Gerbier, G

Mallet, J

Mosca, L

Tao, Charling

CERN Document Server

Pulse Shape Discrimination and Dark MatterSearch with NaI(Tl) Scintillator at theLaboratoire Souterrain de ModaneG. Gerbier1, J. Mallet1, L. Mosca1, and C. Tao21DSM/DAPNIA/SPP, C.E.A. Saclay, F-91191 Gif-sur-Yvette, France2CPPM,IN2P3/CNRS and Universite Aix-Marseille II, 163 av. de Luminy, case 907,F-13288 Marseille Cedex 09, FranceAbstract. Underground data for WIMP search have been obtained from a 83 days runof a 10 kg NaI(Tl) detector in a low activity environment at the Laboratoire Souterrainde Modane (LSM). Emphasis is put in this paper on the understanding of the 2-5 keVenergy region, crucial both at establishing limits and at analyzing annual modulationof Spin Independent WIMPs. Part of the PMT noise, dominant in that energy regioncould induce errors in the results of the modulation analysis. Pulse shape analysis ofthe data at higher energies have shown an unexpected population of "U" events, alsoobserved by another experiment at the same rate. Additional data obtained at LSM on"U" events are reported. A possible interpretation of these events is related to alphadecays of Radon descendants near the surface of the crystal.1 IntroductionThe results of the Saclay/Lyon groups NaI(Tl) experiment performed at theLaboratoire Souterrain de Modane have been recently published in [1]. The goalof the contribution to this conference is rst to underline some basic propertiesof NaI(Tl) which determine the way its sensitivity depends on the type of theWIMP interaction, Spin Independent (SI) or Spin Dependent (SD), second toshow the importance of the low energy spectrum shape in the SI case, in particu-lar the role of the PMT noise in the 2-5 keV region, third to discuss the origin ofan unexpected "Bump/U event" population shown by the pulse shape analysis.2 Sensitivity of NaI(Tl) to SI and SD Interacting WIMPsThe sensitivity of NaI(Tl) to SI and SD WIMP couplings depends on variousexperimental and theoretical parameters which determine the shape of the ex-pected energy spectra of the WIMP interactions with Na and I recoils.First, the scintillation light eciency or "quenching factor" is smaller forIodine recoils (8 %) than for Sodium recoils (25 %). It induces a shrinking of therecoil spectra from Iodine at low energies.Second, the high mass of the Iodine nucleus induces a loss of coherence inthe interaction with a WIMP at the typical energy transfers involved, leading toa reduction of the cross section at the highest energies.2 G. Gerbier et al.A third important parameter is the relative height of the two I and Na recoilenergy spectra in SD and SI cases. This parameter depends on the relativecross sections on Na and I nuclei. For the SI case, the expected cross section isroughly proportional to the square of the atomic mass so the cross section onIodine largely dominates, while for the SD case, it depends on the nuclear spinfactors.0,0010,010,11100 5 10 15 20      Iodine recoils   Sodium recoils60 GeV Spin Dependent WIMP           with         p   = 1pbsdN/dE (evts/kg/keV/day)Energy (keV electron equivalent)Fig. 1. Energy spectra of I and Na recoils of a 60 GeV SD WIMP signal in NaI(Tl),taking into account all experimental eectsThe calculations of the expected energy spectra shown in gures 1 and 2 fora typical 60 GeV mass WIMP include all the above theoretical and experimentalparameters. It is clear that above the experimental energy threshold of 2 keV,a NaI(Tl) experiment is sensitive to Na recoils in the SD case (gure 1). Forthe SI case the Iodine recoils dominate over the Na recoils below 6 keV by morethan one order of magnitude (gure 2) and experiments with a typical "at"spectrum at say 1 to 2 evts/kg/keV/day will be sensitive to Iodine recoils. Thebest sensitivity is then obtained at the lowest energy bin (gure 3).Another experimental proccedure involved in the determination of the sen-sitivity is the Pulse Shape Discrimination (PSD) which allows to reduce thebackground of electrons through the analysis of the decay time of the scintil-lation pulses. The reduction factors have been measured by dierent groups,depend on the temperature, on the light collection eciency, but all agree thatthis analysis is only statistical (not event by event) and possible only above 4keV electron equivalent energy. Typical statistical rejection factors of the back-ground for an experiment at room temperature of 1000 kg.day are shown ongure 3. They increase as the square root of the exposure.Also reported on gure 3 are the typical experimental spectra obtained bythe present experiment and the DAMA experiment [2].PSD and Dark Matter Search with NaI(Tl) at LSM 30,0010,010,11101000 5 10 15 20dN/dE (evts/kg/keV/day)Energy (keV electron equivalent)60 GeV Spin Independent WIMP        with       p  = 10-5 pb      Iodine recoils   Sodium recoilssFig. 