- Email: [email protected]
Dark matter search in the Fréjus Underground Laboratory EDELWEISS experiment
Nuclear
Instruments
and Methods in Physics
Research
A 370
( 1996) 230-232
NUCLEAR INSTRUMENTS a METHODS IN PHYSICS RESEARCH SectjonA
-_ l!B ELSEVIEK
Dark matter search in the Fr6jus Underground Laboratory EDELWEISS experiment A. de Bellefon”, L. Bergeb, I. Berkks’, D. Broszkiewicz”, B. Chambon’, M. Chapellier”‘“, G. Chardind, Ph. Charvin’, V. Chazal’, N. Coronh, M. De Jesus’, D. Drain’, L. Dumoulin”, Y. Giraud-H&raud”, G. Guerrier’, C. Goldbachg, J.P. Hadjoutc, J. Leblancg, Y. MessousC, X.F. Navick’, G. Nollezg, C. Pastore, P. Pari”, I. Prostakovd, M.C. Perillo-Isaac”, D. Yvond “Lahorutoire hC&tre ‘Institut
de Physiyue
de Sprctrometrir de Physique
Nuckaire
“Service
de Physique
‘Service
Corpusculaire.
Nucl4aire
de Physique
de Lyon.
‘Institut
(Uni\~ersitd
de France
(CNRS, IN2P3),
de Masse Orsay
C. Bernard
de I’Etat
Saclay,
CondensPlDRECAMIDSMlCEA-CE Souterrain
d’Astrophy.vique
d’ Astrophysique
de Modane,
de Paris. Spatiale.
France
(CNRS. INZP_ZJ. France
Lyon I & CNRS. INZP3).
des ParticuleslDAPNIAIDSMICEA-CE
‘Laboratoire “lnstitut
Co&ge
et de SpectromL;trie
France
France
Saclay.
France
France
(CNRS, INSU).
France
Orsay (CNRS, INSU).
France
Abstract The status of the EDELWEISS experiment (Experience pour DEtecter Les Wimps En Site Souterrain) in the Frkjus Underground Laboratory is reported. The cryostat is described with the main lines of low radioactivity design and readout system. The first results using bolometer detectors together with the measurement of the internal radioactive background using a 100 cm’ classical Ge crystal are reported. The future program of the experiment will be outlined.
1. Introduction The cold dark matter, if due to supersymmetric particles. requires for its detection with bolometers a well protected site, a very low radioactive cryostat, a sensitive very massive bolometer. and last but not least the possibility to discriminate ordinary events from cosmic events [l]. This paper describes first steps on this long trail.
2. FrCjus Underground
Laboratory
This site is well known and we should only briefly summarize its qualities. It offers a rock coverage of 1780 m, mostly composed of lustrated schists, equivalent to 4800 m of water. Its rapid neutrons flux (energy 1 to 5 MeV) was recently measured and is about 170 neutrons m-‘day-‘. The muons flux is 4 rr-*day ’ The average radon content under normal cleaned air renewal, is 14 Bql ml. *Corresponding author. Tel. +33 I 69087235, fax +33 1 69088786, e-mail [email protected].
A lead and copper castle have been built around a specially designed cryostat, with nitrogen flux to eliminate the presence of residual radon.
3. Cryogenics a) “He cryostat: the upper part containing nitrogen and most of the He is more than 600 mm away from the bolometer. It is built in stainless steel with low content of cobalt. Its activity was estimated to be <0.03 Bq/kg and should be negligible. The lower part is made only of three cylinders of clean copper. Note that liquid He is surrounding the bolometer. The holding time for liquid nitrogen is two weeks, the He should be transferred every 5 days (the corresponding volumes are respectively 41 and 19 I). b) The dilution refrigerator is a home made system with a lower part made of selected materials, copper, nylon, silver powder for the exchangers. The only measurable activity is that of the silver powder with a small radiogenitally content of ‘“‘Ag (4 dpm/kg, energies of y 434, 614 and 723 keV). The dilution chamber is coated with 3.7 g of this powder. The distance to the bolometer is 23 cm and is
016%9002/96/$015.00 0 1996 Elsevier Science B.V. All rights reserved SSDf 0168-9002(95)01093-9
A.
de
Bellefm
et al.
