Results of CRESST phase I

SUPPLEMENTS ELSEVIER

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Suppl.)

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Results of CRESST Phase I F. PrBbsta, G. Angloherb, M. Bruckmayera, C. BucciC, S. Cooperb, P. Di Stefanoa, F. von Feilitzschd, T. Franka, D. Haup, Th. Jagemannd, J. Jochumd, R. Keelingb H. Krausb, J. Marcheseb, Y. Ramachersb, J. Schnagld, W. Seidela, I. Sergeyeva, M. Starkd, L. Stodolskya, H. Wulandarid aMax-Planck-Institut

fur Physik,

bUniversity

of Oxford,

Physics

CLaboratori

Nazionali

de1 Gran

dTechnische

Universitat

Fijhringer

Department

Ring 6, D-80805 Munich, , Oxford

OX1 3RH, U.K.

Sasso, I-67010 Assergi,

Miinchen,

Physik

Results of the CRESST experiment transition thermometers are presented.

Department,

Germany

Italy D-85747 Munich,

Germany

at Gran Sass0 using 262 g sapphire calorimeters with tungsten phase Calibration and analysis methods are described. Data taken in 2000

have been used to place limits on WIMP dark matter particles in the galactic halo. The sapphire detectors are especially sensitive for low-mass WIMPS with spin-dependent interaction and improve on existing limits in this region.

1. Set-up and Detectors The CRESST Collaboration has installed a low-background facility [l] in the Gran Sasso Underground Laboratory (LNGS) and in Phase I of the project has used 262g sapphire cryogenic calorimeters with W superconducting phase transition thermometers [2]. More details are given in those references. This paper presents the resulting dark matter limits. 2. Data taken in Gran Sasso The data used to set dark matter limits were taken during a week in October 2000, with a few short interruptions to re-fill the cryostat with liquid helium. The data consist of a lo-hour calibration run with an external 57Co source, 138.8 hours of data without source (of which 0.6 hours is dead time following triggers) and finally another calibration run. The data from detector # 8, which had the lowest threshold, is used to set our dark matter limits. A second detector was used to eliminate coincident events. The performance of each detector was monitored by injecting heater pulses into a small heater wire bonded to the W thermometer. Their shape was adjusted to create a detector response

similar to that of a particle interaction. A pulse was sent every 30 s throughout both dark matter and calibration runs. The height of the pulses was varied to cover the whole dynamic range, with more pulses in the low energy region. This method provides a monitor of the stability of the detectors, an extrapolation of the energy calibration over the whole dynamic range, and a measure of their trigger efficiency as a function of deposited energy. The amplitude of each pulse was determined by fitting it with a template. This avoids the bias of picking the highest point of suitably filtered pulses, which is systematically pulled by noise fluctuations to larger values. The absence of any bias is important for a precise definition of the threshold. To calibrate the energy scale a 57Co source (122 and 136 keV y lines) was inserted inside the shielding via a removable plug, illuminating the cold box from below. Data were taken with this source along with the heater pulses. A comparison of heater pulse amplitudes with those from the 122 keV y line of the calibration source provides an absolute calibration of the heater pulses in terms of equivalent y energy. A separate heater pulse template was made

0920-5632/02/.$ - see front matter 0 2002 Elsevier Science B.V. All rights reserved. PI1 SO920-5632(02)01453-6

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Figure 1. Energy spectrum of events in detector # 8 in the dark matter run (without source) in 200eV bins. The upper histogram shows the uncut data, the middle histogram the data after coincident events are rejected, and the lower histogram after the pulse-shape cut.

for each amplitude of heater pulses by averaging many pulses from that heater pulse voltage. To extrapolate the energy calibration over the whole dynamic range, we used the heater pulses and plotted their fitted amplitude versus the injected energy. These data were fit with a polynomial function to give the detector response as a function of the deposited energy. For the dark matter data, the response function determined above was used to convert each recorded event pulse to energy in each time bin. A template was made from similarly-converted calibration-source pulses around the Compton edge (30-35 keV). This template could be used to fit the pulse height, but it was found that an optimal filter gave a slightly better resolution. The optimal filter was calculated using the template and randomly sampled baseline noise. A comparison to the template fit showed that the optimal filter applied did not introduce an energy bias. The reliability of the energy calibration method to low energy was later checked in a dedicated run where a low-activity 57Co source was mounted inside the cryostat directly facing the crystals. Besides the 122 and 136 keV y lines, this source gave

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a 14.4 keV y line and a 6.4 keV Fe X-ray line. The source was chosen to be very weak to reduce the chance of contamination, with the result that the one week run gave low statistics in the 14.4 and 6.4 keV lines. After applying the standard calibration method of extrapolation from the 122 keV line as described above, the measured energies for the 14.4 and 6.4keV lines were 15.16+g:$i and +‘.07 keV, respectively, with the fit errors cor6.70-0.05 responding to 90 % CL. Our calibration procedure puts the 14.4 and the 6.4 keV lines 5.3 % and 5.4 % too high. Since it is the lower energies which most affect our dark matter limits, this tendency to shift events up in energy puts our limits on the conservative side. 3. Limits on WIMP 3.1.

