WARP Wimp ARgon Programme Un rivelatore ad argon liquido per la ricerca della materia oscura
M. Cambiaghi, C. DeVecchi, A. Ferrari,
L. Grandi, C. Montanari, M. Rossella, C. Rubbia
L. Grandi, INFN Pavia Introduction
¸ Assuming that General Relativity is correct also on cosmic scale and that Supernovae Ia can be considered as standard candles, the density of energy and matter contained in the Universe can be summarized as follows:
 Ω0=1.04±0.05: CMB mapping experiment [Boomerang]
ΩLª2/3: Dark Energy term (studies on SNIa [Perlmutter et al.])
Ωmª1/3: Matter term (studies on cluster evolution and on Lya forest spectrum)
The majority of matter is not-visible, the so called Dark Matter. It has to be:
¸ non-baryonic since Ωb<<Ωm (strict constraints from BBN); ¸ weakly interacting (otherwise already detected); ¸ cold (for actual structure formation). Such particle is called WIMP. Brief parenthesis on dark Energy problem
¸ All the forms of matter and energy contained in the Universe can be described as
perfect fluid (Pi=wi˙ri): “standard” forms present a non-negative wi. In the flat universe described before the deceleration parameter can be expressed as
q0 ª 1/2 Ω0 +(3/2) Ωx wm +(3/2) ΩL wL=1/2+ wL
and, since observations on SNIa suggest an accelerating universe (q0<0), this implies
wL<-1/2
This fluid is called Dark Energy and its real nature is not really understood (Constant? Rolling field? Vacuum energy? Quintessence?) Susy WIMP: Theoretical constraints
¸ Most promising WIMP candidate is the LSP (lightest supersymmetric particle), that should be stable (if R-parity is conserved) and weakly interacting. WIMPs should be trapped in a dark halo around the galaxy center (larger than the visible matter one) and, in the “standard” model, possess a mean velocity of bª10-3 and an energy density of the order of 0.3 GeV/cm3. Their elastic interaction with ordinary matter nuclei should produce typical atomic recoils of energy up to few hundred of KeV.
Parameters space allowed by DM experiment, by unitarity request (Higgs
Mass < 1 TeV) and by (g-2)m measures in the hypothesis of a susy WIMP. The most probable region provides a WIMP mass in the range of 40÷400 GeV and a nucleon-WIMP x-section from 10-6 pb up to 10-11 pb. Direct detection technique
¸ If WIMPs exist they should interact elastically with ordinary matter nuclei and hence, a direct measure of their existence can rely on the study of nuclear recoils spectrum. The predicted differential cross section, assuming a maxwellian WIMPs velocity distribution can be expressed as follows
2 2 dR/dEr = dR/dEr|ideal x [S(Er) x F (q ) x I]
where dR/dEr|ideal have a typical exponential behavior, S(Er) represents a correction due to the fact that the detector is moving with respect to the galactic center, F2(q2) is the nuclear form factor and I is a factor concerning the spin-dependence.
¸ The choice of the energy threshold and the nuclear form factor plays a crucial role in such kind of experiments. In high A nuclei, the high energy recoils are strongly suppressed. DM 2ND generation experiment ¸ Due to the large theoretical uncertainties (halo models, WIMP nature, ect…) that
afflict the calculation of dR/dEr|ideal and I of the previous formula, the second generation DM experiments must provide a high sensitivity in order to span all the possible predicted rate from about 1 to 10-6 events/day/kg: such an aim could be reached providing very large detectors mass.
Iso-event rate curves are plotted in the parameter space as function of the WIMP mass for argon detectors and Xenon detectors with 30 KeV energy threshold. Argon compares favou- rably with Xenon. DM 2ND generation experiment (cont.)
¸ Since the background in the recoils energy range could be substantial (due to radioactivity), such detectors must perform also a highly efficient discrimination.
Argon technology provides both these characteristics: ß large mass detector (several hundred tons) have been already realized by the ICARUS collaboration (industrially supported, efficient argon purification system, ect...); ß the detection of both scintillation and ionization provides a highly efficient discrimination method as pointed out by the 2.3 litre chamber tests. In this tests neutrons have been used to produce nuclear argon recoils and to study the discrimination technique. 2.3 litre test chamber
Idea:
¸ Scintillation is directly detected by PMT (primary signal,
S1); ¸ Ionization electrons are drifted from the interaction point towards the liquid-gas interface, extracted into gas and strongly accelerated in order to produce
multiplication light (secondary signal S2), detected by the same PMT.