I Nucl.
ltwtr.
atzd
separated by a copper rod or by ancient lead, both with more than 10 cm in thickness so the effect should be negligible. The circulation rate is only 20 kmoles/s, the lowest temperature around 32 mK with all coaxial cables installed. The present available volume is a cylinder of 80 mm diameter and 300 mm in length (without intermediate temperature or radioactivity shield). It will be upgraded within this year to a circulation rate of 100 pmoles/s, a base temperature of IO mK and a diameter of 90 mm. c) Gas handling system and control: a home design, manual or semi-automatic system, remote control, allows us to reduce the start to a few operations. Later on, a fully automatic system will be implemented. Note that we used ‘He as exchange gas. d) Internal electrical connections: the principle used is multiple shielding which is difficult when in the same time we need to carefully thermalize all the coaxial cables.
4. Read-out system and bolometer The read out system consists of the preamplifier designed according to Ref. [2], and FET heated to l20I50 K enclosed in boxes maintained at 4.2 K. They stand 60 cm away from the bolometer. It gives a noise level close to the thermodynamic limit (- 2 nV Hz-“’ in running conditions). The signal is further amplified, filtered by a low pass filter of I kHz to 5 kHz and then digitalized in a LeCroy 6810. and analysed with a home made program which compares each signal with a template. The bolometer is that described in Ref. [3], a 24g sapphire equipped with NTD sensor with typically 260 nV/keV at 45 mK (impedance at zero bias 700 kfI). Lowering the temperature improves the sensitivity but not the energy threshold caused by microphonics. The energy calibration was obtained by an external source of “Co. The spectrum observed was fitted to a Monte Carlo, the calibration is believed to be better than 5%.
Meth.
in Phy.
Res.
results
After an analysis on the shape of signals obtained during I25 hours, rejecting at least electromagnetic spurious signals above 5 keV, we obtained the spectrum presented in Fig. I. compared to an early attempt with the same bolometer in a standard cryostat. There is a definite improvement but still far below what is needed. Part of the problem comes from the small dimension of that bolometer which enhances the low energy part of the spectrum. We note that the bolometer itself and its very surrounding may be improved in radioactivity quality. We have then tried to measure the radioactivity coming from the cryostat with a standard Ge detector ( I20 cm’) removing the lead shield to enhanced the detector sensitivity but keeping liquid He
(1996)
231
?30-232
Radioactive
background
spectrum
1000
2 2 5 e 0 z
600
600
400
E f 3 0
200
E
0 10
0
Fig. I. Radioactive
50
20
60
70
spectrum normalised to event rate line: result of the 1991 attempt 151, solid line: this experiment. An average event rate of 25 events kg~‘keV~‘day~’ above 16keV is observed. The insert shows more accurately the radioactive background at energies above 15 keV
in events
background
kgm’keVm’daym’.
Dashed
around the cryostat. Many lines appear (h”Co. “‘Cs, “‘Sb) which are rather difficult to explain. Those coming from the radon need further experiments with a more careful avoidance of possible trapping of radon in and around the cryostat. Nevertheless this very preliminary experiment excludes a new little phase space for axially coupled WIMPS as shown on Fig. 2. To achieve a comparison between experiments we normalized the cross section on WIMP-proton cross sections [6].
6. Conclusion
and future program
There is obviously many aspects which needs serious improvements. The next trial will be a 80 g Ge with both ionisation [4] and heat which may shed light on the too high residual radioactivity and in the same time will
loocm
5. Preliminary
A 370
;
Exclusion .-..-., I
!5 i=
:
for axially 7
1000:
100
\‘$.;
/
7. ...
1
“.....i
10
(92) !Na .:’
,.:
._j
; .Y’
:
-“caF2
‘, ‘,. 10
/’
,’ ,&S
A,’ $1 _ /,,/ I L\ , ‘,, . - - r ).:
.E
m% E‘0 z
WIMPS T ..,..
