The ICARUS group put forward the idea of constructing liquid argon temporal projection chamber, and made attempt to construct liquid argon temporal projection chamber in 1977. The scintillation light signals were collected for the first time in a liquid argon temporal projection chamber in 1999. Thus, the drift time of the particle can be obtained to determine the particle track. After development, single-phase liquid argon scintillator detector and two-phase argon time projection chamber have become two common types of liquid argon detectors, and have been extensively used in rare event detection experiments in recent years. For dark matter detection, the DEAP group and DarkSide group have achieved good results with single-phase liquid argon scintillation detector and two-phase argon time projection chamber, respectively. For neutrino-free double beta decay experiments, the GERDA group has done a lot of researches of liquid argon anti-coincidence system and applied the said system to experiments. The LEGEND group, which is the combination of GERDA and MAJORANA experimental group, upgraded the liquid argon anti-coincidence system which was applied to the following LEGEND-200 project. For neutrino-nucleon elastic scattering experiments, COHERENT obtained the latest results by using the liquid argon detectors. The Taishan neutrino-nucleon coherent elastic scattering project of the High Energy Institute of Chinese Academy of Sciences has also begun to study the feasibility of liquid argon anti-coincidence system. Finally, this paper discusses the direction of optimizing the liquid argon detector, such as exposure, background level and optical readout scheme, and gives a good prospect of liquid argon detector applied to rare event detection in the future.

Development of an Argon Light Source as a Calibration and Quality Control Device for Liquid Argon Light Detectors

The majority of future large-scale neutrino and dark matter experiments are based on liquid argon detectors. Since liquid argon is also a very effective scintillator, these experiments also have light detection systems. 127 nm wavelength of the liquid argon scintillation leads to the development of specialized light detectors, mostly based on wavelength shifters, and recently photodetectors sensitive to deeper UV. The effective calibration and quality control of these newly developed detectors is still a persisting problem. In order to respond to this need, we developed an argon light source which is based on plasma generation and light transfer across a MgF2 window. The light source is designed as a small, portable and easy to operate device to enable the acquisition of performance characteristics of several square meters of light detectors at once. Here we report on the development of the light source and its preliminary performance characteristics.

Development of an Argon Light Source as a Calibration and Quality Control Device for Liquid Argon Light Detectors

The majority of future large-scale neutrino and dark matter experiments are based on liquid argon detectors. Since liquid argon is also a very effective scintillator, these experiments also have light detection systems. The liquid argon scintillation wavelength of 127 nm is most commonly shifted to the visible range by special wavelength shifters or read out by the 127 nm sensitive photodetectors that are under development. The effective calibration and quality control of these active media is still a persisting problem. In order to respond to this need, we developed an argon light source which is based on plasma generation and light transfer across a MgF2 window. The light source was designed as a small, portable and easy-to-operate device to enable the acquisition of performance characteristics of several square meters of light detectors. Here, we report on the development of the light source and its performance characteristics.

Development and characterization of a slow wavelength shifting coating for background rejection in liquid argon detectors

We describe a technique, applicable to liquid-argon-based dark matter detectors, allowing for discrimination of alpha-decays in detector regions with incomplete light collection from nuclear-recoil-like events.Nuclear recoils and alpha events preferentially excite the liquid argon (LAr) singlet state, which has a decay time of ∼6ns. The wavelength-shifter TPB, which is typically applied to the inside of the active detector volume to make the LAr scintillation photons visible, has a short re-emission time that preserves the LAr scintillation timing. We describe the production method and characterization of a wavelength-shifting polymeric film – pyrene-doped polystyrene – which we developed for the DEAP-3600 detector. At liquid argon temperature, the film’s re-emission timing is dominated by a modified exponential decay with a time constant of 279(14)ns, and has a wavelength-shifting efficiency of 46.4(2.9)% relative to TPB, measured at room temperature. By coating the detector neck (a region outside the active volume where the scintillation light collection efficiency is low) with this film, the visible energy and the scintillation pulse shape of alpha events in the neck region are modified, and we predict that through pulse shape discrimination, the coating will afford a suppression factor of O (105) against these events.

