ELSEVIER Nuclear Physics B (Proc. Suppl.) 87 (2000) 117-119

PROCEEDINGS SUPPLEMENTS www.dsevier.nl/Iocate/npe

The ORPHEUS Dark Matter Experiment B. van den Brandt b, S. Casalbuoni a, G. Czapek a, T. Ebert ¢, F. Hasenbalg a, M. Hauser a, S. Janos a, K. U. Kainer c, K. M. Knoop c, J. A. Konter b, S. Mango b, U. Moser a, K. Pretzl a, and B. Sahli a

a Laboratory for High Energy Physics, University of Bern, Sidlerstrasse 5, CH 3012 Bern, Switzerland

b Paul Scherrer Institute, CH 5232 Villigen, Switzerland

c Institut fiir Werkstoffkunde und Werkstofftechnik, Technical University of Clausthal, Agricolastrasse 6, D 38678 Clansthal-Zellerfeld, Germany

The ORPHEUS dark matter experiment is being built and most of it is already installed at our underground facility in Bern (70 m.w.e). The detector relies in an initial phase on 0.45 kg (1.6 kg maximum capacity) superheated superconducting tin granules (SSG) to measure recoils from weakly interacting massive particles (WIMPs). The actual status of the installation as well as parallel ongoing studies are presented.

1. Introduct ion

The ORPHEUS detector is made of a homoge- neous mixture of SSG in a dielectric filling mate- rial immersed in an external magnetic field. The type I superconductor granules are kept slightly below the boundary of their superconducting-to- normal phase transition in a metastable state. The recoil energy released by a particle interact- ing with a granule causes a temperature increase inversely proportional to the specific heat of the granule. Due to the rather small value of the spe- cific heat below 500 mK, the energy deposited is enough to make a granule normal inducing a flux change because of the disappearence of the Meiss- ner effect. The flux change, or"flip", is measured by a sensitive magnetometer, e.g. an LRC circuit. A general description of SSG detectors is given in Refs. [1,2] and of their readout electronics in Ref. [3].

2. Expe r imen t a l setup

ORPHEUS is being built in the underground facility of the University of Bern (70 m.w.e.). A scheme of the experimental setup is shown in Fig. 1. Active shielding is provided by 2 cm thick plastic scintillators, followed by a passive shield: 15 cm normal lead, 4 cm OFHC-copper, and 18 cm of boron-doped (5%) polyethylene. The

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whole shielding is mounted on rails to open it in two halves and to access the detector. The cold box consists of concentric copper thermal shields held at temperatures of 2.5, 4.5, and 80 K. The detector chamber is installed inside and is made of electroformed copper. For an initial trial, the detector will be filled with Sn grains between 30- 34 #m in diameter mixed with PTFE (Teflon) powder as a filling dielectric material at 10% fill- ing factor. For reading out the flipping granules, 56 pickup coils, 1.8 cm in diameter, 6.8 cm long and roughly 1500 windings will be used. The de- tector chamber will be maintained at a base tem- perature of ~300 mK using an Oxford dilution refrigerator (300 #W cooling power).

Last year the dilution refrigerator and the side access have successfully been tested with a small prototype cold box at a base temperature of 200 mK [4]. At present, the dilution refrigera- tor, side access, cold box, detector chamber, and most of the shielding are already underground. The pickup coil bodies and solenoid are being built and from the shielding only the active veto is missing. Before starting the first trial runs, how- ever, a measurement of the actual background inside the shield with a high purity germanium detector is foreseen.

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118 B. van den Brandt et al./Nuclear Physics B (Proc. Suppl.) 87 (2000) 117-119

Figure 1. O R P H E U S experimental setup.

3. Granule s tudies

Improvement of the phase transition homo- geneity in specially treated tin granules produced by gas atomization has been observed. In a re- cent study [5], granules melted with a laser beam and fast cooled in a liquid nitrogen bath, exhib- ited better properties over untreated granules. In- teresting results were obtained when the super- heating critical field of single grains are measured at several angles with respect to the external magnetic field. While previous measurements of this kind [6] presented large variations (~20%) of the superheating fields, the latest measurements show that the treated granules possess rather fiat (~2%) curves of superheating fields as a function of the rotation angle.