Al (94),
g?? gg 73’;
coupled . . . ..I /
\I’
:a VIE
2
plot
...‘.BPRS(94)
....._I
100
“.._--1
1000
“I
lo4
WIMP mass (GeV)
Fig. 2. Exclusion plot in the hypothesis of an axially coupled WIMP extracted from the background spectrum shown on Fig. I. This experiment is labelled as Al(94). For comparison, we plotted the best previous results extracted from Ref. [6]. Courtesy of C. Tao.
VI. APPLICATIONS
232
A. dr Brll&~
et ul. I Nucl. Instr. md Meth. irl Phys. Res. A 370 (19961
prepare us to a more ambitious experiment after the upgrading of the dilution fridge. In the same time EDELWEISS team is involved in R&D in mostly thin films for detection of ballistic phonons and light and heat simultaneous detection.
230-232
References
[II For a review talk about direct detection of WIMPS, L. Mosca. Rencontres de Moriond on Particle Astrophysics. Atomic Physics and Gravitation (Editions Frontiire, Gif sur Proc.
Yvette, 1994) and references
Acknowledgements Special thanks are due to technical staff for their help, J.P. Soirat, P. Forget (Saclay), M. Martin (IAP). Useful discussion with P. Garoche (URA2). J.P. Terre (LPSP), J.L. Bret, A. Benoit (CRTBT) are acknowledged. L. Mosca and G. Gerbier (Saclay). J.L. Reyss (CFR), F. and P. Hubert (CENBG) measured material with low radioactivity used in this experiment.
therein.
121 D. Yvon et al.. accepted in Nucl. Ins&. and Meth. A. [31 P. de Marcillac et al., in: Low Temperature Detectors for Neutrinos and Dark Matter IV, eds. N.E. Booth and G.L. Salmon (Editions Frontitre. Gif sur Yvette. 1992) p. 81. [41 See D. L’Hote et al., these Proceedings (Workshop on Low Temperature Detectors (LTD6), Beatenberg/Interlaken, Switzerland, 1995) Nucl. Instr. and Meth. A 370 (1996) 193. [51 N. Coron et al.. Astron Astrophys. 278 (1993) L31. 161C. Bacci et al.. Astroparticle Physics 2 (1994) 117.
Instruments
and Methods in Physics
Research
A 370
( 1996) 230-232
NUCLEAR INSTRUMENTS a METHODS IN PHYSICS RESEARCH SectjonA
-_ l!B ELSEVIEK
Dark matter search in the Fr6jus Underground Laboratory EDELWEISS experiment A. de Bellefon”, L. Bergeb, I. Berkks’, D. Broszkiewicz”, B. Chambon’, M. Chapellier”‘“, G. Chardind, Ph. Charvin’, V. Chazal’, N. Coronh, M. De Jesus’, D. Drain’, L. Dumoulin”, Y. Giraud-H&raud”, G. Guerrier’, C. Goldbachg, J.P. Hadjoutc, J. Leblancg, Y. MessousC, X.F. Navick’, G. Nollezg, C. Pastore, P. Pari”, I. Prostakovd, M.C. Perillo-Isaac”, D. Yvond “Lahorutoire hC&tre ‘Institut
de Physiyue
de Sprctrometrir de Physique
Nuckaire
“Service
de Physique
‘Service
Corpusculaire.
Nucl4aire
de Physique
de Lyon.
‘Institut
(Uni\~ersitd
de France
(CNRS, IN2P3),
de Masse Orsay
C. Bernard
de I’Etat
Saclay,
CondensPlDRECAMIDSMlCEA-CE Souterrain
d’Astrophy.vique
d’ Astrophysique
de Modane,
de Paris. Spatiale.
France
(CNRS. INZP_ZJ. France
Lyon I & CNRS. INZP3).
des ParticuleslDAPNIAIDSMICEA-CE
‘Laboratoire “lnstitut
Co&ge
et de SpectromL;trie
France
France
Saclay.
France
France
(CNRS, INSU).
France
Orsay (CNRS, INSU).
France
Abstract The status of the EDELWEISS experiment (Experience pour DEtecter Les Wimps En Site Souterrain) in the Frkjus Underground Laboratory is reported. The cryostat is described with the main lines of low radioactivity design and readout system. The first results using bolometer detectors together with the measurement of the internal radioactive background using a 100 cm’ classical Ge crystal are reported. The future program of the experiment will be outlined.
