ELSEVIER Nuclear Physics B (Proc. Suppl.) 70 (1999) 101-105

PROCEEDHUGS SUPPLEMENTS

Status report on the ORPHEUS dark matter detector and on its SQUID readout system B. van den Brandtb, S. Casalbuonia, G. Czapeka, U. Diggelmanna, T. EbertC, D. Hubera*, S. Janosa, K.U. KainerC, K.-M. KnoopC, J.A. Konterb, S. Mangob, U. Mosera, V.G. Palmieria, K. Pretzla

aLaboratory for High Energy Physics, University of Bern, Switzerland

bPaul Scherrer Institute, Villigen, Switzerland

‘Technical University of Clausthal, Germany

We report the status of the ORPHEUS dark matter experiment. The detector will consist of about 1 kg of superheated superconducting Tin granules and will be operated below 0.5 K. Measurements with a 32 cm3 target, read by a single SQUID channel, are presented. Experience has been gained with cryogenic tests, performed with a test detector chamber, mounted via a side access to the cryostat.

1. INTRODUCTION

Our Superheated Superconducting Granule de- tector (SSG) consists of micrometer sized type I- superconductor spheres embedded in dielectric filling material. A particle interaction can trigger a phase transition of a granule from the metastable superconducting to the normal state, which can be sensed with a magnetome- ter through the disappearance of the Meissner- Ochsenfeld effect. General descriptions of SSG detectors are given in ref. [l].

We have performed an experiment with a SSG detector in a 70 MeV neutron beam at the Paul Scherrer Institute (Switzerland), which has clearly demonstrated the sensitivity to nuclear re- coils [2]. Moreover, we confirmed the predicted sensitivity to 7 and cosmic muon radiation with a 13 g detector [3,4].

The planned WIMP detector ORPHEUS (con- sisting of about 1 kg Tin granules) is described in ref. [5] and illustrated in fig. 1. The detector chamber houses 19 SQUID readout or 57 conven- tional readout channels and a superconducting solenoid. It will be filled with liquid Helium to cool down the granules below 0.5K. Background considerations are reported elsewhere [S]. Here we present recent studies concerning the readout sys-

*present address: University of Geneva, Switzerland

0920~5632/98/% 19.00 0 1998 El sevier Science B.V. All rights resel PI1 SO920-5632(98)00396-X

tern and the actual status of the cryogenic parts of the detector.

2. STATUS OF THE SQUID READOUT SYSTEM

It is our aim to find a readout system that, com- pared to the well developed conventional read- out [7], allows to improve the sensitivity of the ORPHEUS detector and to reduce the number of readout channels. Within this context, we are testing a SQUID magnetometer, which includes several features of the ORPHEUS detector cham- ber, namely a superconducting solenoid and two SSG targets immersed in liquid Helium (fig. 2). However, the dimensions are smaller. The test setup is mounted directly to the mixing chamber of a small dilution refrigerator.

Sensitive measurements with SQUID-magneto- meters require a perfect shielding against external electromagnetic disturbances. Superconducting Faraday cages can fulfill these requirements. Fur- ther, significant noise contributions can arise from mechanical vibrations of the pickup coil in an in- homogeneous magnetic field inside the shielding and from temperature fluctuations. A rigid con- struction and a careful material selection are thus mandatory to obtain a maximum sensitivity of the system.

rved

B. van den Bmndt et al. /Nuclear Physics B (Proc. Suppl.) 70 (1999) 101-105

I Dilution refrioerator

, PIelVln “A J””

I Scintillator Lead Paraffin Copper

Paraffin

Signal cable ,, Mixing chamber

I Radiation shields I 1 2

Superconducting solenoid Detector chamber

Figure 1. Schematic view of the ORPHEUS setup.

A SQUID readout system for SSG detectors was previously studied by the British Columbia University group [8].

