PoS(EPS-HEP 2009)110 http://pos.sissa.it/ † ∗ [email protected] Speaker. On behalf the ArDM collaboration: Andrzej Soltan Institute (Warsaw), CIEMAT (Madrid), ETH-Zürich, Univer- Weak Interacting Massive Particles (WIMPs)universe. are The ArDM prime experiment candidates consiststeracting for of elastically a the with 1t argon dark nuclei. liquid matter Both argon scintillationrecoils detector in light are to and the measured. search ionization charge for The from WIMPs nuclear 128length in- shifting nm foils light and is detected reflected bybe and 8" detected shifted photomultipliers above immersed into the in the the liquid visible liquid. surfacebeing range developed by The by at charge a CERN wave- will double on stage the large surfacefilled before electron for being the multiplier. moved first to The time an detector in undergroundlead is spring site. to 2009 ArDM and a was operated light for collection moreshows efficiency than that of one our month. about goal of Preliminary 0.3 reaching results photoelectrons a / detection kev threshold of for 30 nuclear keV recoils, can be which achieved, ∗ † Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

sities of Granada, Sheffield and Zürich. The 2009 Europhysics Conference on HighJuly Energy 16 Physics, - 22 2009 Krakow, Poland C. Amsler Physik-Institut der Universität Zürich E-mail: ArDM, a 1t liquid argon detectorsearches for dark matter PoS(EPS-HEP 2009)110 + 2 C. Amsler ’s, the ratio is γ Ar isotope. 39 ]. Hence nuclear recoils from 2 -emitter 4 [ β ≈ ]. However, impurities such as water, air 2 ]. The goal of ArDM is to improve on the 1 ]. The population ratio singlet/triplet depends 3 [ pb [ 2 2 8 τ − ) are coated to cover the cylindrical walls inside the 2 s in liquid [ 10 µ × 6 . 1 25 cm ' × 2 (left). The working principle is as follows: in liquid argon a τ 1 5 ns and ' 1 τ ’s and nuclear recoils one finds a ratio of α Left: sketch of the ArDM detector and its purification system on the left. Right: photograph of can absorb the VUV light and reduce ), with the lowest singlet and the triplet excited states decaying into the ground state (two 2 ∗ 2 A sketch of the ArDM detector, as it is now installed in building 182 at CERN (RE18- Charged particles lead to ionization and excitation of argon atoms forming excimers (Ar The current upper limit for the elastic cross section between weak interacting massive parti- 3, while for . 0 electric field shaping rings.spherical The Hamamatsu light R5912-MOD detection manufactured system with consists Pt-underlays) of at 14 the photomultipliers bottom (8” of hemi- the vessel. WIMP collision leading to 30together with keV a nuclear few free recoils electrons. producesin The about the latter drift 400 gas in VUV phase a strong by (128VUV vertical scintillation a nm) electric light field large photons, from and electron are argon multiplierorated detected is (LEM) on shifted above into Tetratex the foils blue light surface andvessel. by of detected Fifteen tetraphenylbutadiene the by Tetratex (TPB) sheets liquid, cryogenic evap- (120 while photomultipliers the (PMT) at the bottom of the experiment), is shown in fig. Figure 1: the detector showing the Greinacher HV divider, the WLS foils and the photomultiplier support mechanics. sensitivity to WIMPs by moreliquid argon than detector one using the order two-phase of techniqueproduced to magnitude. by detect the both We recoil the are charge nuclei andcollect constructing following the and a at luminescence detect WIMP CERN the interaction. VUV a light Webackground 1t to have from reach studied neutrons a ways and detection to electrons, threshold in efficiently of particular 30 from keV the in argon andand to Ar suppress independent atoms) by VUVby photon argon emission atoms is in energetically atimes, suppressed. narrow respectively The band singlet around and the 128and triplet CO nm. states have Reabsorption different decay ≈ on ionization density. For minimum ionizing projectiles, such as electrons and cles (WIMPs) and nucleons is around 4 ArDM, a 1t liquid argon detector for dark matter searches WIMPs populate mostly theionization fast yield decaying is singlet much state.quick higher recombination for In which nuclear addition, decreases than the the chargeof for ratio and light minimum enhances of to the charge ionizing scintillation production luminescence. particles. for to The nuclearcan higher recoils This both ratio and be is the used higher due to population reduce to of background the in fast WIMP decaying searches. state PoS(EPS-HEP 2009)110 ’s α C. Amsler ) from the 1 − s 7 = 0.8. This can r 3 is due to electrons. . ’s and up to 1.2 MeV 0 α = around r 2 of TPB) was evaporated on reflective foils 2 50 ns) component to enhance nuclear recoils. . 