First results of the IGEX dark matter experiment at the Canfranc Underground Laboratory

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SUPPLEMENTS Nuclear Physics B (Proc. Suppl.) 95 (2001) 229-232

ELSEVIER

First results of the IGEX Dark Matter Underground Laboratory

experiment

www.elsevier.nlllocate/npe

at the Canfranc

S. Cebribn”, A.Moralesa, C. E. Aalsethb, F. T. Avignone IIIb, R. L. Brodzinskic, E. Garcia=, D. A. A. Klimenko”, H. S. MileyC, J. Gonzblez”, W. K HensleyC, I. G. Irastorzaa, I. V. Kirpichniko#, S. B. Osetrove, V. S. Pogosovf, J. Puimedbna, J. H. Reevesc, M. L Moralesa, A. Ortiz de Sol&zanoa, Sarsaa, S. Scopel”, A. A. Smolnikov”, A. G. Tamanyanf, A. A. Vasenkoe, S. I. Vasilieve, J. A. Villar” ‘Laboratory bUniversity ‘Pacific

of Nuclear and High Physics, University of Zaragoza, 50009 Spain of South Carolina, Columbia, South Carolina 29208 USA

Northwest

National

Laboratory,

Richland,

and Experimental

Physics,

Washington

‘Institute

for Theoretical

%stitute fYerevan

for Nuclear Research, Baksan Neutrino Observatory, Physical Institute, 375 036 Yerevan, Armenia

99352 USA

117 259 Moscow,

Russia

361 609 Neutrino,

Russia

The enriched 7”Ge double-beta decay detectors from the International Germanium Experiment (IGEX), operating in the Canfranc Underground Laboratory with an overbuden of 2450 m.w.e., were recently upgraded to use them also in a search for WIMPS. This paper presents a description of the experiment and the data analysis as well as a new exclusion plot, g(m), derived from the IGEX data for WIMP-nucleon spin-independent interaction. To obtain this result, 30 days of data from one 2-kg IGEX detector, with an energy threshold &hr N 4 keV, have been considered. These results improve the exclusion limits derived from other conventional ionization germanium experiments in the N 50 Gev DAMA region

1. INTRODUCTION There

is a substantial

evidence

that most

of

the matter in the Universe must be dark, and there are also compelling reasons to believe that it consists mainly of non-baryonic particles. From the cosmological point of view, two big categories of non-baryonic dark matter have been proposed: cold (CDM) and hot (HDM) according to whether they were slow or fast movig at the time of galaxy formation. HDM candidates are light neutrinos, while axions and the secalled Weakly Interacting Massive Particles (WIMPS) are among the leading CDM candidates. WIMPS are heavy (lolo3 GeV), neutral and non-relativistic relic nonbaryonic particles, with a very feeble interaction with ordinary matter, which are supposedly filling (at least partially) est stable particles like the neutralino,

the galactic halo. The lightof supersymmetric theories, describe a particular class of

WIMPS [l]. The direct detection of WIMPS relies on detecting the nuclear recoil produced by the WIMP

elastic scattering off target nuclei in a suitable detector [2]. Because of the low interaction rate (lo@ to 10 events/(kg day)) and small energy deposition (
to double-beta decay experiments have one of the lowest background levels and

have a reasonable 0.3).

with very diodes de-

Thus,

ionization

with sufficiently

yield

(from

low energy

0.2 to thresh-

olds, they are attractive devices for direct WIMP detection. This paper describes the results of one attempt to search for WIMPS with a Ge ionization detector. One usually compares

the expected

signal with

the observed background spectrum getting so constraints on the WIMP-nucleus cross-section as a function of the WIMP mass that is expressed in terms of exclusion plots g(m): if the predicted event rate is larger than the measured one, the particle under consideration can be ruled out as a dark matter component. One should note how-

0920-S632/01/$ - see front matter 8 2001 Elsevier Science B.V. All rights reserved PII SO920-S632(Ol)Olb86-6

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et al. /Nuclear

Physics B (Proc.

ever that this is not a discovery experiment but a method to select the region g(m) where a WIMP could be found by searches for one of their distinctive signatures, like the annual modulation due to the relative Earth-halo motion [3]. 2. THE

