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Low Radioactivity Techniques 2015

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Description

The next LRT workshop, LRT2017, will be held in Seoul, Korea from May 24-26, 2017. LRT2017 webpage: http://lrt2017.ibs.re.kr

LRT 2015 CO-HOSTED BY Pacific Northwest National Laboratory and the University of Washington

The Low Radioactivity Techniques (LRT) workshop series examines topics in low radioactivity materials and techniques, a fundamental aspect of rare event searches. Topics include dark matter, solar neutrinos, double-beta decay, and long half-life phenomena. This conference’s wide scope includes all aspects of the development of low background detectors and techniques.

The goal of this workshop series is to bring together experts in this field for presentations and discussion covering broadly the issues of low radioactivity techniques. The intention is to foster and continue the collaboration and resource sharing required for new generations of detectors to be developed at underground facilities.

This is the fifth workshop in the LRT series. Previous LRT workshops have been held around the world, including LRT2013 at LNGS (Assergi, Italy), LRT2010 at SNOLAB (Sudbury, Canada), LRT2006 at Modane (Aussois, France), and the inaugural LRT2004 hosted at Laurentian University (Sudbury, Canada).

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Poster
  • Wednesday 18 March
    • Session 1: Underground Laboratories and Screening Facilities
      Convener: Prof. Mark Chen (Queen's University)
      • 1
        Welcome
        Slides
      • 2
        Underground Laboratories in North America
        I shall review the major underground research facilities in North America and their physics programs. Underground physics research began in North American in the 1960s. The research initially focused on neutrino studies and searches for proton decay. It has subsequently branched out to cover neutrinoless double beta decay, direct detection of dark matter, and completing our understanding of neutrino properties. Additional efforts in nuclear astrophysics, gravity waves detection, state-of-the-art assay facilities, geoneutrino investigations and even modest applied efforts are aggressively developing. The facilities in North American continue to grow in number and capabilities to accommodate this increasing demand to host deep underground research.
        Speaker: Dr Kevin Lesko (LBNL)
        Slides
      • 3
        Status and prospects of the underground Laboratories in Asia
        Speaker: Prof. Shin-Ted LIN (Sichuan University)
        Slides
      • 4
        Underground Laboratories in Europe
        Speaker: Prof. Stefano Ragazzi (LNGS)
        Slides
      • 5
        Low Background Screening Capability in the UK
        The LZ dark matter experiment will require an unprecedented low background rate within its fiducial volume, defining strict constraints on radioactivity from construction materials that is further mitigated through the combination of powerful self-shielding from liquid xenon, 3D event vertex reconstruction, and external veto detector systems. An aggressive screening campaign with cutting-edge instrumentation will be performed, prior to construction, to measure the trace levels of radioisotopes in materials and construct the experiments background model. To provide sufficient sensitivity for LZ we have been re-developing the UK’s low-background screening capability. New ultra-low background HPGe and BEGe detectors have been installed at the Boulby Underground Laboratory, itself undergoing substantial facility re-furbishment, to provide high sensitivity gamma spectroscopy for all DMUK and 0vBB interests. Dedicated low-activity mass spectrometry instrumentation has been developed at UCL for part per trillion level contaminant identification to complement underground screening, complete U/Th assays, and meet throughput demands. Finally, radon emanation screening at UCL measures radon background inaccessible to gamma or mass spectrometry techniques. With gamma spectroscopy, mass spectrometry and radon screening to world-class sensitivities, the UK will deliver half of the radioactivity screening for LZ.
        Speaker: Dr Chamkaur Chamkaur Ghag (UCL)
        Slides
      • 6
        The Berkeley Low Background Facility
        The Berkeley Low Background Facility (BLBF) at Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California provides low background gamma spectroscopy services to a wide array of experiments and projects. The analysis of samples takes place within two unique facilities; locally within a carefully-constructed, low background laboratory on the surface at LBNL and at a recently established underground location (4300 m.w.e) at the Sanford Underground Research Facility (SURF) in Lead, SD (relocated from Oroville, CA). These facilities provide a variety of gamma spectroscopy services to low background experiments primarily in the form of passive material screening for primordial radioisotopes (U, Th, K) or common cosmogenic/anthropogenic products; active screening via neutron activation analysis for U,Th, and K as well as a variety of stable isotopes; and neutron flux/beam characterization through the use of monitors. A general overview of the facilities, services, and sensitivities will be presented. Recent activities and upgrades will also be described including an overview of the recently installed counting system at SURF and future plans. The BLBF is open to any users for counting services or collaboration on any experiments and projects.
        Speaker: Keenan Thomas (LBNL / UC Berkeley)
        Slides
    • 10:20
      Coffee Break
    • Session 2: Assay and Screening Techniques
      Convener: Dr Kevin Lesko (LBNL)
      • 7
        Trace Assay of Metals Used in the Construction of Low Background Detector Systems for Thorium and Uranium
        The variety of next generation highly sensitive radiation detectors searching for trace radioactive isotopes, dark matter, and neutrinoless double beta decay must be constructed using ultra-low background materials. Because of the cost and complexity of these new detector systems, foreseeable backgrounds can no longer merely be discovered when the detector is first operated, essentially allowing the detector to be the primary assay tool, and then hopefully mitigated later. Rather, backgrounds must be anticipated and eliminated in the design phase. Inductively coupled plasma mass spectrometry is an analytical tool with the sensitivity necessary to assay materials in a timely and cost effective manner before their use as a construction material. Assay methods employed for a few of the metals used in detector and shielding construction are presented. Although necessary in many applications for their high strength, the relatively high concentrations of thorium and uranium found in currently available stainless steel and titanium are reviewed. Assay of lead and commercial copper used for shielding and some detector components is also discussed. Finally assay of electroformed copper of extreme purity is shown.
        Speaker: Eric Hoppe (Pacific Northwest National Laboratory)
        Slides
      • 8
        The Ultratrace Determination of Thorium and Uranium in a Variety of Detector Materials and Components for Use in Ultralow Background Applications
        This paper discusses the analytical methods developed for determining the radiopurity, specifically the U and Th content, of a variety of materials considered for use in rare-event and ultralow background physics experiments, such as dark matter and neutrino studies. Due to the extremely stringent background budgets for these fundamental science studies, all materials and components must be evaluated for their inclusion in the detector. This work focuses on the analysis of a diverse set of materials and components, including linear alkyl benzene, an assortment of electrical components, fused silica, and foreign debris captured in filters. Quantification and analyses were performed using isotope dilution inductively coupled plasma mass spectrometry. Results and the specifically-tailored approaches to preparing and/or digesting these challenging samples at ultratrace levels will be discussed.