2.1. Experimental setup The experimental setup is illustrated in fig. 2.

An Aluminum alloy can (AIMgSil, Tc 21 1.05 K), inside coated with 1.5pm thick sputtered Nio- bium and directly mounted to the mixing cham- ber, serves as a superconducting shield. Setting the temperature above the critical temperature of Aluminum allows to study the shielding proper- ties of the Niobium film. A three layer /.J metal shield is also mounted around the vacuum cham- ber of the refrigerator.

Inside the superconducting shield a cylindrical chamber, made out of Delrin and mounted with Niobium screws to the Aluminum alloy can, holds the two symmetrically arranged targets with a total target volume of 32 cm3. Either target is filled only with 0.44 g of 0 20-23 pm Tin granules mixed with Teflon powder, resulting in a 0.4% volume filling factor. The granules are immersed in liquid Helium for cooling purposes. The tem-

-access

/ Inner vacuum pump tube

Outer vacuum pump tube

y pickup coil

solenoid

1 SSG targets

\ Aluminum can

Figure 2. Schematic view of the test set,up for the SQUID readout system. The arrows indicate the position and the winding-sense of the pickup coil turns.

B. van den Brandt et al. /Nuclear Physics B (Proc. Suppl.) 70 (1999) 101-105 103

perature is measured with a RuO2 sensor. A first-order gradiometer pickup coil is wound

directly around the Delrin chamber, having five windings around either target and a diameter of 40mm. It is connected via twisted and shielded wires to the SQUID, which is mounted at the 1 K stage of the refrigerator. The superconducting solenoid is placed around the Delrin chamber. We estimate the magnetic field in the solenoid using the formulae in ref. [9].

The phase transitions of the granules were in- duced by particles from a Cesium-137 source out- side the cryostat and by the background radioac- tivity of the detector material.

2.2. Results The magnetic shielding properties of the sput-

tered Niobium film are quite similar to those of the reference Aluminum alloy can. The Alu- minum can reduces the signal of an external per- manent magnet by 82 dB, while the Niobium film reduces the same signal by 69dB. This difference may be due to intrinsic material properties or it could result from the different feedthrough ge- ometries of the two shields. The shielding factors turned out to be independent of the magnitude of the inner magnetic field at least up to 16 mT. The two noise spectra shown in fig. 3 were recorded at 1.7 K (where only the Niobium film is supercon- ducting) with and without an applied magnetic field.

The noise peak around 10 Hz in the upper spec- trum of fig. 3 corresponds to the resonant fre- quency of the dilution unit and arises from its mechanical vibrations. This limits the measure- ments to a lower bandwidth of 1Hz in order to assure a good signal-to-noise ratio. The other dominant peak around 550Hz is clearly related to an acoustic resonance caused by a Helium-4 pump. Underlaying this pump with Polystyrene reduced that peak to 10V3 cPe/&% Fig. 4 shows some phase transitions of the granules in 30mT.

In order to show, that the recorded signals are due to phase transitions of the granules, the event rate was recorded in presence and absence of a Cesium-137 source. In fig. 5 the recorded rates are plotted versus the magnetic threshold. The shape of these curves is in good agreement with

10L frequency/Hz

Figure 3. Noise spectrum as detected by the SQUID. The applied field was 31mT for the up- per spectrum and zero for the lower one. @s is the magnetic fluxquantum.

-0.02 ;

-0.03 L, 1 . * 1 1 . . . 1 . . . 1 . . . . 1 . . * . : . 10 20 30 40 50 60

time/s

Figure 4. Phase transitions of granules in 30mT. The units refer to the SQUID signal. The differ- ent polarities of the signals result from the gra- diometer configuration of the pickup coil.

the predictions of Monte Carlo simulations of the SSG detector response [3]. However, the absolute rates are difficult to predict, since the source is located outside the cryostat.

For a given granule size, the present SQUID setup can read about 30 times the volume com- pared to the conventional readout system[7] (fig.6). The target volume of this test setup is limited by the small volume available in the cryo- stat. In a longer solenoid with a more homoge- neous field, the target volume could be increased

104 B. van den Brandt et al. /Nuclear Physics B (Proc. Suppl.) 70 (1999) 101-105

. 0

0 +

5-

0 *

0 .

