Light Detection In Noble Elements (LIDINE 2013)

The article 2013 JINST 8 C09012 has been retracted as it is the accidental duplicate publication of 2013 JINST 8 C09006 (doi:10.1088/1748-0221/8/09/C09006), which was published in this same Journal on September 12, 2013.

The Long-Baseline Neutrino Experiment (LBNE) Project is expected to provide facilities that will enable a program in neutrino physics that can measure fundamental physical parameters, explore physics beyond the Standard Model and better elucidate the nature of matter and anti-matter. The LBNE Photon Detection subsystem is primarily designed to detect the scintillation photons produced at 128 nm as ionizing particles traverse the liquid argon. The LBNE reference design for the photon detector subsystem uses adiabatic light guides consisting of cast acrylic bars whose surface is embedded with waveshifter to convert the Vacuum Ultraviolet (VUV) 128 nm photons into the optical bandpass of silicon photomultipliers (SiPMs). In this investigation, we describe comparative studies of two VUV waveshifters — TPB and bis-MSB. We find that bis-MSB is more efficient than TPB at 128 nm. We also find that the efficiency of converting VUV photons into the optical for both waveshifters rises from 170–200 nm. Studies of the long wavelength behavior of the waveshifters supports the result that the efficiency is rising.

The DarkSide staged program utilizes a two-phase time projection chamber (TPC) with liquid argon as the target material for the scattering of dark matter particles. Efficient background reduction is achieved using low radioactivity underground argon as well as several experimental handles such as pulse shape, ratio of ionization over scintillation signal, 3D event reconstruction, and active neutron and muon vetos. The DarkSide-10 prototype detector has proven high scintillation light yield, which is a particularly important parameter as it sets the energy threshold for the pulse shape discrimination technique. The DarkSide-50 detector system, currently in commissioning phase at the Gran Sasso Underground Laboratory, will reach a sensitivity to dark matter spin-independent scattering cross section of 10⁻⁴⁵ cm² within 3 years of operation.

This note is provided as a supplementary section to accompany the paper [1] which has been included in these proceedings. It describes some simple simulations which were performed in order to understand the attenuation behaviors of acrylic light-guides operated in air and argon, which were characterized in [2]. Whilst these simulations are only at the level of sophistication of a toy model, they illustrate interesting non-exponential light attenuation effects and the differences between operating light-guide based detectors in argon and air environments. We investigate the effects of surface absorption, surface roughness and wavelength dependence, and use a model tuned on the light-guide attenuation curve measured in air to make a prediction of the light-guide attenuation curve in argon. This curve is compared with data from a liquid argon test stand, and an improvement over a simple exponential model is observed.

MicroBooNE is a neutrino experiment located on axis in the Booster Neutrino Beamline (BNB), at Fermi National Accelerator Laboratory, scheduled to begin data collection in 2014. The MicroBooNE detector consists of two main components: a large liquid argon TPC, and a light collection system. Thirty-two 8-inch diameter cryogenic photomultiplier tubes (PMTs) will detect the scintillation light generated in the liquid argon. In this article, we describe the basic features of the system and current status of MicroBooNE light collection system.

The proposed LBNE experiment will employ liquid argon TPCs for the far detector. We are developing a photon detector prototype based on wavelength-shifting fibers and utilizing silicon photomultipliers for potential use in the LBNE far detector. This paper describes progress and plans of the prototype development. An update on the development of a cryogenic detector development test facility, which includes a 500 L cryostat designed for testing full-scale photon detector components for LBNE will also be covered.

Silicon photosensors represent a viable alternative to standard photomultipliers in fields such as communications and medical imaging. We explored the interesting possibility of using these sensors in combination with liquid argon (LAr) for astroparticle physics applications such as neutrino, dark matter and double beta decay experiments. In fact, silicon photosensors have detection efficiencies comparable with those of the highest performance PMTs and can be manufactured with high level of radiopurity. In particular within the on-going R&D activity of the SILENT project (Low background and low noise techniques for double beta decay physics funded by ASPERA) a large area SiPM (Silicon PhotoMultiplier - Hamamatsu S11828-3344M - 1.7 cm² area) has been installed in a LAr scintillation chamber of 0.5 liters volume together with a cryogenic photomultiplier tube (Hamamatsu R11065) used as a reference. The liquid argon chamber has been exposed to many gamma sources of different energies and single photoelectron response and light yield for the SiPM and PMT have been measured and compared.

