Presented at the Cracow Epiphany Conference on Physics in Underground Laboratories and Its Connection with LHC, Cracow, Poland, January 5–8, 2010.The aim of the ArDM project is the development and operation of a one ton double-phase liquid argon detector for direct Dark Matter searches. The detector measures both the scintillation light and the ionization charge from ionizing radiation using two independent readout systems. This paper briefly describes the detector concept and presents preliminary results from the ArDM R&D program, including a 3 l prototype developed to test the charge readout system

Bueno Villar, Antonio

Carmona-Benítez, María del Carmen

Harańczyk, M.

Lozano Bahílo, Julio

Melgarejo-Fernández, Antonio Jesús

Navas Concha, Sergio

Acta Physica Polonica B Proceedings Supplement

English

Navas, Sergio

Institutional Repository of the University of Granada

The ArDM Experiment

Repositorio Institucional Universidad de Granada

Vol. 41 (2010) ACTA PHYSICA POLONICA B No 7THE ArDM EXPERIMENT∗M. Harańczykh, C. Amslerb, A. Badertschera, V. BocconebN. Bourgeoisk, A. Buenoc, M.C. Carmona-BenitezcM. Chorowskig, W. Creusb, A. Curionia, E. Dawf , U. DegundaaA. Dell’Antoneb, M. Drögea, L. Epprechta, C. HalleraS. Horikawaa, L. Kaufmanna, J. Kisieli, L. KnechtaM. Laffranchia, J. Lagodae, C. Lazzaroa, P. LightfootfJ. Lozanoc, D. Lussia, G. Mairek, S. Maniai, A. MarchionniaK. Mavrokoridisj, A. Melgarejoc, P. Mijakowskie, G. NattereraS. Navas-Conchac, P. Otiougovab, A. Piotrowskag, J. PolińskigM. de Pradod, P. Przewłockie, S. Ravatk, C. RegenfusbF. Resnatia, M. Robinsonf , J. Rochetb, L. Romerod, E. RondioeA. Rubbiaa, L. Scotto-Lavinab, N. Spoonerf , T. ViantaA. Trawińskie, J. Ulbrichta, A. ZalewskahaETH Zurich, SwitzerlandbZurich University, SwitzerlandcUniversity of Granada, SpaindCIEMAT, SpaineA. Sołtan Institute for Nuclear Studies, Warszawa, PolandfUniversity of Sheffield, United KingdomgWrocław University of Technology, Wrocław, PolandhH. Niewodniczański Institute of Nuclear Physics PAN, Kraków, PolandiUniversity of Silesia, Katowice, PolandjUniversity of Liverpool, United KingdomkCERN, Switzerland(Received May 14, 2010)The aim of the ArDM project is the development and operation of aone ton double-phase liquid argon detector for direct Dark Matter searches.The detector measures both the scintillation light and the ionization chargefrom ionizing radiation using two independent readout systems. This paperbriefly describes the detector concept and presents preliminary results fromthe ArDM R&D program, including a 3 l prototype developed to test thecharge readout system.PACS numbers: 95.35.+d∗ Presented at the Cracow Epiphany Conference on Physics in UndergroundLaboratories and Its Connection with LHC, Cracow, Poland, January 5–8, 2010.(1441)1442 M. Harańczyk et al.1. IntroductionUnderstanding the nature of Dark Matter is a key issue in contempo-rary particle physics and astrophysics. One of the prime candidates are theWeakly Interacting Massive Particles — WIMPs. Most of the Dark Mat-ter models predict that WIMPs interact only gravitationally and weaklywith matter, thus forming cold relic halos around massive astronomical ob-jects. Though there is convincing evidence for Dark Matter from cosmologyand astronomy, WIMP interactions on ordinary matter are exceedingly rareand therefore direct detection of Dark Matter particles is very difficult andto-date no uncontroversial signal has been discovered [1,2]. From the exper-imental point of view, noble liquids, such as argon and xenon, are amongthe best options for large WIMP direct detection experiments. They com-bine the possibility of reaching ton-scale target masses, low energy threshold(in the keV), and the necessary discrimination power against background.The Argon Dark Matter experiment (ArDM) was initiated in 2004 and isbeing developed at CERN. The goal of the project is to assemble and op-erate the 1 ton scale double phase (liquid–gas) argon detector with inde-pendent readout of the primary scintillation light and the ionization charge.The principle of design and operation of the ArDM detector is described inSection 2.2. Design and first test of the ArDM detector2.1. Detector descriptionThe conceptual design of the ArDM experiment is illustrated in Fig. 1.A vacuum insulated, stainless steel dewar, 200 cm high and 100 cm in di-ameter, contains a sensitive mass of 850 kg of liquid argon. In order tocreate a strong uniform electric field along a vertical drift of the length of120 cm, a Greinacher (Cockroft–Walton) chain keeps the cathode, placed atthe bottom of the sensitive region, and the field shaping rings at the propervoltage. Fourteen 8” low background Photo Multiplier Tubes (PMT), suit-able for cryogenic operation, are used for light readout. They are placedbelow the cathode grid. In liquid argon, the scintillation light is producedin a narrow bandwidth around 128 nm, in the VUV region, which is notdirectly detectable by the PMTs and needs to be wavelength-shifted. ThePMTs are coated with TPB (tetraphenylbutadiene) that shifts the wave-length from the VUV to the visible part of the spectrum (420 nm). Mostof the direct scintillation light hits the sides of the detector rather than thePMTs, therefore to enhance the light yield the sides of the detector are cov-ered with TPB coated reflectors. The charge readout system is positionedabove the liquid surface, in the argon vapor. This system extracts the ion-The ArDM Experiment 1443Fig. 1. Design and working principle of the double-phase Argon Dark Matterdetector.ization charge from the liquid phase