2. Energy spectra of I and Na recoils of a 60 GeV SI WIMP signal in NaI(Tl),taking into account all experimental eectsIt can be noticed that the decrease with energy of the Iodine recoil spectrumis faster than the increase of the reduction factors. This means that the pointof maximum sensitivity using the PSD is the lowest energy point, that is at5 keV. From gure 3, it can be seen that this data point has about the samesensitivity as the 2.5 keV point without PSD. In the DAMA experiment [2],which has a luminosity 16 times higher so a rejection factor 4 times higher, thebest sensitivity is still at 2.5 keV -without PSD- because of the falling spectrumat the lowest energy.In both cases the PSD does not help improving the sensitivity, which is thendetermined by the experimental rate at the lowest energy, i.e. at 2.5 keV. Thisemphasizes the importance of understanding the behaviour of the low energyspectrum, discussed in the next section.3 Set-UpThe setup used at LSM consisted of a 10 by 10 by 25 cm long NaI(Tl) low activitycrystal viewed by two EMI 3 inch low activity PMT through 10 cm quartzlight guides. The detector was installed inside a low activity ancient lead shieldenclosed in a plexiglass box continuously ushed with nitrogen to avoid radoncontamination. The presently described detector module is similar to the onesused in the DAMA experiment in size, crystal growing method, manufacturer,housing, light guides, PMT type. The measured photoelectron yield was about7 pe/keV. All pulses above 1.8 keV were digitised at 200 Mhz and recorded ontape. The live time measurement was 83 days.4 G. Gerbier et al.0,0010,010,11101000 5 10 15 20dN/dE (evts/kg/keV/day)Energy (keV electron equivalent)60 GeV SI WIMP : Iodine recoils        with       p  = 10-5 pbsTypical PSD reduction factors    for 1000 kg.d (only stat)Data from this exp :  805 kg.d- all events (with PMT noise)- after PMT noise removalData from DAMA    best crystalFitted Sm  DAMAFig. 3. Energy spectra of a) expected signal in NaI(Tl) from a 60 GeV SI WIMP, b) ofthe experimental spectra measured by this experiment (raw and cleaned up) and theDAMA one of their best crystal, together with typical reduction factors obtained withPSD c) of the Sm annual modulation signal of DAMA. See text for details4 Low Energy SpectrumOn gure 3 is shown the obtained raw spectrum of a subsample of the totalexposure.4.1 PMT NoiseThe 2 to 5 keV energy region is dominated by PMT noise pulses. The total rate inthe 2-3 keV bin is 80 evts/kg/keV/day. They have characteristic sharper shapesthan scintillation light and strong asymetry between the two PMT signals. Figure4 shows a scatter diagramof a "sharpness" parameter discriminating between thePMT noise and true NaI(Tl) pulses as a function of energy for all the recordedpulses.By applying cuts on this parameter and on asymetry, PMT noise can bemostly rejected. We found that we keep 0.5 % of PMT noise in the 2-3 keV bin,that is 0.4 evts/kg/keV/day. On the other hand the eciency for keeping the"good" scintillation pulses is 75 % in this same bin of energy and rises up to 99% at 4 keV.4.2 Comment on the Annual Modulation SearchThis gure of the contamination of PMT pulses is very important since mostof the annual modulation signal claimed by the DAMA group [5] comes fromPSD and Dark Matter Search with NaI(Tl) at LSM 5the very rst keV energy bin. Varying with the cuts applied in the analysis,a contamination by PMT noise is unavoiddably present among the scintillationpulses. It thus should be prooved that this noise is stable and does not modulate.This was not specically considered in ref [5].On the other hand, it can be observed on gure 3 that the energy spectrumshape of the claimed Sm signal is indeed very close to the initial PMT noiseshape spectrum.4.3 Comment on the Limit Extracted in the WIMP Sigma MassPlaneComing back to the spectrum obtained after cleaning up of the PMT noise, anaccumulation is observed at 3 keV (gure 4) in the NaI(Tl) pulse region, andexplains the rising up of spectrum (shown on gure 3) at the lowest energy, upto 10 evts/kg/kev/day at 2.5 keV. This contrasts with the spectrum observedby DAMA in particular for their best crystal [2](see gure 3), which is roughlyat the same level above 10 keV, but lowers to 0.5 evts/kg/keV/day at 2.5 keV.This mostly explains the dierence in sensitivity of the two experiments inthe cross section mass exclusion diagram for SI WIMP : of about 13 10 5pb forthis experiment as compared to 0.5 10 5pb for DAMA at a WIMP mass of 100GeV.