Liquid Argon XUV scintillation light detection for direct Dark Matter search: the WArP Experiment

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1 Frascati (Italy), October Liquid Argon XUV scintillation light detection for direct Dark Matter search: the WArP Experiment Francesco Di Pompeo* University of L Aquila & INFN-Gran Sasso National Laboratory, Italy *on behalf of WArP collaboration

2 F. Di WUTA08 The WArP Collaboration Università degli Studi di Pavia e INFN P.Benetti, E.Calligarich, M.Cambiaghi, C.Montanari, G.L.Raselli, M.Roncadelli, M.Rossella, C.Vignoli Laboratori Nazionali del Gran Sasso (INFN-LNGS) L.Grandi(*), O.Palamara, L.Pandola, C.Rubbia, E.Segreto, F.Tortorici (* also at Univ. dell'aquila) Università degli Studi dell'aquila e INFN R.Acciarri, M. Antonello, N. Canci, F.Cavanna, F.Di Pompeo(**) (** also at INFN-LNGS) Università degli Studi di Napoli e INFN F.Carbonara, A.Cocco, G.Fiorillo Princeton University Department of Physics F.Calaprice, C.Galbiati, B.Loer, R.Saldanha IFJ PAN, Krakow A.M.Szelc(++) (++ also at INFN-LNGS and Univ. dell'aquila) Università degli Studi di Padova e INFN B.Baibussinov, S.Centro, M.B.Ceolin, G.Meng, F.Pietropaolo,S.Ventura

3 Dark Matter direct detection: Dark Matter search in form of hypothesized Weakly Interacting Massive Particles (WIMPs) is of primary interest in astroparticle physics WIMP interaction with ordinary matter nuclei is expected to be very rare (below 0.1 event per kg of target per day) Underground site is needed in order to prevent cosmic ray interactions (order of 10 event per m 2 per second) Passive and active shielding for the environmental Gamma radioactivity (order of 1-10 event per Kg per second) Gran Sasso mountain Surviving internal background events (order of 0.1 event per Kg per second) shielding Signal to Background ratio still < 10-6 Discrimination techniques are needed in order to distinguish Dark Matter signals by the even more frequent background WArP experiment F. Di WUTA08

4 Dark Matter & Liquid Argon Direct detection of Dark Matter with noble gases liquified as target medium is one of the most promising line of development in experimental technology. A particle interacting in noble liquid produce both atomic excitation and ionization inducing the emission of scintillation light. Simultaneous measurements of free electron charge and light is at the basis of a strong discrimination power Argon is an ideal medium for Dark Matter search and the feasibility of Ar-based detectors has been firmly proved by the WArP Collaboration [Astropart. Phys. 28 (2008), 495]. Why noble liquids? Scintillator NaI(Tl) Liquid Argon Liquid Xenon High scintillation Yield Simultaneous measurement of scintillation and ionization (particle discrimination) Potentiality to be extended to multi-ton volumes Photon Yield [ph/mev] Fast Decay Time [ns] Slow Decay Time [ns] 4.3x x x Why liquid Argon? Scintillation decay times very different (τf 6 ns, τs ns) Argon Technology fully operational Easily available (1% of atmosphere) Two independent discrimination tech. very efficient background reduction! low cost F. Di WUTA08

5 F. Di WUTA08 Liquid Argon scintillation light emission An interaction in argon produces Atomic excitation ionization emission of 128 nm luminescence through 2 processes self trapped exciton luminescence 1) 2) recombination luminescence both processes ending up with the same radiative reaction inducing the emission of a 128 nm UV photon

6 Recent Results: N2 and O2 effect on scintillation light Dedicated tests of the effects of Oxygen and Nitrogen contaminations in liquid Argon has been performed at the INFN-Gran Sasso Laboratory within the WArP R&D program arxiv: v1 arxiv: v ns 850 ns 1280 ns 190 ns 500 ns 560 ns Source spectra at different contaminations are analyzed, showing sensitive reduction of the scintillation yield at increasing concentrations. Direct PMT signals acquisition shows a decreasing behavior in lifetime and relative amplitude of the slow component The effects are appreciable from O(1 ppm) of Nitrogen and O(0.1 ppm) of Oxygen concentrations F. Di WUTA08

7 Effect of ionization density on time dependence of luminescence The effect of ionization density on the scintillation time dependence is shown for gamma and neutron events Gamma and beta radiations induce low ionization energy depositions (e-recoils) Argon Recoils due to Neutron elastic scatterings induces high ionization density (nuclear recoils) experimental results 1. decay constants are not functions of ionization density Amplitude (A.U.) Gamma and Beta Neutron induced Argon Recoils 2. for high ionization density the fast components intensity increases at expense of the slow one Very efficient way to discriminate nuclear recoils from other events F. Di WUTA Time (nsec)

8 Dark Matter Puzzle Gravitational Lensing Observations Lens Dynamic of galaxies in Clusters Merger Cluster Galaxy Rotation Curves F. Di WUTA08

9 F. Di WUTA08 Universe CMB Supernovae Ia Gravitational lensing Dark Energy Matter M. Roos, astro-ph/

10 What is Dark Matter made of? Dark Matter Non-Baryonic Neutral Relic Cold SUSY extension of standard model provides a Good candidate WIMP Dark Energy Baryons Dark Matter M. Roos, astro-ph/ F. Di WUTA08

11 F. Di WUTA08 WIMP 1. They are expected to weakly Weakly Interacting Massive Particle WIMP Direct Detection i n t e r a c t w i t h n u c l e i o f ordinary matter. expected rate below 0.1 event per kg of target per day 3. Energy range of interest is dominated by natural radioactivity 2. Ionizing particle is the recoiling nucleus recoil energy very low ( kev) Need of very low radioactivity material and particle discrimination techniques to reject background events Irreducible Background: Argon recoils induced by Neutron elastic scatterings

