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					HEP 2005 Lisboa                                         21-07-2005




                    Alfredo G. Cocco
            Istituto Nazionale di Fisica Nucleare (Italy)
           WARP Collaboration
          E.Calligarich, M.Cambiaghi, C.DeVecchi,
   R.Dolfini, L.Grandi, A.Menegolli, C.Montanari, M.Prata,
    A.Rappoldi, G.L.Raselli, M.Roncadelli, M.Rossella,
                     C.Rubbia*,C.Vignoli
INFN and Department of Physics at University of Pavia (Italy)

      F.Carbonara, A.G.Cocco, A.Ereditato, G.Fiorillo,
                 G.Mangano,R.Santorelli
INFN and Department of Physics at University of Napoli (Italy)

             F.Cavanna, N.Ferrari, O.Palamara
      Laboratori Nazionali del Gran Sasso, INFN (Italy)

                         C.Galbiati
                 Princeton University (USA)

                         A.Szelc
                 Cracow University (Poland)
         The WARP Experiment

Shed light on the Cold Dark Matter problem aiming at
the detection of WIMP elastic scattering on Argon nuclei

SUSY: the neutralino (LSP) can have a mass up to 1TeV
     impossible to detect at LHC but
     within the reach of an underground experiment

SUSY: massive relic particles are expected to have
      weak-like interaction properties
           WIMP-Ar elastic scattering




  ER  10 100 keV




Spin-independent form factor
      WARP: Ar double phase technique
• Detect primary UV (128nm) scintillation light
  on PMTs (using TPB as wavelength shifter)

• Primary ionization electrons are drifted to the
  liquid-gas interface with a field of 1 kV/cm

• Electron are extracted into gas phase
  by an electric field shaped by wire grids

• Proportional scintillation light production at
  the multiplication grids detected by PMTs

                 About 25 primary phe and 3 free electrons are
                 produced by an Ar recoil of 40 keV !

            Liquid Argon purity suitable to obtain large electron drift
            distances O(1m) are easily achieved using commercial
            Argon (guarenteed by the ICARUS experience)
        Ar-recoil event selection (I)



           S1                                        S2
                                S2
                                      S1



        Ar recoil-like signal              electron-like signal


Selection of events based on the ratio S2/S1, namely the ratio of the
secondary light pulse due to the electron multiplication in gas and
the primary scintillation light, can achieve a background rejection
power larger than 105
              Ar-recoil event selection (II)
Time dependence of scintillation light in high-pressure noble
gas scintillators is known since long time. It has mainly two origins:

• Excitation with fast and slow components corresponding to
 1 (singlet) and 3 (triplet) molecular states transitions



• Recombination (depends on the kinetics in the medium
  and on the ionization density)                                           Triplet
                                                                           1.8 s
                      Isinglet/Itriplet   Singlet
                                            7 ns
  electrons                0.3
  alpha                    1.3
  fission fragments        3.0

          @ zero drift field



• Pulse shape analysis can give an additional rejection power of at
  least 104 (conservative, see next slides)
                     WARP: the detector
              Under construction at Gran Sasso Underground lab

• High sensitive mass
  (140 kg scalable to 1 Ton)

• Active veto (Liquid Ar)

• Gamma shield

• Neutron shield

• Low activity materials

• High background rejection power (108)
                                   inner detector

                            neutron and  shield

                                      active veto
2.3 litre prototype at LNGS

          • PMT: 7 x 2 inches (EMI D749U) and
                 4 x 3 inches (EMI D750U) from 02/2005

          • 10 cm radius

          • 7.5 cm depth (40 s max drift time with 1kV/cm)

          • light yield of 1.0 phe/keV

          • 10 cm Pb shield


           April 2004: Start of underground test-runs

           February 2005: Continous data-taking
                         (about 107 events collected)
2.3 litre prototype at LNGS




 *



                              to Hall A
                   2.3 litre DAQ scheme


• Ortec charge preamplifier
  (Mod. 2005)

• ICARUS flash ADC boards
   (50 MHz sampling)