EXPERIMENT

IGEX is a high sensitivity experiment optimized for detecting 76Ge double-beta decay [4,5], but it was recently upgraded to look also for WIMPS 16). Three - 2 kg enriched detectors are currently operating in the Canfranc Underground Laboratory, located in an old railway tunnel in the Spanish Pyrenees, with an overburden of 2450 m.w.e., which reduces the muon flux to 2 x 10-7cn-zs-1. IGEX detectors were grown at Oxford Instruments with Russian GeO2 powder enriched up to 86% in 76Ge. Copper parts in the cryostat were produced by special techniques to eliminate Thorium and Radium impurities. The feedback resistor is placed close to the crystal to reduce noisecapacitance effects, but separated by a 2.5-cmthick disk of archaeological lead. Other front-end electronics is placed outside the shield and the preamplifiers were modified for Pulse Shape Discrimination (PSD). The IGEX detector employed for WIMP searches, RG-II, has a measured active mass of -2.0 kg. The innermost part of the shield consists of a 60-cm cube of archaeological lead (2.5 tons, 210Pb content
Suppl.)

95 (2001)

229-232

berra 2020 linear amplifiers; the first one registers the energy spectrum up to 3 MeV, while the other two record the low energy region (up to -620 keV) having different shaping time in order to filter noise. Amplifier outputs are converted using Wilkinson- type Canberra analogtodigital converters (ADCs), controlled by a PC through parallel interfaces. The veto signal is obtained from the logical OR of all scintillator outputs. At present, the second amplifier pulse output is routed to a LeCroy 9362 digital oscilloscope to be recorded. Consequently, time information, energy and the pulse shape are collected for each event. Dead time is negligible since the acquisition of each event takes 120 ps and the total counting rate is smaller than 1 Hz. The data analysis deals first with the microphonic noise rejection; two different and independent methods are used, the microphonic suppression obtained being practically the same. Since microphonic generated noise arrives to the readout system in bursts, events which do not follow a Poissonian time distribution, as it is expected for radioactive background, are disregarded. Taking into account the measured rate for noise (R,) and background (Rb), the probability of erroneously eliminate background events is negligible: either because they are consecutive events in a time interval shorter than the considered one, At =0.25 s, (P = 1 - ezp(-RbAt) = 0.0002) or because of a coincidence with noise (P = 2AtR, = 0.001). The other method of rejecting noise is based on the different response of the microphonics to different shaping times in amplifiers, first applied in [7]. The ratio of energies obtained using different shaping times is expected to be close to 1 for background events but not for noise. More precisely, events whose energy ratio falls far from 0.8 (at 4 keV) to 0.99 (at 50 keV) are supressed. The second step in the data analysis is to eliminate off-line events which are produced in coincidence with a veto signal (time window of 240 /ls). Although for the first batch of data (30 days) the record of pulses was not implemented, an eventby-event pulse shape discrimination is now underway. The effectiveness of the whole filtering procedure is shown in Fig. 1, where spectra after each step in the data analysis for the second set of

S. Cebridn et al. /Nuclear

Physics B (Proc. Suppl.) 95 (2001) 229-232

Figure 1. Effectiveness of the filtering procedure in the low-enregy region of the spectrum.

data are depicted. The Pulse Shape Discrimination results specially effective in the lowest energy bins. 3. RESULTS The IGEX-DM results presented here correspond to 30 days of analyzed data (Mt=60 kg days) from the IGEX detector RGII, even though at present 68 days are already available. The de’ tector has an energy threshold of -4 keV and a full width at half maximum FWHM energy resolution of 0.8 keV at the 75 keV Pb X-ray line. The background rate recorded during the first run was 0.3 c/(keV kg day) between 4-10 keV, 0.07 c/(keV kg day) between 10-20 keV and 0.05 c/(keV kg day) between 20-40 keV. Fig. 2 shows both RG-II 30-day (solid line) and 68-day (dotted line) final spectra. The exclusion plot is derived from the recorded spectrum in one-keV bins from 4 to 50 keV, by requiring the predicted signal in an energy bin to be less than or equal to the (90% C.L.) upper limit of the (Poisson) recorded counts. The galactic halo is supposed to be isotropic, isothermal and non-rotating, assuming a density of p=O.3 GeV/cm3, a Maxwellian velocity distribution with v,,,=270 km/s (with an up per cut corresponding to an escape velocity of 650 km/s) and a relative Earth-halo velocity of