12 F. Di WUTA08 WArP Detection Technique Each interaction in liquid argon induce both atomic excitation and ionization Integrated light signal S1 t drift S2 By means of electric fields free ionization electrons are drifted, extracted and accelerated in the gas phase to produce electroluminescence prompt (S1) & electroluminescence (S2) signals are detected through the same set of PMTs. Gas-Liq interface Race Tracks Cathode PMT PMT S2 t drift S1 PMT PMT GAr LAr

13 F. Di WUTA08 WArP Detection Technique For a given energy deposition, the nature of the primary recoiling particle affects both: Integrated light signal S1 t drift S2 The ratio between fast/low light intensity and hence the shape of the S1 signal PMT PMT PMT PMT The amount of ionization surviving recombination and hence the ratio between S1 and S2 signals Double discrimination technique: unique feature of Argon due to the wide difference between the two decay times. Gas-Liq interface Race Tracks Cathode S2 t drift S1 GAr LAr

14 Detection Technique: results Double discrimination power estimated using a 2.3 liters detector inducing argon recoils by means of neutron source (Astropart. Phys. 28 (2008), 495). Fig. (a): Argon recoils populate the red box region Fig. (b): After removing neutron source we acquired 30 million of background events and no event fall in the argon recoil region Log (S2/S1) (a)neutron induced ion recoils kev Pulse Shape Discrimination Parameter (F) Log (S2/S1) (b)wimp Exposure of 96.5 kg day kev >2.8x10 7 Triggers Pulse Shape Discrimination Parameter (F) F. Di WUTA08

15 light collection system Discrimination power is very sensitive to the number of collected photons Argon scintillation light is emitted with a narrow spectrum 128 nm PMTs efficiency is very low in the XUV wavelength region (glass transmittivity) Wavelength shift is needed. Solution we choose is Tetra-Phenyl-Butadiene (TPB) relative to sodium salicylate F. Di WUTA08 TPB: high XUV conversion λ = 128 nm probably higher than 1 (depending on TPB thickness)

16 F. Di WUTA08 light collection system Two kind of layers PMTs TPB evaporated on VM2000*: an high reflective layer in order to improve light collection efficiency TPB-Polystyrene mixture: an high transparence layer for coating the PMT windows in order to shift direct XUV light and transmit optical photons *dielectric mirror In order to characterize the optical properties of the light collection system a series of measurements h a v e b e e n p e r f o r m e d i n collaboration with ENEA

17 F. Di WUTA08 Substrate Characterizations In order to optimize the light collection system dedicated measurements have been done Measured samples can be divided as follows: TPB evaporated over glass TPB evaporated over VM2000 TPB-Polystyrene mixture over glass VM2000 Measurements done*: Photoluminescence (PL) Photoluminescence Excitation (PLE) Direct Transmittance Hemispheric Transmittance & Reflectance * in collaboration with ENEA: VM2000 R.M. Montereali and M.A. Vincenti Dip. Tecnologie Fisiche e Nuovi Materiali, FIM-FISACC, ENEA C.R. Frascati E. Nichelatti Dip. Tecnologie Fisiche e Nuovi Materiali, FIM-FISOTT, ENEA C.R. Casaccia TPB on VM2000

18 F. Di WUTA08 Evaporation Procedure Internal Diameter 63 cm Volume 186 lt Evacuation time 30 min. heater filament: tantalum foil graphite furnace temperature monitored by a thermocouple Evaporation procedure is LNGS by members of the WArP collaboration

19 F. Di WUTA08 Experimental Setup HeT and HeR measures Spectrophotometer UV-Vis-NIR Perkin-Elmer mod. Lambda 19 equipped with a integrating sphere (150 mm diameter). Wavelength experimental window: nm (reduced to nm). Filter: bandpass filter ( nm), thickness 2.1 mm. PL and PLE measures Spectrophotometer UV-Vis-NIR Jobin Yvon Fluorolog-3 FL-1 DT measures Spectrophotometer UV-Vis-NIR Perkin-Elmer mod. Lambda 19 Wavelength experimental window: nm

20 TPB PL and PLE TPB PLE is very large up to 420 nm nm: TPB PL and PLE overlaps TPB PL and PLE VM2000 exhibit a similar behavior PL PLE Need: filter to remove spurious PL TPB PL filter RE filter TE arbitrary units % wavelength (nm) wavelength (nm) F. Di WUTA08

21 Evaporated TPB on VM2000 PL affects strongly hemispheric properties measurements pass band filter ( nm) Linear interpolation ( nm) Hemispheric T and R VM2000 TPB+VM2000 % TPB PL filter RE filter TE transmittance/reflectance wavelength (nm) wavelength (nm) F. Di WUTA08

22 Evaporated TPB on VM2000 PL affects strongly hemispheric properties measurements pass band filter ( nm) Linear interpolation ( nm) TPB-VM2000 sample TPB PL filter RE filter TE % wavelength (nm) F. Di WUTA08

23 F. Di WUTA08 Evaporated TPB on VM2000! TPB-VM2000 sample!

24 In collaboration with ENEA, measurements, in a wide wavelength range spanning from UV to visible, have been done on different substrates used in the WArP light collection system They are of crucial importance in order to improve the collection efficiency and to setup a reliable MonteCarlo simulation tool In fact, high collection efficiency is needed to efficiently discriminate few WIMP signals from much more frequent background events Conclusions Further measurements should be done in order to characterize the optical properties at even low wavelength and at liquid Argon temperature in the meantime we are waiting for the first XUV signals from WIMP... F. Di WUTA08

25 it can happen!