        Speaker: Dr Isaac Arnquist (PNNL)
        Slides
      • 9
        The BetaCage, an Ultra-sensitive Screener for Surface Contamination
        The BetaCage is a proposed neon time-projection chamber for the ultra-sensitive (and nondestructive) screening of material surfaces for alpha- and beta-emitting contaminants, such as those due to implantation of radon progeny. The expected sensitivity is 0.1 betas per keV-m^2-day and 0.1 alphas per m^2-day. I will describe the detector design, results from background simulations, and results from a prototype.
        Speaker: Prof. Richard Schnee (South Dakota School of Mines & Technology)
        Slides
      • 10
        The BiPo-3 detector for the measurement of ultra high radiopurities in Tl-208 and Bi-214 of thin materials
        The BiPo-3 detector is running in Canfranc Underground Laboratory (LSC, Spain) since 2013. It is a new generation detector, dedicated to measure ultra high natural radiopurity in Tl-208 and Bi-214 of thin materials, with a total surface area of measurement of 3.6 m2. The Tl-208 and Bi-214 isotopes are the two main background natural isotopes in the search for the neutrinoless double decay. The BiPo-3 detector has been initialy developped to measure the radiopurity of the selenium double $\beta$ source foils of the SuperNEMO experiment. It is today a generic detector which measures with an unprecedented sensitivity, materials of other double beta experiments. For the LRT workshop, we will present the experimental design and performances of the detector and the main results of the first measurements. An ultra low level of background has been measured in Tl-208 and Bi-214: after several months of background measurement, surface activities of the scintillators of A(Tl)= 1.0 \pm 0.2 microBq/m2 and A(Bi-214)= 1.8 \pm 0.4 microBq/m2 are measured. Given these levels of background, the BiPo-3 detector is able to qualify the radiopurity of a SuperNEMO selenium foil with a sensitivity of about 3 microBq/kg in Tl-208 and 30 microBq/kg in Bi-214 (90\% C.L.) with a six month measurement. The detection efficiency and the Monte-Carlo simulations of the BiPo-3 detector have been validated by measuring a calibrated aluminium foil. First samples of various double beta experiments (mostly SuperNEMO but also CUORE, Lumineu and Zaragoza low radioactive TPC) have been measured with the BiPo-3 detector. The first SuperNEMO enriched selenium foils are under measurement and the preliminary results of this measurment will been presented.
        Speaker: Dr Xavier Sarazin (Laboratoire de l'Accélérateur Linéaire, IN2P3-CNRS / Université Paris-Sud)
        Slides
    • 12:10
      Lunch
    • Session 3: Community Coordination and Clean Electronics
      Convener: Prof. Frank Calaprice (Princeton)
      • 11
        Measurement of radioactive contamination with charge-coupled devices (CCDs)
        I will present the techniques we have developed to measure trace contamination of uranium, thorium, lead-210 and silicon-32 in the CCDs for the DAMIC experiment. I will introduce the devices, which consist of a square array of 16 million, 15 um x 15 um pixels, and a substrate thickness of 675 um. I will discuss the identification and spectroscopy (down to 100 eV) of alpha and beta particles based on the ionization patterns they produce in the substrate of the device. I will describe the algorithms that we have developed to reconstruct the end-points of electron tracks observed in a CCD. Furthermore, I will demonstrate how the high spatial resolution of the CCD allows us to reliably identify radioactive decay sequences with characteristic times between decays of up to weeks. Finally, I will describe how the CCD can also perform with high efficiency spectroscopy of alpha particles and X-rays impinging on its surface, and how this may be exploited for radioactive screening of other materials and surfaces.
        Speaker: Dr Alvaro Chavarria (University of Chicago)
        Slides
      • 12
        Progress toward Cooperative Low Background Infrastructure
        The Assay and Acquisition of Radiopure Materials (AARM) collaboration includes representatives from dark matter and neutrinoless double beta decay experiments. A series of workshops have fostered communication among collaborations with mutual interests in low background studies. Within this context, we have worked to create a model for sustainable and cost-effective low background infrastructure, including simulation tools and an integration website. An automated system has been developed which matches users to screening facilities and provides broad dissemination via the community materials database. A roadmap toward improved resource-sharing in an international context has been developed at AARM workshops and in planning exercises such as Snowmass.
        Speaker: Prof. Priscilla Cushman (University of Minnesota)
        Slides
      • 13
        “radiopurity.org”: A Community Assays Database
        The community of researchers working on experiments that require ultra-low backgrounds has an abundance of material assay results. These results are available through publications, electronic databases and through informal communications. Through a consortium of researchers, we have addressed the long standing problem through the creation of a concise and flexible data format and a database to use it. Here I report our progress on this community assays database.
        Speaker: Jodi Cooley (SMU)
        Slides
      • 14
        Topics on low activity electronics of the GERDA project
        Front End electronics is a crucial and critical item in any experiment aiming at ultra Low Background. All the Dark Matter and Double Beta Decay experiments have to face the fact that the FE devices and electronic, beside being properly electronically tailored to readout the detector, hence having the proper gain, dynamic range, and resolution, must accomplish the same demanding low activity requirements. This is complex and difficult to achieve as the front end electronics and related cabling are not monolithic systems and involve the use of several composite materials. In case of cryogenic environment the problem is further constrained, because devices and materials must accomplish to the cryogenic requirements. Sample solutions for Ge detector readout and for Liquid Argon scintillation light readout by Silicon PMTs and their related cabling will be presented. The adopted devices, electronics architecture, substrate materials, and the radioactive trace contamination analysis techniques adopted to assess their radiopurity will be discussed. The performances of the presented sample solutions will be also reviewed.
        Speaker: Dr Carla Maria Cattadori (INFN Milano Bicocca)
        Slides
      • 15
        Low Background Signal Readout Electronics for the MAJORANA DEMONSTRATOR
        The MAJORANA Collaboration will seek neutrinoless double beta decay in Ge-76 using isotopically enriched p-type point contact (PPC) high purity Germanium (HPGe) detectors. A tonne-scale array of HPGe detectors would require background levels below 1 count/ROI-tonne-year in the 4 keV region of interest (ROI) around the 2039 keV Q-value of the decay. In order to demonstrate the feasibility of such an experiment, the MAJORANA DEMONSTRATOR, a 40 kg HPGe detector array, is being constructed. The Demonstrator has a background goal of <3 counts/ROI-tonne-year, which is expected to scale down to <1 count/ROI-tonne-year for a one tonne experiment. The signal readout electronics, which must be placed in close proximity to the detectors, present a challenge toward reaching this background goal. This talk will discuss the novel materials and design used to construct signal readout electronics with low enough backgrounds for the MAJORANA DEMONSTRATOR.
