Luca Grandi.

Transcription

1 Luca Grandi idm September 2004

2 Wimp Argon Programme Collaboration R. Brunetti, E. Calligarich, M. Cambiaghi, C. De Vecchi, R. Dolfini, L. Grandi, A. Menegolli, C. Montanari, M. Prata, A. Rappoldi, G.L. Raselli, M. Roncadelli, M. Rossella, C. Rubbia*, C. Vignoli INFN e Pavia University F. Carbonara, A. Cocco, A. Ereditato, G. Fiorillo, G. Mangano, R. Santorelli INFN e Naples University F. Cavanna, N. Ferrari, O. Palamara * Spokesman Gran Sasso National Laboratory

3 WIMp detection A WIMP could be directly detected through its elastic scattering on target nuclei. The nuclear recoil spectrum can be expressed as: dr/de r = dr/de r ideal x [S(E r ) x I x F 2 (q 2 )] Ideal spectrum assuming a WIMPs maxwellian velocity distribution in a frame at rest with respect to the galaxy center Detector not at rest with respect to the the galaxy comoving frame Spin dependent factor Nuclear form factor (Lewin et al.) F 2 (q 2 ) Xe 20 KeV thr. 20 KeV thr. 30 KeV thr. 30 KeV thr Recoil energy (kev) The nuclear recoil mean energy is typically in the range 0 100keV. The energy threshold plays a crucial role: for high A target the high energy nuclear recoils are strongly depleted by the nuclear form factor and this effect, for typical energy threshold, tends to cancel the rate enhancement due to coherence effect. S.I. Si Ar Ge

4 cross section on nucleon (pb) 1x10-3 1x10-4 1x10-5 1x10-6 1x10-7 1x10-8 1x10-9 1x x x10-12 Kim et al (2002) GENERAL MSSM DAMA UKDMC (Xe)~ prelim Excluded by (g-2) at 1 Excluded if <1 TeV Mass of WIMP (GeV/c2) CDMS >0 EDELWEISS Excluded by (g-2) at 2 par. indep. lower limit General MSSM allowed region in the parameter space; The favoured region: M w = GeV s nucleon-w =10-6 pb pb

5 cross section on nucleon (pb) 1x10-3 1x10-4 1x10-5 1x10-6 1x10-7 1x10-8 1x10-9 1x x x10-12 (g µ - 2) preferred x10-2 1x10-2 1x10-3 1x10-3 1x10-4 1x10-4 1x10-5 1x10-5 1x10-6 1x10-6 1x10-7 1x Mass of WIMP (GeV/c2) Events/kg/day x10-2 1x10-2 1x10-3 1x10-3 1x10-4 1x10-4 1x10-5 1x10-5 1x10-6 1x10-6 par. indep. lower limit Argon, E thr =30 kev Xenon, E thr =30 kev Germanium, E thr =20 kev µ < 1 TeV µ > 1 TeV 1x10-7 1x events/kton/day -7 Iso-event rate curve for different target Argon (30keV), Xenon (30keV) e Germanium (20keV). Ar and Xenon performance with 30kev realistic energy threshold are comparable. 1 In priciple, the detection technique should be able to span the whole allowed region ( event/day/kg): HIGH SENSITIVITY; possibility to EXTEND it to DETECTORs MASSES of the order of few ktons.

6 2 WIMP dector should be characterized by a HIGHLY EFFICIENT DISCRIMINATION TECHNIQUE. ARGON TECHNOLOGY provides both these aspects: LARGE MASS detectors (600 ton) already realized by ICARUS collaboration (industrially supported, purification,...); The contemporaneous detection of EXCITATION and IONIZATION produced by an interaction in the medium could be used to efficiently discriminate nuclear recoil events from the dominant gamma background. WARP Double Phase Argon detector

7 Double phase prototype (2.3 liters) Primary scintillation produced by an interaction in the liquid argon region is directly detected by the PMTs (S1). It is produced by the de-excitation of argon nuclei (excited by the impinging particle) and by the recombination of ionization electrons to argon ions not drifted by the electric field; Through opportunely set electric fields the surviving ionization electrons are drifted toward the liquid-gas interface, extracted into gaseous phase and then accelerated to produce multiplication light in the linear regime, detected by the same PMTs (S2). Liquid Gas E-Field PMTs Interaction Secondary scintillation Ionization electrons Primary scintillation

8 Double phase prototype (2.3 liters) Primary scintillation produced by an interaction in the liquid argon region is directly detected by the PMTs (S1). It is produced by the de-excitation of argon nuclei (excited by the impinging particle) and by the recombination of ionization electrons to argon ions not drifted by the electric field; Through opportunely set electric fields the surviving ionization electrons are The drifted ratio toward S2/S1 the is function liquid-gas of the interface, exciting particle extracted (different into gaseous recombination phase and effects) then and accelerated should be to used produce to discriminate multiplication nuclear recoils eventually induced by WIMP light in the linear regime, detected by interactions. Signal S3 that is not used is the same PMTs (S2). produced by electronic multiplication near the grids wire. Liquid Gas E-Field PMTs Interaction Secondary scintillation Ionization electrons Primary scintillation