Grids for electrons Extraction and S2 Multiplication S1
Drift (75 mm)
Reflective + 109 Cd Source Shifting Coating 2.3 litre test chamber Calibration with g-source
109 ¸ A Cd48 source (g-rays, X, conversion electrons), inserted in the middle of the drift region, has been used to study the response of the prototype to gammas, estimate the light yield and the extraction and multiplication processes.
¸ The light yield obtained working @Eext=0 and @Emult=0 is
@Edrift=0 G(0)=2.8 ÷2.9 phe/KeV (from 22KeV peak and compton end-point)
@Edrift=0.5 KV/cm G(0.5)=2.5 phe/KeV
@Edrift=1 KV/cm G(1)=2.4 phe/KeV ¸ Using the signal from the 22 KeV peak (selected by drift time), extraction and multiplication processes have been studied as function of electric fields and the following curves have been obtained: 109Ca source measurements
¸ In order to verify that low energy electrons can be well discriminated from nuclear recoils, the following scatter plot has been realized. Events most probably produced by the source, are selected through drift time cuts. Primary and secondary signals present a strong correlation.
¸ The delay time between S1 and S2 signal presents a clear peak, above a diffuse flat background, exactly at half maximum drift time (where events produced by the source, positioned in the middle of drift region, are expected) compatible with background 14 MeV neutron gun measurements
¸ Fast neutrons have been chosen since, in this energy range, the elastic cross section is about half of the total one: consequently a large number of nuclear recoils are induced in the detector.
30% of the population behaves in a different way with respect to the gammas: we will call events falling in this The D-T neutron generator region as “neutron- provides a trigger signal used as like” events. coincidence. The above time distribution of the scintillation 398 events have zero secondary signal. light events is strongly correlated to the trigger signal arrival time: practically all of the events are associated to neutron gun. Neutron gun data analysis: ionization
Cut S <60 phe 4 peaks resolved 1 S2<20 phe
Secondary signal amplitudes distribution of neutron-like events, with S1<60phe and S2<20phe (corresponding to nuclear recoils up to 80 KeV), is represented in the histogram below. They are clustered around discrete multiples of a value m: such behavior can be associated to the discrete number of electrons extracted and multiplied in gas. The fit provides a value of m=3.35 phe: this means that the multiplication factor is about 3.35 phe/e. Neutron gun data analysis: ionization (cont.)
S1<60 phe S2<20 phe The number of events as function of the number of
ionization electrons (ne) is well represented by a poisson fit with
mean
¸ The measured mean value is in very good agreement with the used theoretical formalism (Lindhard, Box Model of Thomas and Imel and grid transparency effects) based on experimental data on slow ions recoils, that provides a mean value of about
¸ Primary scintillation light spectrum in the nuclear recoil region has been deeply studied in order to compare it with the one expected from Lindhard theory and to estimate
the nuclear recoil quenching factor fN. In Lindhard theory the number of photo-
electrons associated to a recoil Erec is expressed as
p(Erec)=G ⋅ fN(Erec) ⋅ Erec fN(Erec)=l ⋅ LAr(Erec) light efficiency for minimum ionizing scale correction Lindhard electron tracks (2.4phe/KeV parameter efficiency production @ 1KV/cm) ¸ The spectrum have been fitted with a function derived from the Lindhard formalism (opportunely corrected with the scale parameter l) and from the nuclear recoil distribution for elastic scattering. The best fit technique provides
l=0.507±0.018 fi fN(40 KeV)=0.28±0.02 This means that @1kV/cm the light yield for a nuclear recoil of 40 KeV is 0.67phe/KeV. Data analysis: conclusion
¸ Clear separation between the g- Neutron gun induced events (S2/S1>1) and nuclear recoil events (S2/S1<1).
¸ For g-induced events S2/S1ªconst. (low energy higher recombination);
Most likely due to (n,a), (n,p) interactions Data analysis (cont.)
Am-Be source ¸ Measure without neutron source: the neutron-like events detected are compatible with background in the hall.
no source
¸ Data with neutrons from Am-Be source of energy peaked in the range 2-6 MeV (high elastic cross section) provides similar results. Conclusions
¸ The proposed technique provides the detection of nuclear recoils in the energy region (10-100 KeV) of interest, although the prototype was not optimized. The measured photo-electron yields are 2.4 phe/KeV @ 1kV/cm for minimum ionizing tracks 0.67 phe/KeV @ 1kV/cm for Argon recoils of 40 keV
¸ The discrimination method allows a high neutron-g separation (at least 10-3).
¸ About 80% of the nuclear recoil events present an associated secondary signal; this fact can be used to provide a three-dimensional localization of the recoils.
¸ After doping with 500 ppm of xenon, the photo-electron yield increases of about a factor 2 (4.8 phe/keV @ 1 kV/cm for low energy electrons).