        Speaker: Mr Ian Guinn (University of Washington)
        Slides
    • 16:10
      Coffee Break
    • Poster Session
    • 18:00
      Reception
  • Thursday 19 March
    • Session 4: Pure Materials
      Convener: Dr Reina Maruyama
      • 16
        SABRE -- a test of DAMA/LIBRA with ultra-high purity NaI(Tl) crystals
        The dark matter claim by DAMA/LIBRA is both significant and controversial. Several experiments have claimed to rule out DAMA/LIBRA, but the claims are made based on astrophysical and nuclear models that are unknown. Therefore, an unambiguous test of DAMA/LIBRA can only be made using NaI(Tl) crystals with lower residual background than that of DAMA/LIBRA, and the SABRE experiment is well equipped to achieve this goal. I will report the development of ultra-high purity SABRE NaI(Tl) crystals, the study of NaI(Tl) scintillation efficiency under Na nuclear recoils, and the progress of SABRE toward testing DAMA/LIBRA.
        Speaker: Dr Jingke Xu (Princeton University)
        Slides
      • 17
        Development of low background CsI(Tl) and NaI(Tl) crystals for dark matter search
        The Korea Invisible Mass Search (KIMS) collaboration has performed direct searches for WIMP-type dark matter using ultra-low-background CsI(Tl) crystal detectors, which are similar experimental devices to the NaI(Tl) crystal used in the DAMA experiment. The DAMA experiment has claimed an observation of dark matter with an annual modulation signature of WIMP-nucleon interaction. Recently the KIMS collaboration started development of ultra-low-background NaI(Tl) crystal detectors to identify unambiguously the origin of the DAMA modulation signature. In this presentation, techniques to reduce internal background of CsI(Tl) crystals as well as development status of low-background NaI(Tl) crystals will be reported.
        Speaker: Prof. Hyun Su Lee (Ewha Womans University)
        Slides
      • 18
        Radiopure ZnMoO4 scintillating bolometers for the LUMINEU double-beta experiment
        The main purpose of the LUMINEU project (Luminescent Underground Molybdenum Investigation for NEUtrino mass and nature) is to elaborate a technology suitable for realization of a next-generation neutrinoless double-beta decay experiment to explore the inverted hierarchy region of the neutrino mass. This goal should be achieved through the development of low-temperature scintillating bolometers based on zinc molybdate (ZnMoO4) crystal scintillators enriched in 100Mo. We report a significant progress in producing of radiopure ZnMoO4 crystal scintillators with high bolometric performance. A dedicated molybdenum purification technique based on sublimation of molybdenum oxide and recrystallization from aqueous solutions of ammonium para-molybdate has been developed. The low-thermal-gradient Czochralski technique was utilized to produce high quality ZnMoO4 crystal scintillators. Radiopurity of two detectors with size D5x4 cm was tested in the EDELWEISS low background set-up at the Modane Underground Laboratory (France). The measurements demonstrate excellent energy resolution (FWHM ~ 4–5 keV in the wide energy range) and efficient discrimination between alpha and beta particles in the region of interest (~15*sigma above 2.6 MeV). Only 210Po (with activity on the level of ~1 mBq/kg) was detected in the analyzed alpha background. The activity of 228Th and 226Ra is below 5 uBq/kg. Test of a first 100Mo (99.5%) enriched ZnMoO4 crystal is in progress. The excellent performance of ZnMoO4-based scintillating bolometers, the high radiopurity of the crystals and the possible extension of the technology to a ton-scale experiment make the approach one of the most promising even able to probe the normal hierarchy of the neutrino mass pattern.
        Speaker: Dr Denys Poda (Centre de Sciences Nucléaires et de Sciences de la Matière, CSNSM, 91405 Orsay, France and Institute for Nuclear Research, MSP 03680 Kyiv, Ukraine)
        Slides
      • 19
        Development of low radioactivity 40Ca100MoO4 crystal scintillators for the AMORE double beta decay search
        The AMoRE (Advanced Mo-based Rare process Experiment) collaboration will perform a search for neutrinoless double beta decay of Mo-100 using a cryogenic technique with Mo-100 enriched and Ca-48 depleted calcium molybdate (CaMoO4) crystal scintillators at the deep underground laboratory in Korea. CaMoO4 crystals show brightest scintillation light among variety of molybdate crystals at room and cryogenic temperature. Techniques to reduce internal background of CaMoO4 crystals and low radioactivity measurements at room and milli-Kelvin temperatures as well as expected various background levels using GEANT4 simulation will be presented.
        Speaker: Prof. HongJoo Kim (Kyungpook National University)
        Slides
      • 20
        Polymer Materials and Assay for Low Background Detectors
        Ultra low-background radiation measurements are essential to several large scale physics investigations, including dark matter detection (e.g. SuperCDMS). To drive down background noise levels, radioactive contaminant species in the materials used to make and house radiation detectors must be minimized. Low background materials have been affirmed as an infrastructure requirement at recent workshops.(e.g. Snowmass 2013) The radioisotopic purity required goes well beyond even the best industry practices for chemical purity. Prior work has addressed many issues with semiconductor detector materials, shielding, and metallic conductors(copper). There remain needs for insulating dielectric materials with extremely low background radioactivity, a need best met with polymer plastics. Past plastics found to have reasonable radiopurity, i.e., certain polyethylene and polytetrafluoroethylene (Teflon) materials, have very poor mechanical properties. There is a need to move from materials with milliBq/kg levels toward those with low microBq/kg levels. It is required that the materials be especially low in U, Th, and K. In addition, the assay of polymer materials for extremely low levels of radioactive elements presents new challenges. There are, for example, no certified reference materials for U or Th in thermoplastics. Typical methods for sample preparation for analysis of ppm to ppb levels of metals in plastic are not always directly applicable to the determination of ppt levels of U in plastics, because sample preparation processes can introduce contamination using conventional methods and containers. This talk will describe PNNL’s program in sourcing and analyzing polymers for low background applications. We will describe the multifaceted criteria for selecting potential polymers, and sourcing them to find low contaminant materials that are not subsequently contaminated by post-production processing. We will also describe a variety of assay methods, under use or in development, including radiation counting and various mass spectrometric techniques.