9 Prototype characterization Electrons extraction through liquid gas interface as function of applied electric field; Proportional light production as function of the applied electric field; Detection efficiency 1kV/cm drift field 2.3phe/keV for mip and 0.67phe/keV for nuclear recoils); Detector response to different radioactive sources (alpha, gamma and neutron) and performance of the proposed discrimination 2.5 kv/cm extraction time less than 0.1µs

10 Prototype characterization Electrons extraction through liquid gas interface as function of applied electric field; Proportional light production as function of the applied electric field; Detection efficiency 1kV/cm drift field 2.3phe/keV for mip and 0.67phe/keV for nuclear recoils); Detector response to different radioactive sources (alpha, gamma and neutron) and performance of the proposed discrimination technique. Differently from xenon, low elec. fields (> 1kV/cm) are enough to produce light

11 Prototype characterization Electrons extraction through liquid gas interface as function of applied electric field; Proportional light production as function of the applied electric field; Detection efficiency 1kV/cm drift field 2.3phe/keV for mip and 0.67phe/keV for nuclear recoils); Detector response to different radioactive sources (alpha, gamma and neutron) and performance of the proposed discrimination technique. 109 Cd primary scintillation spectrum 2.3 phe/kev

12 Prototype characterization Electrons extraction through liquid gas interface as function of applied electric field; Proportional light production as function of the applied electric field; Detection efficiency 1kV/cm drift field 2.3phe/keV for mip and 0.67phe/keV for nuclear recoils); Detector response to different radioactive sources (alpha, gamma and neutron) and performance of the proposed discrimination technique. Nuclear recoils primary scintillation spectrum 0.67 phe/kev

13 Prototype characterization Electrons extraction through liquid gas interface as function of applied electric field; Proportional light production as function of the applied electric field; Detection efficiency 1kV/cm drift field 2.3phe/keV for mip and 0.67phe/keV for nuclear recoils); Detector response to different radioactive sources (alpha, gamma and neutron) and performance of the proposed discrimination technique. 222 Rn contained in argon

14 Sharp separation ( 60/1) between the two families. Alpha particles, exactly as nuclear recoils but in a different energy range, are characterized by S2/S1<1 due to an enhancement of the recombination process. PARTICLE S2/S1 (3.5kV/cm) electrons (mip) 11.9 alphas (5-8 MeV) 0.19 nuclear recoils <0.19

15 2.3 Nat. Lab. Gran Sasso LNGS Gallery Background Study

16 2.3 Nat. Lab. Gran Sasso

17 Typical signals LNGS

18 Scintillation LNGS LNGS no gamma shield Surface 40K 208Tl (232Th) 222Rn-218Po 214Po

19 100 liters Chamber A liquid argon full detector has been proposed in March It is mainly constituted by: A sensitive double phase argon volume of 100 liters; An active VETO liquid argon region surrounding the central core; A gamma and neutron shields to reduce gamma background (@ LNGS dominated by Radon chain ) and to absorb or at least thermalize the neutron flux; The proposal have been approved and funded by INFN in June 2004.

20 100 liters Chamber The sensitive volume reproduces the scheme of the 2.3 liters prototype apart from the dimensions and the choice of materials (strict radiopurity requests 1Bq/kg) inches PMTs (10% photcathode coverage). They can be used, detecting S2 signals, to localize the event in x,y plane; A stainless steel cup to form a gas pocket inside the surrounding liquid argon region;

21 A system of copper racetracks deposited onto a Kapton layer (to minimize the mass of material) is used to laterally delimit the drift region and to optically separate it from the VETO region; All the inner surfaces are covered with a reflective layer.

22 ACTIVE VETO The VETO region has been dimensioned in order to reduce the probability that a neutron produces a signal in the central part with no signal over threshold in the VETO to a value < A WIMP candidate event should be a recoil event with no associated signal in the VETO region External Dewar liters; inches PMTs; 10% photocathode coverage; Estimated efficiency for a mip 3 phe/kev; Nominal energetic threshold 20 phe (7.5 kev for mip)

23 cross section on nucleon (pb) 1x10-3 1x10-4 1x10-5 1x10-6 1x10-7 1x10-8 1x10-9 1x x x10-12 GENERAL MSSM DAMA Excluded if <1 TeV Mass of WIMP (GeV/c2) >0 CDMS UKDMC (Xe) EDELWEISS 100 litres sesitivity goal 1 ton sesitivity goal (projected) par. indep. lower limit 2.3 litres cell present Sensitivity gain of two order of magnitude.

24 Thank you