The ORPHEUS dark matter experiment

Nuclear Instruments and Methods in Physics Research A 370 ( 1996) 227-229

NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH SectlonA

ELSEYIER

The ORPHEUS

dark matter experiment

M. Abplanalp”, G. Czapek”, U. Diggelmann”, M. Furlan”, D. Huber”, S. Janos”, U. Moser”, R. Pozzi”, K. Pretzla, K. Schmiemann”‘“, B. van den Brandtb, J.A. Konterb, S. Mangob, D. Perret-Gallix’, K.U. Kainer’, K.-M. Knoopd “Laboratory

for

High

Energy

Physics,

“Paul Schemer ‘LAP, “lnstitut

,fiir Werkstojfkunde

Urziversity Institute.

ofBern. CH 5232

Chemin de Bellerue,

und Werkstoftechnik,

Technical

Sidlerstrasse Villigen,

74941 Annecy,

Urziversity

5, CH 301-7 Bern, Swit:rrland

Swit:erland

of Clausthal.

France Agricolastr.

6, D 38678

Clausthal-Zellerfeld.

German>

Abstract A progress report of the ORPHEUS dark matter experiment in the Bern Underground Laboratory is presented. A description of the ORPHEUS detector and its sensitivity to WIMPS is given. The detector will consist of 1 to 2 kg Sn granules operating in a magnetic field of approximately 320 G and at a temperature of 50 mK. In the first phase. the detector will be read out by conventional pickup coils, followed by a second phase with SQUID loops. Preliminary results on background and radioactivity measurements are shown.

There is striking evidence, e.g. the flat rotation curve of the Milky Way. that about 90% of our cosmic neighbourhood consists of Dark Matter (DM). Searches for baryonic DM indicate that the majority of the galactic DM halo may consist of nonbaryonic particles which are supposed to interact weakly with matter [I]. The coherent scattering rate of Weakly Interacting Massive Particles (WIMPS) can be calculated as a function of recoil energy, WIMP mass. halo density and target material [2]. For a halo density of 0.4 GeV/cm’, the rates in a 1 kg Sn detector are listed in Table 1. The annual modulation of the interaction rate due to the motion of the earth around the sun can be quantified by

- R&x 2lR,““C

a= RJun,+ Rdrc ’ Table 1 The WIMP coherent scattering and energies above 10 keV

(1)

rate R and its annual modulation

Recoil energies above 10 keV

a (as described

30

100

300

1000

a [%I

1530 5.1

2399 2.0

1467 2.9

569 4.7

178 5.3

R [kg-‘day-‘]

0

397 15.6

628 0.4

276 3.4

89 4.5

a [%I

* Corresponding

author. Fax +41 31 631 44 87, e-mail [email protected].

0168-9002/96/$015.00 0 1996 Elsevier Science B.V All rights reserved 0168.9002(95)01092-O

SSDI

in the text) for Sn. The rates are listed for all recoil energies

10 R [kg-‘day-‘]

All recoil energies

with RJunc and Rdcc being the counting rates in June and December, respectively. To achieve high rates and recoil energies of typically a few keV. detector materials with high atomic number are preferable. Detectors made of superheated superconducting granules (SSG) have proven to be able to measure nuclear recoils with these energies [3]. A WIMP detector needs to be shielded against background radiation because the expected number of WIMP interactions is only a few hundred per kg per day (see Table I ). These requirements can be fulfilled by SSG detectors. The basic principle of detectors made of SSG is the phase transition of a granule due to the energy deposited by a particle, which can be sensed as a change in the magnetic flux [4]. The flux change can be read out either by a pickup coil with conventional electronics which

VI. APPLICATIONS

M. Ahiunalp

228

et ul. I Nucl. Instr. and Mrth.

in Phys. Res. A 370 (1996)

_?27-229

provides an induced voltage proportional to d@ldt or by a SQUID measuring Q(r). The readout sensitivity of a pickup coil with conventional electronics is limited by thermal noise. The detector volume which can be read by one pickup coil with conventional electronics should not exceed [5]

with r [km] being the granule radius and B [G] being the magnetic field. The WIMP counting rates strongly depend on the uniform behaviour of the granules in the detector. However, as found earlier, the phase transition fields of the granules are not identical [4]. Measurements on SSG detectors showed a field distribution with a standard deviation (T of typically 10%. This can be reduced to a (T of 57~ with special surface treatment of the granules [6]. The size distribution of the granules, which can be defined by sieving within a relative range of 5% has a negligible effect on the detection rate. The ORPHEUS experiment, which will be performed in the Bern Underground Laboratory in a depth of 70 mwe, is sketched in Fig. I. The cold box with the SSG detector will be thermally connected via a side access to a KelvinOX 300 dilution refrigerator. A side access was chosen in order to tit the ORPHEUS detector into the Bern Underground Laboratory. This arrangement allows also for an effective shielding of the detection chamber from external radioactive sources as described below. In a first phase, the SSG detector consists of three cylindrical units each containing I8 targets (Fig. 2). Each

Scint,illator

I

,:.