• Trigger based on NI cRIO-9101
  FPGA module
  (threshold at about 1.5 phe)
2.3 litre preliminary results (I)
      S2/S1 background discrimination




                                induced electron-like events


                                from 222Rn - 218Po decays




 S2/S1 rejection power > 105
             2.3 litre preliminary results (II)
                Pulse shape background discrimination

                                                            17000 electron-like events

• To discriminate between different “mixtures” of
  fast (7ns) and slow (1.8 s) components in the
  primary scintillation signal we use the fraction
  of PMT charge collected in the first 400 ns
                         S1
              R
                   S1(t  400 ns)
                                                     -like events
• Preliminary results obtained on a
  sample of visually checked events show
  no electron-like events in the  peak


                           Rejection power > 104
       2.3 litre preliminary results (III)
        Combined criteria for background discrimination



                                              electron-like events

S2/S1 and R are largely uncorrelated




     Overall rejection power  107                     -like events
            2.3 litre preliminary results (IV)

            Background spectrum in the region 80-7000 keV



                                          222Rn 218Po
  no Pb shielding (blue)
                                                            214Bi   + 214Po
  with 10 cm Pb shielding (red)



222Rn    218Po   (5.59 MeV)

218Po    214Pb   (6.11 MeV)

214Bi    214Po  e- (3.27 MeV)
               210Pb   (7.83 MeV)
             2.3 litre preliminary results (V)
                              39Ar   measurement

                                              Residual spectrum after subtracting
39Ar    39K  e-  e (Q = 565 keV)          222Rn contribution and  estimated

                                              from events without Pb shield
Measured activity in agreement with the
result of H.H.Loosli quoting the 39Ar in
natural Argon as (8.10.3)  10-16
                                                   Measured Activity of 39Ar
The fraction of 39Ar decays producing an              (1.1 ± 0.4) Bq/litre
energy release in the range 30 100 keV is
about 3%

                                                             39Arspectrum convoluted
                                                               with detector resolution

 The expected rate due to 39Ar in the
 ion recoil evergy window from 30 to
 100 keV is given by 1.1  0.03 Hz/litre
        39Ar     background in the 100 litre
                       detector

The rate of 39Ar induced events is expected to be 3.3 Hz


There are two ways to in order to keep this rate below
1 event in 100 days (design sensitivity):


• Total rejection power of about 3107 must be obtained  ACHIEVABLE

• Isotopic separation of 39Ar is under study (Russian lab)
  A depletion of the order 102103 seems feasible.
  In this case a rejection power of 3104 is enough  ALREADY ACHIEVED
WARP 100 litre sensitivity




   100 litre limit
          WARP 100 litre: inner detector

• Sensitive mass = 140 kg                            2 inches PMT

• Height = 60 cm, Radius = 25 cm

• 61  2 inches PMT (30% qe at peak)

• Photocathode coverage = 10%

• Drift field = 1 kV/cm

              Extraction and multiplication grids


                          Field shaping electrodes


                                        Cathode
WARP 100 litre: active veto and external shield
WARP 100 litre: active veto and external shield
                  Top view
WARP 100 litre installation at LNGS


            HALL B




        Detector feet
                          Conclusions
• WARP double phase Ar detector is able to detect very low energy
  Argon recoils (30 100 keV) induced by WIMP elastic scattering

• The ratio between the primary scintillation signal (S1) and the
  secondary signal produced at the multiplication grids (S2) provide
  a powerful tool to select clean signal events

• The pulse shape discrimination represent another (uncorrelated)
  handle to establish the recoil-like nature of WIMP induced
  event candidates

• WARP (140 kg) will be operational in the second half of 2006 in the
  Gran Sasso underground laboratory

• Both external (shield+veto) and internal (S2/S1+pulse shape) noise
  contributions are expected to be reduced to 1 event each 100 days

• DAMA signal (7.2  10-6 pb/nucleon and WIMP mass of 52 GeV)
  would be detected in WARP as 56 ev/day
Neutrino induced background
100 litre detector
Argon properties
Electron extraction from liquid to gas
               phase
WARP 100 litre: inner detector

				
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posted:2/28/2012
language:Italian
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