-500050010001500200025000 1 0 2 0 3 0 4 001 02 03 04 05 00       1      2      3       4      5      6       7      8      9     1020304050Mean amplitude per time bin (au)PMT noise pulsesNaI pulses3 keV peak0Energy (keV electron equivalent)Fig. 4. Scatter plot of the mean amplitude per time bin as a function of energy fordata events before appyling any software cuts. The 3 keV peak can clearly be seen inthe NaI(Tl) pulses region6 G. Gerbier et al.However, the falling down spectrum below 3 keV of DAMA is hard to recon-cile with the expected resolution of the detector [3] and the usual behaviour ofelectromagnetic background spectra, typically at or rising up with decreasingenergy.5 Pulse Shape Analysis and U Event PopulationPulse shape analysis of the 805 kg.day data, at energies higher than 10 keV,have shown an unexpected population of "U" events with even faster decaytimes than nuclear recoil pulses [1]. UKDMC has observed the same kind ofevents, the "Bump" events [4] and similar total rates of about 3 evts/kg/dayand dierential energy spectra were observed, between the present experimentand the 6 kg data, see gure 5.Two other 10 kg crystals, manufactured by BICRON twelve years ago, havebeen recently measured at LSM. Measured U event spectra are shown also ongure 5. They are again very similar to the ones previously obtained.0.0010.010.111 00 1 0 2 0 3 0 4 0 5 0 6 0 7 0Bump events from UKDMCU events from this experimentdN/dE (evts/kg/keV/day)Energy (keV electron equivalent)U events from Bicron 10 kgFig. 5. Energy spectra of U events (from this experiment), Bump events from UKDMCresult [4], and more recent results from Bicron crystals measured at LSMThe most likely explanation is related to the existence of surface events.We found that X-rays exhibited faster decay times than Compton. This wasnot observed by the UKDMC group with enuncapsulated crystals. A possibleexplanation is that the Modane calibration was done at a single spot, througha thin window, where the NaI(Tl) surface may have been deteriorated and theresponse could be dierent from the rest of the surface.On the other hand, the UKDMC group has found that alpha particles, gener-ated by a source outside the crystal and slowed down through absorbers, indeedPSD and Dark Matter Search with NaI(Tl) at LSM 7give faster decay times than recoils at relevant energies, but not as fast as Uevents [6].Indeed, a more likely explanation consists in a radon surface contamination.Recoiling nuclei from descendants of radon implant slightly inside the crystal.Subsequent alpha decays give rise to recoiling nucleus and escaping MeV alpha.Then the low energy U events are the result of the sum of the deposited energyby the recoiling nucleus (100 keV recoil gives a 8 keV equivalent electron energy)and a part of the deposited energy by the alpha before it escapes the crystal (upto 70 keV).However, preliminary calculations by the UK group [7] show that the neededamount of radon accumulated at the surface between the cutting and the sealingof the crystal seems much higher than what can be reasonably inferred from theknown way of manufacturing low activity detectors.Even if this explanation holds, it does not account for the surprising similarityof spectra between crystals of very dierent origins and manufacturers.An experimental investigation is led together with the UKDMC group todisentangle this issue. Various crystals have been tested. In particular, an exper-iment led by the UKDMC group is currently under way to expose a bare crystalto a radon contamination, and measure the shape of the pulses and the energyspectrum of such events.References1. G. Gerbier et al.: Astropart. Phys. 11, 287 (1999)2. R. Bernabei et al.: Phys. Lett. B 450, 448 (1999)3. R. Gaitskell: these proceedings4. P.F. Smith et al.: Phys. Rep. 307, 275 (1998)5. R. Bernabei et al.: Phys. Lett. B 480, 23 (2000)6. V. Kudryavtsev et al.: Phys. Lett. B 452, 167 (1999)7. N.J.C. Spooner: these proceedings

Pulse shape discrimination and Dark Matter Search with NaI(Tl) Scintillator at the Laboratoire Souterrain de Modane

Abstract

Similar works

Full text

Available Versions

CERN Document Server