        Speaker: Dr Jay W. Grate (Pacific Northwest National Laboratory)
        Slides
    • 10:10
      Coffee Break
    • Session 5: Pure Materials (continued)
      Convener: Dr Tad Kishimoto
      • 21
        Electroforming Copper for the MAJORANA DEMONSTRATOR Experiment
        Locating neutrinoless double beta decay experiments deep underground mitigates backgrounds originating from cosmic rays. Of equal importance is constructing the inner shielding and detector components using ultra-low-background materials that contain the lowest levels of radio-contaminates to aid in the mitigation of backgrounds from natural sources. Because of the favorable thermal and electrical properties, ultrapure copper is an ideal material when levels of naturally occurring radioactive uranium and thorium, or troublesome cosmogenically induced radioisotopes such as Co-60, have been greatly reduced. Producing such copper is possible under stringently clean conditions within an underground electroforming facility. All copper comprising the inner shield and detector components in the MAJORANA DEMONSTRATOR experiment were electroformed at the 4850 level of the Sanford Underground Research Facility and the Pacific Northwest National Laboratory shallow underground facility. The material produced has exceeded the 10ksi tensile strength design criteria and the purity is better than the 0.3 µBq Th or U/kg Cu determined to be below the levels required for use in the DEMONSTRATOR.
        Speaker: Ms Cabot-Ann Christofferson (South Dakota School of Mines of Technology)
        Slides
      • 22
        Study of the Kroll-Process to Produce Ultra-Pure Ti for the Low Background Experiments
        To construct detectors for future low background experiments, a lot of ultra-pure construction materials will be needed. Our studies of various Ti samples have shown that the levels of radioactive contaminations (U and Th) in commercially available industrial titanium can vary from 0.2 to 100 mBq/kg. If the way to produce titanium in the tons scale with very low level of contaminations (below ~1 mBq/kg of U and Th) would be developed, than the titanium will become extremely promising material. Our study of the titanium sponge production process (Kroll-process) aims at finding sources of U and Th contaminations and their migration during multistage process. To understand U and Th migration during the Kroll process, we have studied a distribution of other impurities by means of precise ICP-MS analysis. Preliminary results have confirmed that the Kroll process could be used for ultra pure titanium sponge production. The further production stages of metal titanium parts with necessary mechanical properties from titanium sponge need to be additionally studied.
        Speaker: Dr Alexander Chepurnov (s: Skobeltsyn Institute of Nuclear Physics Lomonosov Moscow State University (SINP MSU))
      • 23
        Radon emanation based material measurement and selection for the SuperNEMO double beta experiment
        The SuperNEMO Demonstrator experiment aim to study the neutrino less double beta decay of 7kg of 82Se in order to reach a limit on the light Majorana neutrino mass mechanism T1/2(0nubb) > 6.5 . 10^24 years (90%CL) equivalent to a mass sensitivity m_bb < 0.20 - 0.40 eV (90%CL) in two years of data taking. The detector construction started in 2014 and the installation in the Laboratoire Souterrain de Modane (LSM) is expected during the course of 2015. The remaining level of 226Ra (238U chain) in the detector components can lead to the emanation of 222Rn gas. This isotope should be controlled and reduced down to the level of a 150 µBq/m3 in the tracker chamber of the detector to achieve the physic goals. Besides the HpGe selection of the detector materials for their radiopurity, the most critical materials have been tested and selected in a dedicated setup facility able to measure their 222Rn emanation level. The operating principle relies on a large emanation tank (0.7m^3) that allows to measure large material surfaces or large number of construction pieces. The emanation tank is coupled to an electrostatic chamber equipped with a silicon diode to perform the alpha spectroscopy of the gas and extract the 222Rn daughters. The transfer efficiency and the detector efficiency have been carefully calibrated through different methods. The intrinsic background of the system allows one to measure 222Rn activities down to 3mBq, leading to a typical emanation sensitivity of 20 µBq/m^2/day for a 30m^2 surface sample. Several construction materials have been measured and selected, such as nylon and aluminized mylar films, photomultipliers and tracking chamber parts.
        Speaker: Dr Cédric Cerna (Centre d'Etudes Nucléaires de Bordeaux Gradignan)
        Transparents
      • 24
        Low-Background Acrylic in the DEAP-3600 Dark Matter Experiment
        The DEAP-3600 experiment will search for dark matter particle interactions on 3.6 tonnes of liquid argon at SNOLAB. The argon is contained in a large low-background acrylic vessel viewed by 255 8-inch photomultiplier tubes. Very good pulse-shape discrimination has been demonstrated for scintillation in argon, and the detector has been designed for a total background budget, including (alpha,n) and external neutron recoils, surface contamination from 210Pb and radon daughters, of 0.2 events per tonne-year, allowing an ultimate sensitivity to spin-independent scattering of 10^{-46} cm^{2} per nucleon at 100 GeV mass. Details of the fabrication and radioactive background control of the acrylic will be presented, along with the current status of the experiment.
        Speaker: Aksel Hallin (University of Alberta)
        Slides
      • 25
        Surface purity control during XMASS detector refurbishment
        The XMASS project aims at detecting dark matter, pp and 7Be solar neutrinos, and neutrino less double beta decay using large volume of pure liquid xenon. The first physics target of the XMASS project is to detect dark matter with 835kg liquid xenon. After the commissioning runs, XMASS detector was refurbished to minimize the background contribution mainly from PMT sealing material and we restarted data taking in November 2013. Surface purity control during XMASS detector refurbishment and the result is reported. Status and future plan of XMASS are also reported.
        Speaker: Dr Kazuyoshi Kobayashi (ICRR, University of Tokyo)
        Slides
    • 12:20
      Lunch
    • Session 6: Liquid and Gas Purity
      Convener: Prof. Stefano Ragazzi
      • 26
        Radon and its Progeny: A pervasive problem for low-background counting
        As neutrino and dark matter experiments push to higher sensitivity for detecting rare events, background from radon (Rn-222) and its progeny can present serious problems. In some cases, background decays from one or more of the progenies limit the ultimate sensitivity of the experiment. Radon is produced in the decay chain of U-238, present in the Earth's crust at ppm levels, and diffuses from its origin in rock into the environment. The radiation from radon progeny includes high energy gamma rays from Bi-214, and beta and alpha radioactivity from the final decay sequence, Pb-222(22y β) → Bi-210(β,γ) → Po-210(α) → Pb-206(stable). Radon’s mobility and its long-lived progeny make this a dangerous source of background. In this talk I will summarize recent studies of radon daughters, and the possible relevance of these studies for Borexino and other low-background experiments.
        Speaker: Prof. Frank Calaprice (Princeton)
        Slides
      • 27
        Liquid Xenon Purification and De-Radonation
        Liquid xenon detectors are at the forefront of rare event physics, including searches for neutrino-less double beta decay and WIMP dark matter. The xenon for these experiments needs to be purified from chemical impurities such as electronegative atoms and molecules, which absorb ionization electrons, and VUV (178 nm) scintillation light-absorbing chemical species. In addition, superb purification from radioactive impurities is required. Particularly challenging are radioactive noble isotopes (Kr-85, Ar-39/42, Rn-222/220). Radon is a particularly universal problem, due to the extended decay sequence of its daughters and it’s ubiquitous presence in detector materials. Purification and de-radonation of liquid xenon will be addressed with particular focus on the experience gained with the EXO-200 neutrino-less double beta decay detector. Related issues with radon in the vicinity of the detectors will be discussed as time allows.