In this contribution the results of the tests, and the ongoing R&D to optimize the SiPM for cryogenic and for ultralow background applications, are reported, as well as the possible application in the GERDA experiment on Double Beta Decay Searches of ⁷⁶Ge.

In these proceedings I discuss results of a degradation mechanism study of tetraphenyl butadiene (TPB) coatings of the type used in neutrino and dark matter liquid argon experiments. Using gas chromatography coupled to mass spectrometry (GCMS) we identified the UV blocking impurity benzophenone. We monitored the drop in performance and increase of benzophenone concentration in TPB plates with exposure to ultraviolet (UV) light, and demonstrate the correlation between these two variables. We show promisng results obtained by adding a free radical inhibiting stabilizing compound, which improves the initial performance of light-guide coatings by up to 20% and significantly improves their UV stability. These proceedings summarize work previously published in JINST [1].

Light detection systems in Liquid Argon Time Projection Chambers (LArTPCs) require the detection of the 128 nm light produced during argon scintillation. Most detectors use Tetraphenyl Butadiene (TPB) to shift the wavelength of the light into a range visible to Photomultiplier Tubes (PMTs). These proceedings summarize characterizations of light-guides coated with a matrix of TPB in UV transmitting acrylic which are more compact than existing LArTPC light collection systems.

A simple model for the estimation of the light yield of a scintillation detector is developed under general assumptions and relying exclusively on the knowledge of its optical properties. The model allows one to easily incorporate effects related to Rayleigh scattering and absorption of the photons. The predictions of the model are benchmarked with the outcomes of Monte Carlo simulations of specific scintillation detectors. An accuracy at the level of few percent is achieved. The case of a real liquid argon based detector is explicitly treated and the predicted light yield is compared with the measured value.

The Noble Element Simulation Technique (NEST) is an extensive collection of models explaining both the scintillation light and ionization yields of noble elements as a function of particle type (nuclear recoils, electron recoils, alphas), electric field, and incident energy or energy loss (dE/dx). It is packaged as C++ code for Geant4 that implements said models, overriding the default model which does not account for certain complexities, such as the reduction in yields for nuclear recoils (NR) compared to electron recoils (ER). We present here improvements to the existing NEST models and updates to the code which make the package even more realistic and turn it into a more full-fledged Monte Carlo simulation. All available liquid xenon data on NR and ER to date have been taken into consideration in arriving at the current models. Furthermore, NEST addresses the question of the magnitude of the light and charge yields of nuclear recoils, including their electric field dependence, thereby helping to understand the capabilities of liquid xenon detectors for detection or exclusion of a low-mass dark matter WIMP.

Tetra-Phenyl Butadiene (TPB) is the most commonly used compound to wave-shift the 128 nm scintillation light of liquid Argon down to the visible spectrum. We present a study on the loss of conversion efficiency of thin TPB films evaporated on reflective foils when exposed to light and atmosphere. The efficiency of the films is measured and monitored with a dedicated set-up that uses gaseous Argon excited by alpha particles to produce 128 nm photons and working at room temperature. In particular we performed a two years long exposure of the samples to lab diffuse light and atmosphere. We also performed more controlled aging tests to investigate the effect of storing samples in a inert atmosphere.

Xenon is an especially attractive candidate for both direct WIMP and 0νββ decay searches. Although the current trend has exploited the liquid phase, the gas phase xenon offers remarkable performance advantages for: energy resolution, topology visualization, and discrimination between electron and nuclear recoils. The NEXT-100 experiment, now under construction in the Canfranc Underground Laboratory, Spain, will operate at ∼ 15 bars with 100 kg of ¹³⁶Xe for the 0νββ decay search. We will describe recent results with small prototypes, indicating that NEXT-100 can provide about 0.5% FWHM energy resolution at the decay's Q value (2457.83 keV), as well as rejection of γ-rays with topological cuts. However, sensitivity goals for WIMP dark matter and 0νββ decay searches indicate the probable need for ton-scale active masses. NEXT-100 provides the springboard to reach this scale with xenon gas. We describe a scenario for performing both searches in a single, high-pressure, ton-scale xenon gas detector, without significant compromise to either. In addition, even in a single ton-scale, high-pressure xenon gas TPC, an intrinsic sensitivity to the nuclear recoil direction may exist. This plausibly offers an advance of more than two orders of magnitude relative to current low-pressure TPC concepts. We argue that, in an era of deepening fiscal austerity, such a dual-purpose detector may be possible at acceptable cost, within the time frame of interest, and deserves our collective attention.