Performance of Hamamatsu VUV4 SiPMs for detecting liquid argon scintillation

Detection of light signals is crucial to a wide range of particle detectors. In particular, efficient detection of vacuum ultraviolet (VUV) light will provide new opportunities for some novel detectors currently being developed, but is technically challenging. In this article, we characterized the performance of Hamamatsu VUV4 silicon photomultipliers (SiPMs) for detecting VUV argon scintillation light without wavelength shifting. Using a customized cryogenic amplifier design, we operated two models of VUV4 SiPMs inside liquid argon and thoroughly examined their direct sensitivities to liquid argon scintillation. In addition to describing their cryogenic performance, we measured a photon detection efficiency of 14.7+1.1 -2.4% and 17.2+1.6 -3.0% at 128 nm for these two VUV4 models for operation at 4 V of overvoltage, with the main uncertainty arising from the SiPM reflectivity for VUV light.

Scintillation and optical properties of xenon-doped liquid argon

Liquid argon (LAr) is a common choice as detection medium in particle physics and rare-event searches. Challenges of LAr scintillation light detection include its short emission wavelength, long scintillation time and short attenuation length. The addition of small amounts of xenon to LAr is known to improve the scintillation and optical properties. We present a characterization campaign on xenon-doped liquid argon (XeDLAr) with target xenon concentrations ranging from 0 to 300 ppm by mass encompassing the measurement of the photoelectron yield Y, effective triplet lifetime τ 3 and effective attenuation length λ att. The measurements were conducted in the Subterranean Cryogenic ARgon Facility, Scarf, a 1 t (XeD)LAr test stand in the shallow underground laboratory (UGL) of TU-Munich. These three scintillation and optical parameters were observed simultaneously with a single setup, the Legend Liquid Argon Monitoring Apparatus, Llama. The actual xenon concentrations in the liquid and gaseous phases were determined with the Impurity DEtector For Investigation of Xenon, Idefix, a mass spectrometer setup, and successful doping was confirmed. At the highest dopant concentration we find a doubling of Y, a tenfold reduction of τ 3 to ∼90 ns and a tenfold increase of λ att to over 6 m.

Measurement of the angular distribution of wavelength-shifted light emitted by TPB

We present measurements of the angular distribution of re-emitted light from tetraphenyl butadiene thin films when exposed to 128 nm light, which is the peak of the liquid Argon (LAr) scintillation spectrum, in vacuum. Films ranging from 250 nm to 5.5 μm in thickness are measured. All films were fabricated by evaporation deposition on ultraviolet transmitting (UVT) acrylic substrates. Preliminary comparisons of the angular distribution to that produced by a detailed Monte Carlo model are also presented. The current shortcomings of the model are discussed and future plans briefly outlined.

Properties of liquid argon scintillation light emission

Liquid argon is used as active medium in a variety of neutrino and dark matter experiments thanks to its excellent properties of charge yield and transport and as a scintillator. Liquid argon scintillation photons are emitted in a narrow band of 10 nm centered around 127 nm and with a characteristic time profile made by two components originated by the decay of the lowest lying singlet, ${^{1}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, and triplet states, ${^{3}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, of the excimer ${\mathrm{Ar}}_{2}^{*}$ to the dissociative ground state. A model is proposed which takes into account the quenching of the long lived triplet states through the self-interaction with other triplet states or through the interaction with molecular ${\mathrm{Ar}}_{2}^{+}$ ions. The model predicts the time profile of the scintillation signals and its dependence on the intensity of an external electric field and on the density of deposited energy, if the relative abundance of the unquenched fast and slow components is know. The model successfully explains the experimentally observed dependence of the characteristic time of the slow component on the intensity of the applied electric field and the increase of photon yield of liquid argon when doped with small quantities of xenon (at the part per million level). The model also predicts the dependence of the pulse shape parameter, ${F}_{\text{prompt}}$, for electron and nuclear recoils on the recoil energy and the behavior of the relative light yield of nuclear recoils in liquid argon, ${\mathcal{L}}_{\mathrm{eff}}$.