        Speaker: Andrea Pocar (UMass Amherst)
        Slides
      • 28
        Radon and material radiopurity assessment for the NEXT neutrinoless double beta decay experiment
        The “Neutrino Experiment with a Xenon TPC” (NEXT), intended to investigate neutrinoless double beta decay using a high-pressure xenon gas TPC filled with Xe enriched in 136Xe at the Canfranc Underground Laboratory (LSC, Laboratorio Subterráneo de Canfranc) in Spain, requires ultra-low background conditions demanding an exhaustive control of material radiopurity and environmental radon levels. An extensive material screening process is underway for several years based mainly on gamma-ray spectroscopy using ultra-low background germanium detectors at LSC but also on mass spectrometry techniques like GDMS and ICPMS. Components from shielding, pressure vessel, electroluminescence and high voltage elements and energy and tracking readout planes have been analyzed, helping in the final design of the experiment and in the construction of the background model. The most relevant measurements carried out will be presented. The commissioning of the NEW detector, as a first step towards NEXT, has started in Canfranc; in-situ measurements of radon levels are being taken and the development of a sensitive detector based on micromegas readout for airborne radon levels and radon emanations is underway. The status of these radon measurements as well as plans for radon mitigation in the experiment will be summarized. Finally, the implications on NEXT sensitivity for all these backgrund sources will be discussed.
        Speaker: Dr Susana Cebrian (University of Zaragoza)
        Slides
      • 29
        The NEST simulation framework for low energy response in liquid xenon
        Direct searches for dark matter require ultra-low-radioactivity techniques. In recent years, such experiments using noble liquids (especially liquid xenon) have obtained the lowest background levels in the field. Along with the development of this technology, there has been a continued effort in the community to better understand the detailed scintillation and ionization responses of noble liquids in the presence of low-energy ionizing radiation. As this body of knowledge is reaching a mature state, a unified software framework for simulating scintillation and ionization production in these detectors is strongly needed. In this talk, I introduce NEST: Noble Element Simulation Technique, which is an open-source simulation package based on physics models informed by the world's best data on the subject. I will show how the technique handles both electronic recoils (which comprise most of the background in a dark-matter search) and nuclear recoils (which are the expected signal), and their differing features that lead to the ability to perform background rejection.
        Speaker: Mr Brian Lenardo (Lawrence Livermore National Laboratory)
        Slides
      • 30
        Expected background in the LZ experiment
        The LZ experiment, featuring a 7-tonne active liquid xenon target, is aimed at achieving unprecedented sensitivity to WIMPs where the background in the nuclear recoil band is expected to be dominated by neutrinos. To reach this goal, extensive simulations are carried out to accurately calculate the electron recoil and nuclear recoil rates in LZ. Both internal and external backgrounds are considered. A very efficient suppression of background rate is achieved with an outer liquid scintillator veto, water Cherenkov detector, xenon skin and fiducialisation. Based on the current measurements of radioactivity of different materials, it is shown that we are on track to reduce the background from radioactivity for a WIMP search down to < 1 event in 1000 days for 5.6 tonne fiducial mass.
        Speaker: Dr Vitaly Kudryavtsev (University of Sheffield)
        Slides
    • 16:10
      Coffee Break
    • Session 7: Liquid and Gas Purity (continued)
      Convener: Dr Vitaly Kudryavtsev (University of Sheffield)
      • 31
        Measurements and understanding of radon adsorption in nanoporous materials
        For many experiments working at low energy and very low counting rate in particle and astroparticle physics, the background from the Radon decay chain is one of the strongest constraints. Most of the time, activated charcoal filters are used to dynamically capture the radon from the air or from the gas of the detectors. In general case activated charcoal is a good adsorptive material. It has large effective surface and broad porosity, going from macro to nanopores. However, the big constraints from futures experiments need ad hoc radon capture filters. The optimal adsorption depends on various parameters such as the correct pore size and shape, the temperature, the microscopic structure of the adsorbent or the competition between radon and carrier gas. In this context, we have developed at Centre de Physique des Particules de Marseille (CPPM), a test bench to study the radon capture in various porous materials1. Several very interesting results have been already obtained with non-standard, commercially and research adsorbents like some Carbon Molecular Sieves (CMS), organic molecular cage (CC3)2, or carbon aerogels. In this talk we present a global quantitative and qualitative study of radon adsorption in porous materials. This work is the results of the analysis of more than 30 porous materials in the framework of the collaboration between particle physicist and chemist from several universities. 1. Busto, J. & Collaboration, S. Radon adsorption in nanoporous carbon materials. AIP Conference Proceedings 1549, 112–115 (AIP Publishing, 2013). 2. Chen, L. et al. Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nature Materials 13, 954–960 (2014).
        Speaker: Mr Raymond NOEL (Centre de Physique des Particules de Marseille (CPPM))
        Slides
      • 32
        Ultra-low Level Radon Assays in Gases
        A "Radon Concentration Line" (RnCL) was developed to be used in conjunction with a state-of-the-art radon detector to achieve world leading sensitivities to 222Rn content in large gas volumes to the level of a few μBq/m3. A gas purification system was developed and installed which has demonstrated radon suppression by several orders of magnitude depending on the carrier gas. This apparatus has now been commissioned and measurements of cylindered gas has been made to confirm radon suppression. The results from measurements of radon content in gases used in SuperNEMO using the RnCL will be presented. This system can also be used for radon emanation measurements from construction material samples when coupled to a dedicated emanation chamber. Results from both radon emanation and radon diffusion studies will also be presented.
        Speaker: Mr XinRan Liu (University College London)
        Slides
      • 33
        Background reduction of a spherical gaseous detector
        The Spherical gaseous detector (or Spherical Proportional Counter, SPC) is a novel type of detector. It consists of a large spherical volume filled with gas, using a single detection readout channel. The detector allows ≈100% detection efficiency. SEDINE is a low backg round version of SPC (made of Cu_NOSV) installed at the LSM underground laboratory (4800 m.w.e) looking for rare events at very low energy threshold, below 100 eV. I will present details on the chemical cleaning to reduce internal Pb-210 surface contamination on the copper vessel and the external radon decreasing via circulation of pure air below anti-radon tent. I will also show the radon measurement of pure gases (Ar, N, Ne, etc) which are used in the underground laboratory for the low background experiments.