The solid (crystalline) phase of xenon possesses many of the same advantages of liquid xenon as a particle detector material including good transparency and ionization drift, self-shielding, low intrinsic background, and high scintillation light yield. Many of the properties of solid xenon have been measured previously employing small volumes and thin films. However, few systematic studies have been successfully produced using large volumes of solid xenon. Two major R&D issues must be addressed to make a solid xenon particle detector; the demonstration of the scalability of solid xenon and the capability to readout solid xenon signals. Both issues are being addressed with a dedicated cryogenic system at Fermilab. The first phase of this project entailed growing approximately a kilogram of transparent solid phase xenon and was successfully completed in 2010 at Fermilab. The second phase of this project is underway where the signals from scintillation light and electron drift in solid xenon will be measured. These measurements are expected to be completed this year. In this talk, we will discuss the recent progress of solid xenon detector R&D performed at Fermilab.

The Long-Baseline Neutrino Experiment (LBNE) Project is expected to provide facilities that will enable a program in neutrino physics that can measure fundamental physical parameters, explore physics beyond the Standard Model and better elucidate the nature of matter and anti-matter. The LBNE Photon Detection subsystem is primarily designed to detect the scintillation photons produced at 128 nm as ionizing particles traverse the liquid argon. The LBNE reference design for the photon detector subsystem uses adiabatic light guides consisting of cast acrylic bars whose surface is embedded with waveshifter to convert the VUV 128 nm photons into the sensitive optical range of silicon photomultipliers (SiPMs). In this investigation we first describe our methods for manufacturing light guides. We then describe our testing program to determine the effects of UV radiation on the waveshifter efficiency. We find that UV radiation from fluorescent lights does not degrade waveshifter efficiency significantly in 96 hours of exposure, but that waveshifter quickly deteriorates when exposed to the UV radiation in sunlight.

Most neutrino experiments using liquid argon as a detector medium focus on obtaining information about the interaction from ionization electrons and choose to use the scintillation light as a trigger or an indication of interaction time. On the other hand, experiments investigating lower energy ranges, i.e. Dark Matter searches have shown that there is a wealth of information in the scintillation light, which by itself allows calorimetric reconstruction and particle identification based on the shape of the light signal. LArIAT is a an experiment set to calibrate the LAr Time Projection Chamber technology by placing the detector on a beam of charged particles of known type and momentum. One of its goals is to test a Dark Matter search-like light collection system, which could supplement the calorimetric and particle identification capabilities of the LArTPC. The plans to implement this setup in the LArIAT detector will be presented together with the small set-up being constructed to test the components.

A thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface delimiting the active volume of the detector and/or onto the photosensor's optical window is the most common solution to down convert argon VUV scintillation light in current and planned liquid argon based experiments for dark matter searches and neutrino physics. Characterization of the main features of TPB coatings on different, commonly used substrates is reported, as a result of measurements at the specialized optical metrology labs of ENEA and University of Tor Vergata. Measured features include TPB emission spectra with lineshape and relative intensity variation recorded as a function of the film thickness and for the first time down to LAr temperature, as well as optical reflectance and transmittance spectra of the TPB coated substrates in the wavelength range of the TPB emission.

This paper presents the proposed PMT readout and triggering system that will be used in the MicroBooNE LArTPC experiment. The triggering scheme has been designed to study beam neutrino events as well as fully characterize cosmic rays. In addition, exploration of important physics applications including ''late'' scintillation light in argon and Michel electrons from muon decay will be possible. Various types of triggers and how they will be implemented in the combined PMT+TPC readout electronics system will be discussed.

We review our work in developing a tetraphenyl butadiene (TPB)-based detection system for a measurement of the neutron lifetime using magnetically confined ultracold neutrons (UCN). As part of the development of the detection system for this experiment, we studied the scintillation properties of liquid helium itself, characterized the fluorescent efficiencies of different fluors, and built and tested three detector geometries. We provide an overview of the results from these studies as well as references for additional information.