Liquid argon scintillation response to electronic recoils between 2.8–1275 keV in a high light yield single-phase detector

We measure the liquid argon scintillation response to electronic recoils in the energy range of $2.82$ to $1274.6~{\rm keV}$ at null electric field. The single-phase detector with a large optical coverage used in this measurement yields $12.8 \pm 0.3 ~ (11.2 \pm 0.3)~{\rm photoelectron/keV}$ for $511.0$-${\rm keV}$ $\gamma$-ray events based on a photomultiplier tube single photoelectron response modeling with a Gaussian plus an additional exponential term (with only a Gaussian term). It is exposed to a variety of calibration sources such as $^{22}{\rm Na}$ and $^{241}{\rm Am}$ $\gamma$-ray emitters, and a $^{252}{\rm Cf}$ fast neutron emitter that induces quasimonoenergetic $\gamma$ rays through a $(n, n'\gamma)$ reaction with $^{19}{\rm F}$ in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the $2.82$-${\rm keV}$ peak of $^{37}{\rm Ar}$, a cosmogenic isotope in atmospheric argon. The observed light yield and energy resolution of the detector are obtained by the full-absorption peaks. We find up to approximately $25\%$ shift in the scintillation yield across the energy range and $3\%$ of the energy resolution for the $511.0$-${\rm keV}$ line. The Thomas-Imel box model with its constant parameter $\varsigma=0.033 ^{+0.012} _{-0.008}$ is found to explain the result. For liquid argon, this is the first measurement on the energy-dependent scintillation yield down to a few ${\rm keV}$ at null field and provides essential inputs for tuning the argon response model to be used for physics experiments.

Design and implementation of the new scintillation light detection system of ICARUS T600

ICARUS T600 is the far detector of the Short Baseline Neutrino program at Fermilab (U.S.A.), which foresees three Liquid Argon Time Projection Chambers along the Booster Neutrino Beam line to search for LSND-like sterile neutrino signal. The T600 detector underwent a significant overhauling process at CERN, introducing new technological developments while maintaining the already achieved performances. The realization of a new liquid argon scintillation light detection system is a primary task of the detector overhaul. As the detector will be subject to a huge flux of cosmic rays, the light detection system should allow the 3D reconstruction of events contributing to the identification of neutrino interactions in the beam spill gate. The design and implementation of the new scintillation light detection system of ICARUS T600 is described.

Sensitivity of the COHERENT experiment to accelerator-produced dark matter

The COHERENT experiment is well poised to test sub-GeV dark matter models using low-energy recoil detectors sensitive to coherent elastic neutrino-nucleus scattering (CEvNS) in the $\pi$-DAR neutrino beam produced by the Spallation Neutron Source. We show how a planned 750-kg liquid argon scintillation detector would place leading limits on scalar light dark matter models, over two orders of magnitude of dark matter mass, for dark matter particles produced through vector and leptophobic portals in the absence of other effects beyond the standard model. The characteristic timing structure of a $\pi$-DAR beam allows a unique opportunity for constraining systematic uncertainties on the standard model background in a time window where signal is not expected, enhancing expected sensitivity. Additionally, we discuss future prospects, further increasing the discovery potential of CEvNS detectors. Such methods would test the calculated thermal dark matter abundance for all couplings $\alpha'\leq1$ within the vector portal model over an order of magnitude of dark matter masses.

Development of Liquid Noble Gas Scintillation Detectors for Studying Coherent Elastic Neutrino–Nucleus Scattering

Coherent elastic neutrino–nucleus scattering (CEvNS) process, discovered in 2017, requires a detailed study, since it plays an important role in the formation of the Universe and can be used to improve the safety of nuclear energy and support international programs for the nonproliferation of nuclear weapons. In this paper, we discuss development of the liquid-argon scintillation detector technology carried out by the COHERENT collaboration to create high-efficiency detectors to observe and study CEvNS in an accelerator-based experiment.

Measurements of argon-scintillation and -electroluminescence properties for low mass WIMP dark matter search

It has been demonstrated that the liquid argon scintillation detector is an excellent particle detector for use in various physics experiments, particularly those seeking evidence of WIMPs dark matter. The detector observes primary scintillation signal (S1) and/or secondary electroluminescence signal (S2). It is crucial that the properties of the detector are understood comprehensively so that such projects reduce systematic uncertainty and improve sensitivity. This work covers the recent measurements, taken using the liquid argon test facility at Waseda University, of the liquid argon scintillation and ionization responses for nuclear and electronic recoils. Additionally, a basic study of the gaseous argon luminescence signal, which involves the S2 signal, is presented herein. The luminescence process of the S2 signal, based on this measurement, is then discussed along with a suggested new mechanism called neutral Bremsstrahlung.