0 ‘...‘...“....‘.‘..Q....o....‘.... 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

magnetic threshold 68/B

Figure 5. Event rate versus magnetic threshold at 30 mT and 420 mK with (full circles) and without (open circles) the external Cesium-137 source.

without significant reduction of the signal pulse- height. Therefore, this measurement reflects only a lower limit for the readable target volume with one SQUID channel.

2.3. Implications for ORPHEUS The sputtered Niobium film qualifies to be used

as a superconducting shield in ORPHEUS. How- ever, the Aluminum will be replaced by elec- troformed Copper, in order to reduce the back- ground radioactivity.

The longer solenoid in ORPHEUS will allow to increase the target volume per channel to about 100 cm3, which corresponds to 73 g Tin granules with a 10% filling factor. Being able to read sin- gle granules of 20 pm diameter with a SQUID in- creases the WIMP detection sensitivity by about an order of magnitude compared to a detector with conventional readout, using typically 35pm granules.

We are aware, that, in order to recognize coin- cident events with the muon veto counters, OR- PHEUS requires a faster readout system than the present one. We speculate that in the ORPHEUS

Figure 6. The required granule radius at 30mT and signal/n&se 1 10 versus the target volume per channel for the conventional readout (hatched region). The asterisk indicates the current status of the SQUID readout system.

environment the readout system will pick up less noise because of the more homogeneous magnetic field. This would allow to increase the bandwidth of the SQUID readout system.

3. STATUS OF THE CRYOGENICS

In 1995 we have installed a KelvinOX 300 re- frigerator. Because of the limiting height of our underground lab, the extension to the detector chamber has to be horizontal. This requires flexi- ble connections between the cryostat and the side access because of the thermal stresses. On the other hand, such connections increase the tem- perature gradients between the cold box and the cryostat.

Cryogenic tests, with the side access mounted to the cryostat, started in March 96 with a liq- uid Nitrogen run, followed by four liquid Helium runs. Then we added a cold box with a detector chamber of 600 cm3. Although the pressure of the outer vacuum chamber was below 10m6 mbar,

B. van den Bmndt et al. /Nuclear Physics B (Proc. Suppl.) 70 (1999) 101-105 105

Inner vacuum

5K 300 K the sputtered Niobium shield. Last not least, we would like to acknowledge the engagement for the cryogenics of F. Nydegger and the technical sup- port of S. Lehmann, H. Ruetsch and H.-U. Schiitz from the Bern group.

This work was supported by the Schweizer- ischer Nationalfonds zur Fiirderung der wissen- schaftlichen Forschung and by the Bernische Stiftung zur Forderung der wissenschaftlichen

0.2 K Outer vacuum Forschung an der Universitat Bern.

500 mm REFERENCES

Figure 7. The achieved temperatures of the ther- mal shields and of the detector chamber. The cryostat is located at 2m to the left of the cold box. Two of the thermal shields are additionally cooled with a stream of liquid Nitrogen, respec- tively of liquid Helium.

heat conduction via Copper alone turned out to be insufficient to reach the needed low temper- ature in the detector chamber. However, using additional liquid Nitrogen and liquid Helium cool- ing, it could be shown, that a temperature of 200mK can be reached in the detector chamber (fig. 7).

4. CONCLUSIONS

Significant steps in the ORPHEUS detector de- velopment were made with the SQUID readout system, with the cryogenic construction of the side access and with a small size cold box. A ma- jor problem currently under investigation is the necessary radiopurity of the detector elements. These questions will be addressed with a small size detector chamber, made of electroformed low radioactivity Copper, presently under construc- tion.

ACKNOWLEDGMENTS

We would like to thank C. Benvenuti, S. Ca- iatroni and R. Russo of the EST/SM division at CERN for their advice and for the production of

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