Radiation detectors with noble gasses as the active medium are becoming increasingly common in experimental programs searching for physics beyond the standard model. Nearly all of these experiments rely to some degree on collecting scintillation light from noble gasses. The VUV wavelengths associated with noble gas scintillation mean that most of these experiments use a fluorescent material to shift the direct scintillation light into the visible or near UV band. We present an overview of the R&D program at LBL related to noble gas detectors for neutrino physics, double beta decay, and dark matter. This program ranges from precise measurements of the fluorescence behavior of wavelength shifting films, to the prototyping of large are VUV sensitive light guides for multi-kiloton detectors.

The Long-Baseline Neutrino Experiment (LBNE) Project is expected to provide facilities that will enable a program in neutrino physics that can measure fundamental physical parameters, explore physics beyond the Standard Model and better elucidate the nature of matter and anti-matter. The LBNE Photon Detection subsystem is primarily designed to detect the scintillation photons produced at 128 nm as ionizing particles traverse the liquid argon. The LBNE reference design for the photon detector subsystem uses adiabatic light guides consisting of cast acrylic bars whose surface is embedded with waveshifter to convert the VUV 128 nm photons into the sensitive optical range of silicon photomultipliers (SiPMs). In this investigation we first describe our methods for manufacturing light guides. We then describe our testing program to determine the effects of UV radiation on the waveshifter efficiency. We find that UV radiation from fluorescent lights does not degrade waveshifter efficiency significantly in 96 hours of exposure, but that waveshifter quickly deteriorates when exposed to the UV radiation in sunlight.

The Argon Dark Matter experiment is a ton-scale double phase argon Time Projection Chamber designed for direct Dark Matter searches. It combines the detection of scintillation light together with the ionisation charge in order to discriminate the background (electron recoils) from the WIMP signals (nuclear recoils). After a successful operation on surface at CERN, the detector was recently installed in the underground Laboratorio Subterráneo de Canfranc, and the commissioning phase is ongoing. We describe the status of the installation and present first results from data collected underground with the detector filled with gas argon at room temperature.

Hamamatsu's R5912-02 MOD is an 8 inch diameter cryogenic photomultiplier tube of interest for light detection in large liquid noble dark matter and neutrino detectors. The R5912-02 MOD will be used in the MiniCLEAN single phase liquid argon dark matter detector and has been tested and characterized at cryogenic temperatures in the single photoelectron regime. A detailed model of the single photoelectron timing properties, pulse shape, and charge distribution will be described. The model extracts these parameters from fits to the unique multi-component timing structure of the R5912-02 MOD pulses.

In these proceedings I discuss results from the Bo test stand at the Proton Assembly Building, Fermilab. This test stand has been used to characterize elements of the MicroBooNE optical system as well as to perform studies of processes affecting argon scintillation light such as scintillation quenching and optical absorption by impurities. I review in detail a recent measurement of the absorption of liquid argon scintillation light by dissolved nitrogen at the part-per-million level.

The Cryogenic Apparatus for Precision Tests of Argon Interactions with Neutrinos (CAPTAIN) is being built at Los Alamos National Laboratory. A hexagonal time projection chamber (TPC) with a 1 m drift length will be constructed inside a cryostat containing 7,700L of liquid argon. CAPTAIN will be used to test interactions using beams of neutrons and neutrinos. It will serve as a test bed for various options for the Long Baseline Neutrino Experiment (LBNE) including in the photon detection system. The current photon detection system will be described and future options discussed. The system is composed of sixteen R8520-500 Hamamatsu photomultiplier tubes with a wavelength shifting coating on acrylic in front of the PMT. Various wavelength shifting coatings can be examined with the current default of tetraphenyl butadiene.

Detectors based on liquid or gas xenon have been used and are in use for a number of applications, in particular for the detection of gamma rays. Xenon is a well-suited medium for gamma spectroscopy thanks to its high atomic number and, consequently, large cross-section for photo-electric absorption. This paper presents experimental studies of high pressure xenon as a scintillator, with the aim of developing a gamma ray detector for the detection of Special Nuclear Materials (SNM). The first goal was to study the dependence of the light yield and of the energy resolution on the thermodynamic conditions. We present preliminary results from an optimised version of the detector.