A second sample of regularly spaced tin cylin- ders produced by tin evaporation onto a glass substrate also shows an improved phase transi- tion homogeneity. While ramping up the exter- nal magnetic field ~75% of the phase transitions occurr in a narrow magnetic field range, of the

order of 3%, around the superheating critical field of ~28 mT at 1.4 K [5].

4. C o n v e n t i o n a l readout

The conventional readout consists of a LRC circuit given by a pickup coil L, a cooled shunt resistor R, and a capacitance C determined by the combination of cable capacitances and the in- put capacitance of the low noise amplifier used to measure the signal. To maximize the sensing vol- ume and guarantee a signal to noise ratio (SNR) of ~10, several configurations were tested. Phase transitions of Sn granules 30-34 #m in diameter were used to produce the signals. The noise level was determined measuring the Vrms after pass- ing the amplified signal through an optimized low pass filter. For different coil dimensions and num- bers of windings, the values of R and cutoff fre- quency of the filter which maximized the SNR were found.

The best configuration was obtained for a pick-up coil with L=9.8 mH, number of wind-

B. van den Brandt et aL /Nuclear Physics B (Proc. Suppl.) 87 (2000) 117-119 119

ings=1500 (1.8 cm in diameter and 6.8 cm in length). For this coil, R was equal to 10 k~, the optimized cutoff frequency (-3 dB) of the filter was 15.9 kHz, and SNR=ll .5 .

5. SQUID readout

To gain sensitivity to flux variations of small size granules (,,~20 #in in diameter) and decrease the number of signal channels, a SQUID readout system is foreseen to be implemented in the fu- ture. Preliminary tests of noise levels have started in a copper chamber with dimensions similar to the detector chamber for conventional readout. This chamber contains a superconducting shield made of 1.5 #m-thick Nb film coated on a copper cylinder 15 cm in diameter and 52 cm long. In- side, a superconducting solenoid of similar dimen- sions provides the external magnetic field. Low inductance superconducting coils (~2 #H) will be used to sense the flux changes.

At present, several tests with a flux-gate mag- netometer have been performed to determine the flux attenuation of the superconducting shield as well as the solenoid homogeneity. Preliminary at- tenuation values of - 57 dB have been attained and further tests are expected to obtain a better shielding. The solenoid inhomegenity is less than 2% over ~75% of its volume. Calibration tests with two SQUIDs, one hooked up to a magne- tometer and the other to a gradiometer sensing coil, have already started and are under analysis.

6. Radiopurity measurements

A series of radiopurity measurements were per- formed in the last year at the Gotthard tunnel (3000 m.w.e) with a small high purity germanium detector inside a passive shield. A background level of 10 (5) cts/keV.kg.d, at 15 (45) keV, has been achieved. The main contribution to the background coming from 21°Pb contamination in the inner low activity Pb of the shield. With this Ge detector, sensitivity levels of ~-5, 10, 5, and 100 mBq kg -I have been obtained for the par- ent isotopes of 232Th, 238U, 137Cs, and 4°K, re- spectively. Samples of PTFE powder and polyac- etal polyoxymethylene (POM, Delrin) were mea-

sured with activity levels comparable to the back- ground. Probes of high purity Sn, OFHC Cu, and copper wire insulation are planned.

A major concern is the activity of the normal lead to be used in the ORPHEUS shield. Several probes were measured for their 21°Pb content by c~ and 7 spectroscopy. It was found that normal ORPHEUS lead has an activity of ~200 Bq kg -1 . This implies that an ultra-low activity inner Pb cylinder would be required to reduce the "y ray flux due to the normal lead.

7. Outlook

The actual status of the ORPHEUS project has been presented. We look forward to finish the active shield, superconducting solenoid, and last parts of the detector chamber. We will attempt to characterize the background of the whole shield- ing so that we should be able to start our first data acquisition tests in the forthcoming year.

Acknowledgments

This work was supported by Schweizeri- scher Nationalfonds zur FSrderung der wissen- schaftlichen Forschung and by the Bernische Stiftung zur FSrderung der wissenschaftlichen Forschung an der Universit~t Bern.

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