Development of the Light Collection Module for the Liquid Argon Time Projection Chamber (LArTPC)

A modular liquid argon (LAr) Time Projection Chamber (TPC) with pixelated charge readout is considered as a part of the near-detector for the Deep Underground Neutrino Experiment (DUNE) [1]. Such a TPC is being developed by the ArgonCube collaboration [2]. To provide a trigger for the data acquisition (DAQ) of a neutrino event the LAr scintillation light detection is proposed. The light is a vacuum ultraviolet (UV) with 128 nm wavelength, thus, it is a challenge to register it. The main requirements imposed on the light detection system are a good performance at cryogenic temperatures, non-conductive materials, compact dimensions, and a detection efficiency at a level of percent. A Light Collection Module (LCM) as a candidate for the system has been developed at Joint Institute for Nuclear Research (JINR) in Dubna, Russia. The LCM is based on the wavelength-shifting (WLS) fibers that are coated with Tetraphenyl Butadiene (TPB) and read out by silicon photomultipliers (SiPM). Also at JINR, a full readout chain for the light detection system has been developed, consisting of the front-end electronics, power-supply for the SiPMs, and the DAQ. A cryogenic test setup has been built at JINR to study the performance of the LCM in LAr. A similar study was carried out in the laboratory for high energy physics of Bern University with highly purified LAr. These studies have shown that the photon detection efficiency (PDE) of the LCM for the LAr scintillation light is about 1–2%. Further tests in the ArgonCube prototype TPC will provide the real performance of the LCM system with a full readout chain.

Measurement of liquid argon scintillation light properties by means of an alpha source placed inside the CERN 10-PMT LAr detection system

A particle detection system that exploits the scintillation light produced by ionizing particles in liquid argon (LAr) has been assembled at CERN. The system is based on 10 large-area photomultiplier tubes (PMT) immersed in a 1500-liter dewar filled with liquid argon and equipped with an extendible feed-through and mechanical support for an alpha source (241Am). The position of the source can be changed with respect to the PMT plane in vertical and horizontal directions. Arrays of silicon photomultiplier (SiPM) photodetectors, integrated in the source support, are used for the data acquisition trigger and to define the t0 of the light generation. PMT and SiPM signals can be recorded at different distances and different positions allowing the measurement of some of the LAr scintillation light properties. The system was studied and characterized in detail, and physics results on LAr scintillation properties are expected soon.

Study and characterization of WLS for ARAPUCA to DUNE experiment

The photon detection system of the far detector of the Deep Underground Neutrino Experiment (DUNE) will collect liquid argon scintillation photons with an array of modules whose working principle is based on the ARAPUCA technology. These devices acting as light traps are able to convert VUV scintillation photons into visible ones and capture them inside a reflective cavity observed by an array of SiPMs as active photo-sensitive elements. The use of wavelength shifting techniques in light collecting systems for particle detection in liquid argon is of fundamental importance, since the scintillation photon wavelength is around 127 nm of the photon produced which is extremely challenging to be directly detected. The main objective of the present work was to explore and characterize different substrates, wavelength shifting compounds and deposition methods to optimize and improve the light collection of the ARAPUCA device.

The liquid-argon scintillation pulseshape in DEAP-3600

DEAP-3600 is a liquid-argon scintillation detector looking for dark matter. Scintillation events in the liquid argon (LAr) are registered by 255 photomultiplier tubes (PMTs), and pulseshape discrimination (PSD) is used to suppress electromagnetic background events. The excellent PSD performance of LAr makes it a viable target for dark matter searches, and the LAr scintillation pulseshape discussed here is the basis of PSD. The observed pulseshape is a combination of LAr scintillation physics with detector effects. We present a model for the pulseshape of electromagnetic background events in the energy region of interest for dark matter searches. The model is composed of (a) LAr scintillation physics, including the so-called intermediate component, (b) the time response of the TPB wavelength shifter, including delayed TPB emission at {mathcal {O}}(ms) time-scales, and c) PMT response. TPB is the wavelength shifter of choice in most LAr detectors. We find that approximately 10% of the intensity of the wavelength-shifted light is in a long-lived state of TPB. This causes light from an event to spill into subsequent events to an extent not usually accounted for in the design and data analysis of LAr-based detectors.