        Speaker: Dr Ali DASTGHEIBI FARD (CNRS/IN2P3/LSM)
        Slides
      • 34
        Selective precipitation of potassium in seawater samples for improving the sensitivity of plain γ-ray spectrometry
        An analytical method is presented to reduce the amount of 40K in sea water samples, in order to lower its interference in γ-ray analysis below 1.4 MeV due to the Compton continuum. Sodium tetraphenylborate was used to successfully precipitate 40K in the samples. A custom procedure for precipitation of potassium was developed and it was evaluated for its selectivity, repeatability and efficiency, using conventional analytical techniques such as atomic absorption spectrophotometry and inductively coupled plasma mass spectrometry (ICP-MS). This work has shown that the selective precipitation of potassium with sodium tetraphenylborate has led to a decrease of detection limit of radio nuclides such as 238U,226Ra, 228Ra, 137Cs, 134Cs, 133I, 134I, 60Co in γ-analysis. In particular, the detection limit for nuclides with emissions in the energy window energy below 1400 KeV is improved by almost one order of magnitude. Keywords: Sodium tetraphenylborate, gamma spectroscopy, potassium, precipitation, seawater
        Speakers: Mr Marco Ferrante (Laboratori Nazionali Del Gran Sasso - INFN), Dr Matthias Laubenstein (Laboratori Nazionali Del Gran Sasso - INFN), Mr Stefano Nisi (Laboratori Nazionali del Gran Sasso - INFN)
        Slides
    • 18:00
      Workshop Banquet Ivar's Salmon House

      Ivar's Salmon House

      401 NE Northlake Way, Seattle, WA 98105
  • Friday 20 March
    • Session 8: Liquid Purity
      Convener: Andrea Pocar (UMass Amherst)
      • 35
        Quest for Lowest Energy Neutrinos in Super-Kamiokande
        This presentation is for the lowest energy physics in Super-Kamiokande, solar neutrino. SK detects 8B solar neutrinos through neutrino-electron elastic scattering, where the energy, direction, and time of the recoil electron are measured. Since SK-III started, many efforts to reduce backgrounds have been made. The most serious background comes from the beta decay of 214Bi, which is produced in the decays of radon in the purified air, detector materials, and the purified water. In order to reduce the 214Bi background, the SK water system has been upgraded. First, the cooling power of the heat exchanger for the supply water was increased so as not to induce convection in the tank, which transports radon near the PMTs into the fiducial volume. Then, the water flow in the detector was precisely investigated and optimized to reduce the background contamination in the fiducial volume as much as possible. In order to evaluate the remaining radon concentration, very low background and high precision radon detectors for air and water were newly developed. Not only radon, but other contaminations in the water (bacteria and metal ion) were also investigated.
        Speaker: Dr Hiroyuki Sekiya (ICRR, University of Tokyo)
        Slides
      • 36
        Assay and identification of particulate contaminants from the PICO dark matter search experiment
        An unknown source of bubble nucleation is impacting the sensitivity of the bubble chambers of the PICO dark matter search. After the end of the current PICO runs assays have shown contamination with particulates. The present hypothesis is that these residual particulates which are small enough in size to remain suspended in the liquid are a dominant contributor of this nucleation. We have developed methods to test this hypothesis and to investigate the source of the particulate matter in the chambers. A procedure for the collection of samples from both of the PICO chambers (2L and 60L) was developed to extract particulates on a PTFE filter. The filter was then examined under optical and electron microscopes. A method to estimate the mass of each kind of particulate seen in the filters (steel oxide, silica) was then developed using electron micrographs, x-ray spectroscopy and image analysis. This presentation reports on procedures developed and the studies performed to estimate the mass and to identify the types of materials in the samples and discusses the possible sources of the contaminants.
        Speaker: Mr Pitam Mitra (University of Alberta)
        Slides
      • 37
        Radiopure Metal-doped Liquid Scintillator
        Metal-doped liquid scintillator plays a key role in various particle and nuclear physics experiments. The applications of different metallic ions in solar and reactor neutrinos and in double beta decay searches and their purification methods will be discussed in this talk.
        Speaker: Dr Minfang Yeh (Brookhaven National Laboratory)
        Slides
      • 38
        A Scintillator Purification Plant and Fluid Handling System for SNO+
        A large capacity purification plant and fluid handling system has been constructed for the SNO+ neutrino and double-beta decay experiment, located 6800 feet underground at SNOLAB, Canada. SNO+ is a refurbishment of the SNO detector to fill the acrylic vessel with liquid scintillator based on Linear Alkylbenzene (LAB) and 2 g/L PPO, and also has a phase to load nature tellurium into the scintillator for a double-beta decay experiment with 130Te. The plant includes processes multi-stage dual-stream distillation, column water extraction, steam stripping, and functionalized silica gel adsorption columns. The plant also includes systems for preparing the scintillator and metal-loading the scintillator for double-beta decay exposure. We review the basis of design, the purification principles, specifications for the plant, and the construction and installations. Passivation and high-purity cleaning of the plant is currently in progress, and soon we will begin the plant commissioning stage. We discuss the methods for achieving vacuum leak-tightness, passivation and high-purity cleaning, and the safety system requirements for operations in a deep mine.
        Speaker: Richard Ford (SNOLAB)
        Slides
    • 09:50
      Coffee Break
    • Session 9: Cosmogenics
      Convener: Prof. Richard Schnee (South Dakota School of Mines &amp; Technology)
      • 39
        FLUKA, Predictive Power for Cosmogenic Backgrounds
        The next generation of experiments searching for rare-physics events with increased sensitivity will require precise predictions of cosmogenic backgrounds. Recent high quality deep underground measurements for cosmogenic neutrons in large liquid scintillator targets were used to study the FLUKA simulation package for this purpose. The results and conclusions drawn from a detailed benchmark comparison with data from the Borexino experiment were reported recently and will be summarized with focus on identified issues. Improved physics models already implemented in a beta-version of the FLUKA code, which will be publicly available with the upcoming code release, address the more important identified issues. A careful evaluation of the improved predictions is ongoing. However, the agreement between FLUKA simulation results and the Borexino experimental data is quite excellent at this point. Preliminary new findings will be presented.
        Speaker: Anton Empl (University of Houston)
        Slides
      • 40
        Study of Muon-Induced Neutron Production Using Test Beam Facility at CERN
        In the Fall of 2012, we conducted a beam test at CERN to study the feasibility of using the M2 beam line to systematically study the production properties of muon-induced fast neutrons. The M2 beam line is capable of delivering high intensity muon beam with momentum up to 280 GeV/c. Our experimental setup consists of a lead target and a neutron detector surrounded by veto counters placed at about 1.5 m away from the target. In this talk, we summarize our findings of the beam test..