231

v,.=230 km/s. The cross-sections are normalized to the nucleon, considering a dominant scalar interaction. The Helm parameterization [8] is used for the form factor and a recoil energy dependent ionization yield (E,i, = 0.14(Erecoi1)1.1g) is considered to compare the IGEX exclusion plot with that derived from the latest Heidelberg-Moscow data [9]. Fig. 3 shows the exclusion plots for spin-independent interaction (SI) obtained from the IGEX-DM results (thick solid line) as well as from other experiments: COSME-1 [lo] (thin dashed line), COSMEZ [ll] (thick dashed line) and Heidelberg-Moscow [9] (dot-dashed line). All plots were calculated from the original spectra with the same set of hypotheses and parameters. The DAMA experiment contour line (thin solid line) obtained from PSD spectra [12] and the (30) DAMA region corresponding to its reported annual modulation effect [13] (filled area) are also drawn. IGEX-DM results exclude WIMPnucleon cross-sections above 1.3 x lop8 nb for mssses corresponding to the DAMA region, improving other germanium results included that of Ref. [9]. The IGEX results have been derived using raw data without background substraction. 4.

SUMMARY

AND

OUTLOOK

Data collection is currently in progress imple menting other background reduction strategies. Copper (instead of teflon) tubes for evaporated nitrogen gas have been installed to avoid radon intrusion into the tubes. An exhaustive control of environmental conditions in laboratory is underway aiming to reduce radon levels. An additional neutron moderator (40 cm of water loaded with boron) is being installed. Finally, improved PSD recording of both preamplifier and amplifier pulses is being tested. Summarizing, the IGEX experiment has started to look for WIMPS by employing one of the 2-kg enriched Ge detectors currently in operation in the Canfranc Underground Laboratory. The energy threshold of the chosen detector is less than 4 keV. After 60 kg day of exposure, a background level of 0.3 c/(keV kg day) between 4 and 10 keV have been recorded and about 5 x lo-’ c/(keV kg day), on the average, from 10 to 40

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Physics B (Proc. Suppl.) 95 (2001) 229-232

MwG=V)

Figure 2. Low-energy spectrum of the IGEX RG-II detector for Mt=60 kg d (solid line) and Mt=136 kg d (dotted line).

keV. The derived exclusion plot a(m) for SI interaction improves the exclusion limits from previous conventional germanium diodes data. The data taking is proceeding. Acknowledgements. The Canfranc Astroparticle Underground Laboratory is operated by the University of Zaragoza under contract No. AEN99-1033. This research was partially founded by the Spanish Commission for Science and Technology (CICYT), the US National Science Foundation and the US Department of Energy. The isotopically enriched ‘“Ge was supplied by the Institute for Nuclear Research (INR), Moscow, and the Institute for Theoretical and Experimental Physics (ITEP) , Moscow. REFERENCES

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Figure 3. Exclusion plots for SI interaction in IGEX-DM (thick solid line), COSME-1 [lo] (thin dashed line), COSME-2 [ll] (thick dashed line), Heidelberg-Moscow 19) (dot-dashed line) and DAMA NaI-0 [12] (thin solid line). The filled area corresponds to the DAMA region [13]. IGEX-DM 1 kg y projection (dotted line) is also shown.

6. 7. 8. 9. 10. 11. 12. 13.

A. Morales et al., Phys. Lett. B 489 (2000) 268. J. Morales et al., NIM A 321 (1992) 410. J. Engel. Phys. Lett. B 264 (1991) 114. L. Baudis et al., Phys. Rev. D 59 (1998) 022001. E. Garcia et al., Phys. Rev. D 51 (1995) 1428. S. Cebrian et al., New J. Physics 2 (2000) 12.1-12.20 (http://njp.org) R. Bernabei et al., Phys. Lett. B 379 (1996) 299. R. Bernabei et al., Phys. Lett. B 450 (1999) 448, ROM2F/2000/01, January 2000.