1. Introduction The cold dark matter, if due to supersymmetric particles. requires for its detection with bolometers a well protected site, a very low radioactive cryostat, a sensitive very massive bolometer. and last but not least the possibility to discriminate ordinary events from cosmic events [l]. This paper describes first steps on this long trail.
2. FrCjus Underground
Laboratory
This site is well known and we should only briefly summarize its qualities. It offers a rock coverage of 1780 m, mostly composed of lustrated schists, equivalent to 4800 m of water. Its rapid neutrons flux (energy 1 to 5 MeV) was recently measured and is about 170 neutrons m-‘day-‘. The muons flux is 4 rr-*day ’ The average radon content under normal cleaned air renewal, is 14 Bql ml. *Corresponding author. Tel. +33 I 69087235, fax +33 1 69088786, e-mail [email protected].
A lead and copper castle have been built around a specially designed cryostat, with nitrogen flux to eliminate the presence of residual radon.
3. Cryogenics a) “He cryostat: the upper part containing nitrogen and most of the He is more than 600 mm away from the bolometer. It is built in stainless steel with low content of cobalt. Its activity was estimated to be <0.03 Bq/kg and should be negligible. The lower part is made only of three cylinders of clean copper. Note that liquid He is surrounding the bolometer. The holding time for liquid nitrogen is two weeks, the He should be transferred every 5 days (the corresponding volumes are respectively 41 and 19 I). b) The dilution refrigerator is a home made system with a lower part made of selected materials, copper, nylon, silver powder for the exchangers. The only measurable activity is that of the silver powder with a small radiogenitally content of ‘“‘Ag (4 dpm/kg, energies of y 434, 614 and 723 keV). The dilution chamber is coated with 3.7 g of this powder. The distance to the bolometer is 23 cm and is
016%9002/96/$015.00 0 1996 Elsevier Science B.V. All rights reserved SSDf 0168-9002(95)01093-9
A.
de
Bellefm
et al.
I Nucl.
ltwtr.
atzd
separated by a copper rod or by ancient lead, both with more than 10 cm in thickness so the effect should be negligible. The circulation rate is only 20 kmoles/s, the lowest temperature around 32 mK with all coaxial cables installed. The present available volume is a cylinder of 80 mm diameter and 300 mm in length (without intermediate temperature or radioactivity shield). It will be upgraded within this year to a circulation rate of 100 pmoles/s, a base temperature of IO mK and a diameter of 90 mm. c) Gas handling system and control: a home design, manual or semi-automatic system, remote control, allows us to reduce the start to a few operations. Later on, a fully automatic system will be implemented. Note that we used ‘He as exchange gas. d) Internal electrical connections: the principle used is multiple shielding which is difficult when in the same time we need to carefully thermalize all the coaxial cables.
4. Read-out system and bolometer The read out system consists of the preamplifier designed according to Ref. [2], and FET heated to l20I50 K enclosed in boxes maintained at 4.2 K. They stand 60 cm away from the bolometer. It gives a noise level close to the thermodynamic limit (- 2 nV Hz-“’ in running conditions). The signal is further amplified, filtered by a low pass filter of I kHz to 5 kHz and then digitalized in a LeCroy 6810. and analysed with a home made program which compares each signal with a template. The bolometer is that described in Ref. [3], a 24g sapphire equipped with NTD sensor with typically 260 nV/keV at 45 mK (impedance at zero bias 700 kfI). Lowering the temperature improves the sensitivity but not the energy threshold caused by microphonics. The energy calibration was obtained by an external source of “Co. The spectrum observed was fitted to a Monte Carlo, the calibration is believed to be better than 5%.
Meth.
in Phy.