        Speaker: Dr Yasuhiro Nakajima (LBNL)
        Slides
      • 41
        Monte Carlo Study and Data Analysis of the Neutron Multiplicity Meter
        Muon-induced neutrons are a problematic background for underground experiments searching for rare-event interactions such as WIMP dark-matter scatters, neutrino interactions with matter, neutrinoless double-beta decay, and proton decay. Current knowledge of both the rate and energy spectrum of such cosmogenic neutrons is limited. The Neutron Multiplicity Meter (NMM) is a water Cherenkov detector capable of measuring the cosmogenic neutron flux at the Soudan Underground Laboratory with an overburden of 2090 meters water equivalent. The NMM consists of two 2.2-tonne gadolinium-doped water tanks situated atop a 20-tonne lead target. It detects a high-energy (> ~50 MeV) neutron via moderation and capture of the secondary neutrons released when the former interacts in the lead target. I will present results from Monte Carlo simulation and analysis of the first ~2 year dataset.
        Speaker: Mr Yu Chen (Syracuse University)
        Slides
      • 42
        Comparison of Radiogenic Neutron Background Calculations
        Low background experiments must contend with the intrinsic radioactivity of their detector and surrounding materials, including radiogenic neutrons which may be generated by spontaneous fission and alpha-n reactions. To calculate the expected neutron yield and spectra from alpha-n reactions due to the thorium and uranium decay chains, cross-section databases and nuclear model code systems must be consulted. Here we present a comparison of the neutron spectra and yields generated by two such collections: EMPIRE cross-sections used with SOURCES4 nuclear modeling code, and the TENDL data library used with a TALYS codebase. We focus our comparisons upon materials common to current direct dark matter detectors, and present the implications these differences may have in neutron background predictions.
        Speaker: Kimberly Palladino (SLAC)
        Slides
      • 43
        Fast Neutron Backgrounds As A Function Of Depth Underground
        Fast neutrons can be created by muogenic processes and produce a depth dependent background for rare-event neutral particle detectors. Muogenic fast neutrons can have energies extending past several hundred MeV. Muons that do not pass through but near the active shielding of these detectors are particularly problematic since they may produce high energy neutrons capable of penetrating the detector shielding. At all but the most shallow depths (>200 meter water equivalent (m.w.e.)) the fast neutron background lacks consensus on the spectrum and rate anti-coincident from the initiating muon. A fast neutron spectrometer has been constructed and deployed to measure the fast neutron background above 50 MeV at the Kimballton Underground Research Facility (KURF). The detector uses a lead-based spallation amplifier to convert single fast neutrons into multiple lower energy secondary neutrons. The secondary neutron signal is used to un-fold the incident neutron energy. Measurements have been completed and preliminary results obtained at 380 and 600 m.w.e. Currently the detector is operating at the 1450 m.w.e. depth at KURF.
        Speaker: Mr Caleb Roecker (University California Berkeley)
        Slides
    • 12:00
      Lunch
    • Session 10: Background Modeling and Measurements
      Convener: Prof. Maria Martinez
      • 44
        Background analysis techniques in the CUORE experiment
        The Cryogenic Underground Observatory for Rare Events (CUORE) will search for the neutrinoless double-beta (0νββ) decay of 130Te using an array of 988 high-resolution TeO2 bolometers. 0νββ decay is a long sought-after second-order weak process in which a nucleus (A,Z) transitions to a nucleus (A,Z+2) through the emission of two electrons. Searching for this decay is currently the only practical method to investigate the Majorana nature of neutrinos. Observation of 0νββ decay would not only establish that neutrinos are Majorana fermions, but also constrain the neutrino-mass scale and hierarchy and demonstrate that total lepton number is not conserved. In CUORE, the signature of the decay is a peak at the double-beta decay Q value (Qββ) of 2528 keV. As 0νββ decay would be a rare process, minimizing the background rate is essential for maximizing the experimental sensitivity. CUORE’s goal background rate at Qββ is 0.01 counts/(keV∙kg∙y), which would allow the experiment to reach a half-life sensitivity of 9.5e25 years (90% C.L.), assuming a live time of 5 years and a FWHM energy resolution of 5 keV. To reach this goal, various background analysis techniques have been developed or implemented, including pulse-shape discrimination, Monte Carlo simulations of background sources, and studies of cosmogenic activation of materials. This talk will provide an overview of these techniques. This work was supported by the National Science Foundation under Grant nos. NSF-PHY-0605119, NSF-PHY-0500337, NSF-PHY-0855314, NSF-PHY-0902171, and NSF-PHY-0969852; the Istituto Nazionale di Fisica Nucleare (INFN); the Alfred P. Sloan Foundation; the University of Wisconsin Foundation; Yale University; the US Department of Energy (DOE) Office of Science under Contract nos. DE-AC02-05CH11231 and DE-AC52-07NA27344; and the DOE Office of Science, Office of Nuclear Physics, under Contract nos. DE-FG02-08ER41551 and DEFG03-00ER41138.
        Speaker: Dr Barbara Wang (University of California, Berkeley)
        Slides
      • 45
        Radiation Background of PandaX Experiment at China Jingping Uderground Lab
        Understanding, controlling and reducing experimental background is crucial in dark matter searching experiments. In this report, we review various radiation sources shown in the PandaX experiment[1,2], as well as the strategies of controlling and reducing them. These background sources include cosmic ray, Radon, the passively shielding materials, the vessels holding the Xe, the detector materials, and 85Kr contamination in Xenon itself. All materials used in the experiment were scrutinized before hands. A Monte Carlo code is used to simulate the experiment, and to understand the background. This report will focus on the technics of distillation tower for Kr removal, and homemaking low-background stainless steel used in the PandaX project. 1. PandaX Collabration, First dark matter search results from the PandaX-I experiment, Science China 57, 2024(2014), http://arxiv.org/abs/1408.5114 2. PandaX Collabration, PandaX: A Liquid Xenon Dark Matter Experiment at CJPL, http://arxiv.org/abs/1405.2882 http://arxiv.org/abs/1405.2882
        Speaker: Dr Changbo Fu (Shanghai Jiaotong University)
        Slides
      • 46
        Background analysis and status of the ANAIS dark matter project.
        ANAIS (Annual modulation with NaI Scintillators) is a project aiming to set up at the new facilities of the Canfranc Underground Laboratory (LSC), a large scale NaI(Tl) experiment in order to explore the DAMA/LIBRA annual modulation positive result using the same target and technique. Two 12.5 kg each NaI(Tl) crystals provided by Alpha Spectra are currently taking data at the LSC (ANAIS-25) and a new detector, with improved performances, will be received at the beginning of 2015. The comparison of the background model for the ANAIS-25 prototypes with the experimental results will be presented. ANAIS crystal radiopurity goals have been achieved for Th-232 and U-238 chains, and are at reach for K-40 but a Pb-210 contamination out-of-equilibrium has been identified, whose origin is being studied. The high light collection efficiency obtained with these prototypes allows to anticipate an energy threshold of the order of 1 keVee. Preliminary estimates of trigger efficiency at very low energy will be also presented.