1 − < 3 ’s detected in coincidence in a NaI(Tl) detector. The γ of fast ( r (right) shows the detector inside the vessel. . The accumulation of events around r 1 Pb radioactive source producing 5.3 MeV ’s. α of fast to total amplitude for the Pb-sources (left), with the additional Am- 210 TOT L / ns 50 n. In the 1t detector the probability for two or more neutron interactions is large L 3 = r He n/s. It was contained in a high density polyethylene shielding block with two aper- 4 → Ratio 10 shows the measured ratio × 2 Even small neutron fluxes around 1 MeV are potentially dangerous, since the neutron-argon As pointed out before, heavily ionizing particles populate mostly the fast decaying singlet In parallel to the construction of ArDM we optimized the light output and collection efficiency Figure 2: cross section is some 18investigate the orders response of of magnitude the largerdecisive dark than advantage matter for of detector WIMPs. to monoenergetic It neutronsknown neutrons as is incident over a therefore energy radioactive function essential sources from of to measuring (such recoil which the energy. as the neutron Am-Be) A elastic energy scattering is transferredNSD-Fusion angle. GmbH. the to The We source the have will therefore deliver nucleus monoenergetic ordered 2.45 can a MeV be neutron neutrons (10 source calculated from by states. We therefore define the fraction The signal from recoil nuclei associatedbe with proven by neutrons inserting is a shown in 11(not fig. cm shown). thick polyethylene absorber, while a 4 cm lead sheet has no effect reaction dd enough to be measured precisely.reduce the This fraction neutron can background be during measured WIMP with searches, the since dd-source WIMP and interactions used do to not lead to Be-source (center), and withaffected 11 by ADC cm saturation of from polyethylene absorber (right). Data points above the red line are for charged particles and neutrons.mm high, We 74 used mm a iners diameter) small viewed R6091-02MOD). cylindrical by A cell two wavelength containing PMTs shifter liquid (Hamamatsu (1 low argon mg/cm temperature (78 photomultipli- ArDM, a 1t liquid argon detector for dark matter searches The PMT glass is coated withlight a collection thin efficiency. layer Figure of a transparent TPB-paraloid compound to increase the electrons was held on a stick inobtained the by middle placing of the a active 370 volume.from MBq Measurements americium Am-Be-source with are next neutrons absorbed were to by beryllium, theabout producing experimental 2 2 setup. – 12 The MeV 5.4 neutrons.tures, MeV The one source for produced the neutrons, the secondneutron for flux the at the target position was around 30 s Figure covering the internal wall. A PoS(EPS-HEP 2009)110 , ’s in a 0.3 p.e. γ 102 C. Amsler -rays. ∼ γ s, in accord µ ’s, expressed as γ 0.1 (2009) P06001. 4 ± , Phys. Rev. Lett. = 1.6 2 from Cs is clearly observed. τ γ Development of wavelength shifter (2008) 021303; Z. Ahmed et al. (CDMS 100 , Journal of Instrumentation Na-sources measured with the 1t liquid argon detector, 4 22 First results from the XENON10 Dark Matter Experiment , Phys. Rev. Lett. Cs- and (1983) 5279. 137 shows the energy distribution of the detected 3 B 27 (2008) P02001; V. Boccone et al., 3 ]. Measurements of the light collection efficiency were performed with 2 1 p.e./ kev with the completed detector (14 PMTs). Assuming a quenching -spectrum from ∼ γ Effect of ionization density on the time dependence of luminescence from liquid Luminescence quenching of the triplet excimer state by air traces in gaseous argon , Phys. Rev. Search for Weakly Interacting Massive Particles with the First Five-Tower Data from ). From these measurements one obtains about 0.5 p.e. / kev electron equivalent 3 Na-sources. Figure 22 Preliminary Na measurements were performed by detecting the 511 keV positron annihilation ’s (see fig. argon and xenon at the Gran Sasso National Laboratory collaboration), the Cryogenic Dark Matter Search at(2009) the 011301. Soudan Underground Laboratory Journal of Instrumentation coated reflectors for the ArDM dark matter detector γ ArDM equipped with 7 PMTs was filled for the first time in May 2009 with 2t of liquid argon The charge readout will be also operational early in 2010. Following the extensive perfor- 22 Cs- and [3] C. Amsler et al., [1] J. Angle et al., (XENON-10 collaboration), [2] A. Hitachi et al., For NaI(Tl)-detector, in coincidence with the 1.275keV keV transition, or by triggering on one of the 511 with the literature value [ the number of photoelectrons (p.e.). The signal from the 662 keV (fiducial volume of 1t) and kept fullThe of slow liquid component for was more measured than one to month be without constant purification during system. 20 days at and therefore expects factor of around 30% for nuclear recoils one then derives a light collection efficiency of mance tests which can be most convenientlythe performed detector on to the an surface at underground CERN, location, we most will likely then the move Canfranc UndergroundReferences Laboratory. expressed as a functionliquid. of The the figure number on the of right photoelectrons shows the detected low by energy distribution 7 from photomultipliers the 511 immersed keV in annihilation the 137 / keV for WIMPs. This should be sufficient to detect 30 keV nuclear recoils with good efficiency. Figure 3: ArDM, a 1t liquid argon detector for dark matter searches multiple scattering events.