dark matter

cuts

For the analysis of the dark matter data a software threshold of 600eV was used, above which the trigger efficiency was lOO%, as measured with the heater pulses. The spectrum for detector #8 is shown as the upper histogram in Fig. 1. There are 446 events from the software threshold to 120keV. Events in coincidence in two or more detectors cannot be due to WIMP interactions, and so can be discarded. The coincidence cut was set at f4 ms, removing 76 events and introducing negligible dead time. Considering that only two detectors were active in this run, the coincidence rate of about 17% is consistent with the solid angle for detecting coincidence and thus with all events being background. This shows that the coincidence cut could be very useful in reducing background in a larger segmented detector. The pulse shape of the remaining 370 events was then examined. Some of the events were spurious, induced by mechanical vibration or electronic noise, with an abnormal pulse shape. To judge the correctness of the pulse shape, each event was fitted with a template and the r.m.s. deviation calculated. A cut on this deviation was chosen to be conservative and have a retention efficiency of 100 % at all energies for good events. The effect of these cuts are shown in Fig. 1. After the pulse-shape cut 320 events remain. In the energy range from 15 to 25 keV the background

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Physics B (Proc. Suppl.) 110 (2002) 67-69

Spin Dependent Interaction

I”

1

10

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1000

WIMP lllsss [GeV/c~

Figure 2. Equivalent WIMP-proton cross section limits (90 % CL) for a spin-dependent interaction as a function of the WIMP mass from a 1.51 kgday exposure of a 262g sapphire detector. For comparison we show limits from the EDELWEISS dark matter search with cryogenic sapphire detectors [6] and from DAMA [7] and UKDMC [8] with NaI detectors.

is (0.73 f 0.22) counts/kg/keV/day and drops to about 0.3 counts/kg/keV/day at 100 keV. The spectrum shows a peak at about 5.9 keV with (7.0 f 1.2) counts/day. The position of the peak suggests a contamination with 55Fe in the vicinity of the crystal. However the peak is wider than expected, 572 f 90eV FWHM compared to the 200 f 50eV of the 6.4 keV line from the internal calibration source, so that it may be due to more than a single X-ray line. 3.2. Extraction of limits To extract upper limits for the WIMP interaction cross section we tried the optimal-interval method used by some other groups, but found that it was biased to lower cross sections from picking a ‘lucky’ downward fluctuation. For example for WIMP masses between 30GeV and 1000 GeV the relatively small interval of 22.6 26.4 keV with no counts was selected. Instead we used fit and Monte Carlo methods, as described in Ref. [3]. For comparison with other experiments, our limits for spin-dependent WIMP interactions

69

were converted to the equivalent WIMP-proton cross section using the X2J(J + 1) factors for the odd-group model as given in Ref. [4]. The result is shown in Fig. 2. Using the spin factor from Ref. [5] would shift the whole CRESST exclusion curve down by a factor of l/1.9 to lower cross sections, if one neglects the possible neutron contribution. In either case, we improve existing limits for low-mass WIMPS, which is due to our low threshold and was the goal of the first phase of CRESST. The second phase of the experiment is now being prepared. This will use cryogenic scintillators [9,10], with simultaneous measurement of the phonon and scintillation light to reduce the background from local radioactivity and provide improved sensitivity for high-mass WIMPS. 4. Acknowledgement This work was supported by the DFG SFB 375 “Particle Astrophysics”, the EU Network “Cryogenic Detectors” (contract ERBFMRXCT980167), BMBF, PPARC, and two EU Marie Curie Fellowships. REFERENCES 1. M. Biihler et al. (CRESST), Proc. LTD-6, Nucl. Instr. Meth. A 370 (1996) 237. 2. M. Sisti, 0. Meier et al. (CRESST), Nucl. Instr. Meth. A 466 (2001) 499. et al. (CRESST), astro3. M. Altmann ph/0106314. 4. J. Ellis and R.A. Flores, Phys. Lett. B 263 (1991) 259. 5. J. Engel et al., Phys. Rev. C 52 (1995) 2216. 6. A. de Bellefon et al. (EDELWEISS), Astropart. Phys. 6 (1996) 35. 7. R. Bernabei et al. (DAMA), Phys. Lett. B 389 (1996) 757. 8. N. Spooner et al. (UKDMC), Phys. Lett. B 473 (2000) 330. 9. P. Meunier et al., Appl. Phys. Lett. 75 (1999) 1335. 10. M. Bravin et al. (CRESST), Astropart. Phys. 12 (1999) 107.