Res.
results
After an analysis on the shape of signals obtained during I25 hours, rejecting at least electromagnetic spurious signals above 5 keV, we obtained the spectrum presented in Fig. I. compared to an early attempt with the same bolometer in a standard cryostat. There is a definite improvement but still far below what is needed. Part of the problem comes from the small dimension of that bolometer which enhances the low energy part of the spectrum. We note that the bolometer itself and its very surrounding may be improved in radioactivity quality. We have then tried to measure the radioactivity coming from the cryostat with a standard Ge detector ( I20 cm’) removing the lead shield to enhanced the detector sensitivity but keeping liquid He
(1996)
231
?30-232
Radioactive
background
spectrum
1000
2 2 5 e 0 z
600
600
400
E f 3 0
200
E
0 10
0
Fig. I. Radioactive
50
20
60
70
spectrum normalised to event rate line: result of the 1991 attempt 151, solid line: this experiment. An average event rate of 25 events kg~‘keV~‘day~’ above 16keV is observed. The insert shows more accurately the radioactive background at energies above 15 keV
in events
background
kgm’keVm’daym’.
Dashed
around the cryostat. Many lines appear (h”Co. “‘Cs, “‘Sb) which are rather difficult to explain. Those coming from the radon need further experiments with a more careful avoidance of possible trapping of radon in and around the cryostat. Nevertheless this very preliminary experiment excludes a new little phase space for axially coupled WIMPS as shown on Fig. 2. To achieve a comparison between experiments we normalized the cross section on WIMP-proton cross sections [6].
6. Conclusion
and future program
There is obviously many aspects which needs serious improvements. The next trial will be a 80 g Ge with both ionisation [4] and heat which may shed light on the too high residual radioactivity and in the same time will
loocm
5. Preliminary
A 370
;
Exclusion .-..-., I
!5 i=
:
for axially 7
1000:
100
\‘$.;
/
7. ...
1
“.....i
10
(92) !Na .:’
,.:
._j
; .Y’
:
-“caF2
‘, ‘,. 10
/’
,’ ,&S
A,’ $1 _ /,,/ I L\ , ‘,, . - - r ).:
.E
m% E‘0 z
WIMPS T ..,..
Al (94),
g?? gg 73’;
coupled . . . ..I /
\I’
:a VIE
2
plot
...‘.BPRS(94)
....._I
100
“.._--1
1000
“I
lo4
WIMP mass (GeV)
Fig. 2. Exclusion plot in the hypothesis of an axially coupled WIMP extracted from the background spectrum shown on Fig. I. This experiment is labelled as Al(94). For comparison, we plotted the best previous results extracted from Ref. [6]. Courtesy of C. Tao.
VI. APPLICATIONS
232
A. dr Brll&~
et ul. I Nucl. Instr. md Meth. irl Phys. Res. A 370 (19961
prepare us to a more ambitious experiment after the upgrading of the dilution fridge. In the same time EDELWEISS team is involved in R&D in mostly thin films for detection of ballistic phonons and light and heat simultaneous detection.
230-232
References
[II For a review talk about direct detection of WIMPS, L. Mosca. Rencontres de Moriond on Particle Astrophysics. Atomic Physics and Gravitation (Editions Frontiire, Gif sur Proc.
Yvette, 1994) and references
Acknowledgements Special thanks are due to technical staff for their help, J.P. Soirat, P. Forget (Saclay), M. Martin (IAP). Useful discussion with P. Garoche (URA2). J.P. Terre (LPSP), J.L. Bret, A. Benoit (CRTBT) are acknowledged. L. Mosca and G. Gerbier (Saclay). J.L. Reyss (CFR), F. and P. Hubert (CENBG) measured material with low radioactivity used in this experiment.
therein.
121 D. Yvon et al.. accepted in Nucl. Ins&. and Meth. A. [31 P. de Marcillac et al., in: Low Temperature Detectors for Neutrinos and Dark Matter IV, eds. N.E. Booth and G.L. Salmon (Editions Frontitre. Gif sur Yvette. 1992) p. 81. [41 See D. L’Hote et al., these Proceedings (Workshop on Low Temperature Detectors (LTD6), Beatenberg/Interlaken, Switzerland, 1995) Nucl. Instr. and Meth. A 370 (1996) 193. [51 N. Coron et al.. Astron Astrophys. 278 (1993) L31. 161C. Bacci et al.. Astroparticle Physics 2 (1994) 117.