into the vapor phase, provides chargemultiplication and finally detects the induced signals on a segmented anode.Charge multiplication is necessary in order to produce a detectable signalover noise for ionizing events with energies as low as few keVs. A multi-stage system of Large Electron Multipliers (LEM) can provide the necessarythousandfold charge multiplication. This LEM readout system is being de-veloped on a smaller scale in dedicated setup (see Section 3). In order todrift free electrons in liquid argon over lengths of a meter, the concentra-tion of electronegative impurities in liquid argon must be extremely small [3]better than a part per billion (ppb). This level of purity is obtained by recir-culating the argon through a custom made copper filter. For the completedescription and design details of the ArDM detector see [4, 5].2.2. First test of ArDM with liquid ArThe ArDM detector was filled with liquid argon for the first time in May2009. The run lasted for over a month. The light readout system was onlypartially completed and was deployed with 7 PMTs in place. The goals ofthis month long run were: test of the cryogenic system (cool down and stableoperation), achieve good and stable liquid argon purity, commissioning andstable operation of the available light readout system, provide a preliminarymeasurement of the light yield, develop data reconstruction and providedata for benchmarking the Monte Carlo simulation. Measurements of thelight collection efficiency were performed with two gamma sources, 137Cs and22Na. Data were taken in a variety of trigger configurations. Figure 2 showsthe energy spectrum in units of photoelectrons (p.e.) from the exposure1444 M. Harańczyk et al.to 22Na. The 511 keV gamma rays were externally tagged through detectionof the coincident 1274 keV and 511 keV gammas, the latter emitted back-to-back, with a NaI counter. From these measurements, a light yield of 0.5p.e./keV was determined for this partially instrumented detector. Off-line itwas determined that the rms noise is approximately 2 p.e. for the 7 PMTs,providing an energy threshold of few keVs. For a preliminary description ofthe measurements see also [6].Fig. 2. Energy spectrum from exposure to 511 keV gamma rays (Monte Carlosuperimposed).3. Small setup for charge readout R&DA 3 l setup dedicated to R&D work for the LEM readout has been builtand has been in operation for the past two years [7, 8]. It is a scaled downversion of the full ArDM detector, with a PMT installed below the cathode,extraction grids at the liquid–vapor interface, LEM for charge multiplicationin the Ar vapor and a segmented anode for readout. Charge multiplicationhappens in the LEM holes, where there is a high enough electric field tostart Townsend multiplication [9, 10]. A picture and a sketch of the smallLEM-TPC with one stage LEM device is shown in Fig. 3. A LEM is a thickmacroscopic hole multiplier manufactured with standard PCB techniques.Several different LEMs have been tested in this setup, differing for theirgeometrical parameters, manufacturers, or both. Event shown here (Fig. 4)is for a one-stage LEM, 1 mm thick, double-sided copper-clad plate drilledwith a regular pattern of 500 µm diameter holes, with a distance of 800 µmbetween the centers of the neighboring holes. The LEM itself is segmented(strips with 6 mm pitch), and after the LEM the charge is readout by a seg-mented anode, with strips (6 mm pitch) oriented orthogonally with respectto LEM’s strips, thus providing X–Y readout.The ArDM Experiment 1445Fig. 3. Photograph (left) and schematics (right) of the argon LEM-TPC 3 l setup.This double phase argon LEM-TPC allows the reconstruction of theposition of the ionizing event (the value of third coordinate is assigned basedon drift time). As an example, Fig. 4 shows a typical cosmic muon trackregistered during double phase operation.Fig. 4. Typical cosmic muon event.1446 M. Harańczyk et al.4. Conclusions and outlookThe ArDM detector has had a first run in Spring 2009, which allowed totest and commission the cryogenic system and a partially completed lightreadout system. More tests are foreseen for the remaining of year 2010,with the fully completed light readout system in place. The detector willbe operated as a full double phase LAr TPC, with a direct readout of theionization charge. After completion of the R&D and commissioning phase,ArDM will be moved to an underground, low background location to startthe Dark Matter science phase. Several underground laboratories are underconsideration, among them the SUNLAB laboratory at the Sieroszowice–Polkowice mine in Poland.REFERENCES[1] Y. Mambrini, Acta Phys. Pol. B 41, 1395 (2010), this issue.[2] A. Szelc, Acta Phys. Pol. B 41, 1417 (2010), this issue.[3] S. Amoruso et al., Nucl. Instrum. Methods A516, 68 (2004).[4] A. Rubbia, J. Phys. Conf. Ser. 39, 129 (2006) [arXiv:hep-ph/0510320].[5] L. Kaufmann, A. Rubbia, Nucl. Phys. Proc. Suppl. 173, 141 (2007).[6] Ch. Regenfus, J. Phys. Conf. Ser. 203, 012024 (2010) [arXiv:0912.2962v1].[7] A. Badertscher et al., arXiv:0811.3384v1 [physics.ins-det].[8] A. Badertscher et al., arXiv:0907.2944v1 [physics.ins-det][doi:10.1016/j.nima.2009.10.011].[9] J. Townsend, Electrons in Gases, Hutchinson, London 1947.[10] F. Sauli, Principles of Operation of Multiwire Propotional and Drift Cham-bers, lectures given in the Academic Training Programme of CERN 1975–1976,Geneve 1977.

The ArDM Experiment

Abstract

Similar works

Full text

Available Versions

Institutional Repository of the University of Granada

Repositorio Institucional Universidad de Granada