1 met(er

1s

t
t
Fig. 2. The detector chamber is shown for units equipped conventional readout and SQUID readout.

with

cylindrical target of 20 mm diameter and 140 mm length is filled with Sn granules of 20 p,m radius and surrounded by one pickup coil with conventional readout electronics. Assuming a 10% volume filling factor, the total mass of the granules will be approximately 1.6 kg. The magnetic field will be produced by a superconducting solenoid. In a second phase, a SSG detector with SQUID readout will be installed in the same cold box. The detector will consist of three units each containing seven targets of 25 mm diameter and 160 mm length, filled with Sn granules of 5 pm radius, resulting in a total granule mass of 1.2 kg

Lmd

Jj Fig.

7

I The ORPHEUS dark matter detector.

Copper

M. Ablanalp Table 2 Counting rate per day of the ORPHEUS phase transition field uniformity

229

et al. I Nucl. Imtr. and Meth. in Phys. Res. A 370 (1996) 227-229

detector with conventional

readout CR’) and SQUID readout CR”) as function

of WIMP mass and

mx @VI 10

1% a=58 (T= 10% u=

for a volume

30

sensitivity

1000

R‘

R’

R‘

R’

R’

R’

R‘

R’

R‘

143.2 40.5 20.5

625.0 168.6 85.4

642.6 160.8 81.0

1988.0 767.0 409.8

612.9 145.6 73.3

1343.5 621.7 352.8

259.2 61.2 30.9

528.7 253.4 255.4

82.9 19.5 10.0

165.7 80.1 46.4

filling

factor

of 10%. In order

to shield

the

field fluctuations, it will be surrounded by a superconducting shield and a set of five p-metal cylinders. Because of the higher detector

300

100

R’

chamber

from

of the smaller

environmental

granules

which

magnetic

can be read by the

SQUID system, the WIMP counting rate increases considerably, as shown in Table 2. The shielding against background radiation is sketched in Fig. 1. The muonic background after 70 mwe of rock can be identified by scintillators placed around the cold box and detector signals due to several simultaneously flipping granules. The y shielding will contain about 15 cm lead and 5 cm copper. The remaining y background can be identified by a detector signal of several simultaneously flipping granules [7]. In a simulation of y-background impinging the ORPHEUS detector, one out of ten y’s could fake a WIMP signal, so a radiopurity of the detector chamber and the granules of 1 mBq/kg will lead to a background rate of approximately IO/day. In order to monitor the radiopurity of the materials used for the experiment, a 120 g Ge-detector has been installed in the underground laboratory. A first unselected sample of Sn granules contained about 4 X 10d5 lead impurities. The Sn granules showed a continuous y spectrum with totally 750 counts kg ‘day ’ and the Sn K, line with an activity of 15 mBq/kg. The muon induced neutron background will be effectively reduced by paraffin moderators of about 30 cm around the cold box (Fig. 1) and 5 cm polyethylene in the detector chamber (Fig. 2). From Ref. [8] we estimate a neutron background rate of less than IO/day. This can be reduced by another order of magnitude when using the scintillation counters as muon veto. The present status of the ORPHEUS project is as follows: -A 13 g Sn SSG detector has been tested in a first long-lasting experiment [9] confirming the predicted sensitivity to cosmic muons and y irradiation.

- The low background Ge detector for material selection is already testing candidates for detector chamber components. -The KelvinOX 300 refrigerator has already been installed in the gallery of the Laboratory for High Energy Physics and cooled down to 10 mK. -The side access is under construction. The cold box and the detector chamber are under design.

Acknowledgements We would like to thank the technical staff of the Laboratory for High Energy Physics for the installation and the first operation of the KelvinOX 300 refrigerator. This work was supported by the Schweizer Nationalfonds zur Forderung der wissenschaftlichen Forschung and by the Bemische Stiftung zur Forderung der wissenschaftlichen Forschung an der Universitlt Bern.

References [l] K. Pretzl, Proc. Vulcan0 Workshop 1994: Frontier Objects in Astrophysics and Particle Physics, Vulcano, 23-28 May 1994, eds. F. Giovanelli and G. Mannocchi, p. 89. [2] A. Gabutti and K. Schmiemann. Phys. Lett. B 308 (1993) 411. [3] M. Abplanalp et al., Nucl. Instr. and Meth. A 360 (1995) 616. [4] K. Pretrl, Particle World l(6) (1990) 153. [5] K. Borer and M. Furlan. Nucl. Instr. and Meth. A 365 (1995) 491. [6] R. Pozzi, diploma thesis, Laboratorium fiir Hochenergiephysik Bern, 1995. in German. [7] M. Abplanalp et al.. J. Low Temp. Phys. 93 (1993) 809. [8] A. da Silva et al., Nucl. Instr. and Meth. A 354 (1995) 553. [9] M. Furlan et al., these Proceedings (Workshop on Low Temperature Detectors (LTD6), BeatenbergIInterlaken. 1995) Nucl. Instr. and Meth. A 370 (1996) 17.

VI. APPLICATIONS