        Speaker: Dr Clara Cuesta (University of Washington)
        Slides
      • 47
        Background investigation in EDELWEISS-III
        Protection from and rejection of backgrounds is a key issue for the EDELWEISS-III direct dark matter detection experiment which aims at exploring the 10^-9 - 10^-10 pb cross section region for spin-independent WIMP-nucleon interactions. The experiment is situated in the low radioactivity environment of the Modane Underground Laboratory and consists of 36 advanced FID germanium detectors operating at 18 mK in a dilution refrigerator in order to identify eventual rare nuclear recoils induced by elastic scattering of WIMPs from our Galactic halo. I will discuss the electromagnetic background and the methods of rejecting it with the FID detectors. Detector performances and a first analysis of data acquired in a long-term campaign will be presented as well. The FID detector technology is not limited to EDELWEISS-III but can be further employed in the next generation of cryogenic detector experiments.
        Speaker: Dr Silvia Scorza (KIT)
        Slides
      • 48
        Reduction of Backgrounds for SuperCDMS
        The advanced phases of the Cryogenic Dark Matter Search (SuperCDMS) attempt to directly detect galactic dark matter via keV-scale nuclear recoils in semiconductor detectors located deep underground. I will review the detection technique and the backgrounds observed in the current 9 kg experimental phase, SuperCDMS Soudan. I will then provide an overview of the next-generation SuperCDMS SNOLAB project, emphasizing the features that will reduce backgrounds sufficiently to reach our sensitivity goals. Particularly important is more careful mitigation of radon-related backgrounds. I will discuss the commissioning of a cleanroom with radon level reduced to ~0.2 Bq/m^3 using a vacuum-swing-adsorption radon mitigation system.
        Speaker: Dr Raymond Bunker (South Dakota School of Mines &amp; Technology)
        Slides
    • 15:20
      Coffe Break
    • Session 11: Background Modeling and Measurements (continued)
      Convener: Dr Carla Maria Cattadori (INFN)
      • 49
        Low-background techniques applied in the Borexino experiment
        Borexino, located at the Gran Sasso Laboratory, is a liquid scintillator detector with active mass of 278 tons. The main goal of the experiment is the real-time registration of sub-MeV solar neutrinos through their elastic scattering on the electrons. The lack of directionality of the light emitted by the scintillator makes it impossible to distinguish neutrino-scattered electrons from electrons due to natural radioactivity. This leads to a crucial requirement of the Borexino technology, namely an extremely low radioactive contamination of the detection medium. This has been achieved after extensive R&D studies and presently the detector purity is at an unprecedented level, never achieved so far in any other project. In this sense the Borexino detector is very unique world-wide and allows to study extremely week processes. Thanks to its extremely low background level the collaboration was able to register in real-time almost entire spectrum of the solar neutrinos, including the most fundamental low-energy pp-neutrinos.
        Speaker: Grzegorz Zuzel (Institute of Physics, Jagiellonian University in Krakow, Poland)
        Slides
      • 50
        Background reduction in the SNO+ Experiment
        SNO+ is a large multi-purpose liquid scintillator experiment, which first aim is to detect the neutrinoless double beta decay of Te-130. It is placed at SNOLAB, at 6000 m.w.e. and it is based on the SNO infrastructure. SNO+ will contain approximately 780 tonnes of liquid scintillator, loaded with Te-130 inside an acrylic vessel (AV) with an external volume of ultra pure water to reduce the external backgrounds. Light produced in the scintillator by the interaction of particles will be detected with about 9,500 photomultipliers. For the neutrinoless double beta decay phase, due to its the extremely low rate expected, the control, knowledge and reduction of the background is essential. Moreover, it will also benefit other phases of the experiment focused on the study of solar neutrinos, nucleon decay, geoneutrinos and supernovae. In order to reduce the internal background level, a novel purification technique for tellurium loaded scintillators has been developed by the collaboration that reduces the U/Th concentration and several cosmic-activated isotopes by at least a factor 10^2 -10^3 in a single pass. In addition, different rejection techniques have been developed for the remaining internal backgrounds based on MonteCarlo simulations. In this work, the scintillator purification technique and the levels obtained with it will be discussed. Furthermore, an overview of the different backgrounds for the double beta phase will be presented, highlighting some of the techniques developed to reject the remained ones based on their expected timing differences.
        Speaker: Dr Laura Segui (University of Oxford)
        Slides
      • 51
        Results and perspectives of the GERDA experiment
        The GERDA experiment for the search of neutrinoless double beta decay has finished its first phase of data taking in 2013. A very low background level of ~0.01 Counts/(kg yr keV) corresponding to ~40 Counts/(t yr RoI) has been achieved around Q_bb. Competitive limits on neutrinoless double beta decay of 76Ge have been given. The experiment will be described. Special emphasis will be given to the background understanding of the GERDA phase I data. The preparations for the second phase of data taking with the GERDA experiments will be reported and a projection for the expected background level will be given.
        Speaker: Dr Bela Majorovits (MPI für Physik)
        Slides
      • 52
        Low background techniques in CANDLES
        CANDLES is a double beta decay experiment using 48Ca in CaF2 crystals. Thanks to the highest Q value among all isotope candidates (4.27 MeV), we can measure signals in small background contribution. A remarkable distinction in the measurement is the active veto by liquid scintillator. Because the time constant is largely different in pulse shape between CaF2 and liquid scintillator, large amount of external backgrounds can be removed by choosing only CaF2 scintillation signals. One of the main background around Q value comes from radioactive impurities in CaF2 crystals, especially 232Th decay series. We developed highly pure crystals, and those with least impurities can be chosen in the analysis. Pile-up events of 212Bi and 212Po decays are background, and they can be reduced by pulse shape discrimination. 208Tl is also dominant background, and we apply veto time after tagging parent nuclei 212Bi. Impurities of each crystals and analytical method of background reduction is presented in this talk. The other main background is high energy gamma rays (< 10 MeV) from neutron captures on the material surrounding detector, such as rock. Observed spectrum using neutrons sources are compared with MC, and we discuss the effect of elemental rock composition to the spectrum. Since this is the most serious background in CANDLES, we plan to install the shield for external gamma rays and neutrons.
        Speaker: Dr Kyohei Nakajima (Osaka University)
        Slides
      • 53
        Closing Remarks
        Slides
    • 17:15
      Adjourn
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