C. Amsler Physik-Institut der Universität Zürich

Search for WIMPs 

in liquid argon 

C. Amsler 

Physik-Institut der Universität Zürich

WIN’11, CapeTown, 31 January 2011, C. Amsler 

Abstract: The DARWIN Collaboration has been funded by ASPERA to prepare a proposal for a 

next generation of WIMP searches using noble liquids. The University of Zurich is performing 

R&D to detect the VUV light from recoiling nuclei. I will present results obtained with 1 ton of 

liquid argon (ArDM project) and discuss the response to neutrons using a monochromatic neutron 

source. 

Progress report on some of the developments at CERN towards the 

construction a large liquid argon (LAr) detector for dark matter searches. 

Liquid argon technology for dark matter 

searches is still in its infancy 

1) Brief review on dark matter 

2) Future project, DARWIN consortium 

3) R&D on light readout in liquid argon 

4) First results from 1 ton LAr (ArDM) 

5) Response LAr to neutrons 

6) Conclusions 

C. Amsler et al. (ArDM Coll.) JINST 5 (2010) P11003 

(and ref. therein) 

V. Boccone, PhD thesis, Univ. Zurich, 2010 

2

• Fritz Zwicky (1933) from the movements of galaxies in 

clusters using the virial theorem: Ttotal much too large 

• Vera Rubin (1970) from the speed of stars in spiral galaxies 

Andromeda 

Astrophysical evidences for dark matter 

v 

1/r 1/2 

Milky Way 

5 x more dark matter 

than ordinary matter 

• Most impressive: 

Collision of galaxies in the Bullet Cluster (1E0657-558) 

X-ray emission from baryonic matter slowing 

down (electromagnetic repulsion) 

Center of mass from 

gravitational lensing 

WIN’11, CapeTown, 31 January 2011, C. Amsler 3 

r 

© 1994 Deneba Systems, Inc.

V(baryons)= 4 % 

V(dark matter) = 20% 

Nature of dark matter (DM) 

but only 1% of baryonic mass is visible! 

Energy pizza 

V(dark energy) = 76% 

Nature of DM = elementary particles are stable and weak interacting (survival) 

Interaction: 

• gravitational (+ weak) 

• no electromagnetic interaction 

Neither emission nor absorption of light 

Should be called instead transparent matter 

WIN’11, CapeTown, 31 January 2011, C. Amsler 4

Most popular: 


II) Axions: a -> gg: Mean life ≫ age of universe for M < 10 eV 

M < 0.01 eV otherwise a -> gg enhances cooling in red giants 

M = 10-3 - 10-5 eV 

CAST expriment at CERN (axions from the sun) M < 0.02 eV 

g 

a 

g 

9T 

III) WIMPs: Weakly Interacting Massive Particles 

e.g. the neutralino x1 of predicted by supersymmetry 

excluded 

DM candidates, see e.g. RPP 2010, M. Drees and G. Gerbier 

I) Neutrinos (unlikely) 

Early universe: g e + e - nini until decoupling at T ≈ 1 MeV 

Critical density (V = 1 ) requires mne + mnm + mnt ≃ 50 eV but neutrinos are 

much lighter. Heavy neutrinos should decay 

< 45 GeV (LEP) 

M ≈ 100 GeV - a few TeV 

5

Ar, Xe 


Invisible 

nuclear recoil 0 - 100 keV 

light, ionization, phonons 

Best limits: 

Spin independent ~ A 2 boost 

Current upper limits on sN 

3.4 x 10 -44 cm 2 @ 55 GeV 

(CDMS, XENON-100) 

Weak interaction: needs very large targets 

Laboratory (non-accelerator) searches 

χ χ 

Z 0 , H, h 

q q 

χ χ 

q ~ 

q q 

E. Aprile et al., PRL 105 (2010) 131302 

SUSY 

6

Future laboratory searches: coordination needed!! 

I) Cryogenic detectors 

T < 100 mK (bolometers) 

Phonons and ionization, EDELWEISS, CDMS 

Phonons and scintillation, CRESST 

Future generation: 100 x more sensitive 

e.g. EURECA- consortium: EDELWEISS + CRESST 

10 kg to 1000 kg 

Advantages: high resolution, nearly background free 

Signal proportional to measurement time 

II) Noble liquids 

ZEPLIN, ArDM, WARP, DEAP, MINICLEAN, 

XENON-10,100... 

Future generation: DARWIN-consortium (Xe, Ar) 

LUX in the USA (Xe), CLEAN, DEAP-3000 (Ar) 

Advantage: large masses, Ar is cheap 

However: signal proportional to square root of 

measurement time 


Goal: 

DARWIN = DARk matter WImp search with Noble liquids 

(L. Baudis, Spokesperson) 

(funded by ASPERA) 

Regroups current participants in 

ArDM /WARP / XENON 

8t LAr 

5t LXe 

Laura Baudis: arXiv:1012.4764v1 [astro-ph.IM] 21 Dec 2010 

(Identification of Dark Matter 2010-IDM2010 July 26-30, 2010 Montpellier France) 

Concept of a multiton detector for DM search in Europe using LXe and / 

or LAr: design report by 2013, operation by 2016. 

Target: sensitivity around 10 -47 cm 2 (3 orders of magnitude improvement) 


Rate/day in a 1 ton liquid Ar detector 

r (local) = 0.3 GeV/cm 3 

v = 245 km/s (June), 215 km (December) 

M(WIMP) = 100 GeV, s = 10 -42 cm 2 

•at 10 -45 cm 2 : 0.1 event /day @ 30 

keV threshold in LAr 

•LAr and LXe are complementary 


Liquid argon 

1/v 2 

Rate d -1 

1000 

100 

10 

1 

s[cm 2 ] 

10 -45 

10 -47 

dσ 

dq2 = G2F CF2 (q2 ) 

v2 30 keV 

100 evts/day 

Xenon 

Argon 

Xenon 

0 20 40 60 80 100 

Threshold [keV] 

Argon 

J/D 

20 100 

Nuclear recoil energy [keV] 

Years 

Form 

factor 

10

light 

128 nm 


Density 1.4 g/cm 3 

Refractive index 1.24 

Scintillation yield 

(E=0) 

Boiling point 87 K 

Interaction 

length 

≈50‘000 g/MeV 

@128 nm 

83.6 cm 

Price 1€ / liter 

Radioactive 

isotopes 

39 Ar isotope (b) 

T1/2 = 269 yrs, 

Q = 565 keV, 

1 Bq/kg 

11

Ar2* argon excimers 

1S (t1 = 5ns), 

3S (t3 = 1.6 ms) 

in liquid argon 


Scintillation mechanism in liquid argon 

pressure dependent 

128 nm, 10 nm band 

Tetra-Phenyl-Butadiene (TPB) 

128 nm -> 430 nm 

Wavelength shifter needed 

12

a- source 

Signal Amplitude [V] 

0 

-0.2 

-0.4 

-0.6 

-0.8 

Light readout R&D in gaseous and liquid argon 

1 GHz sampling rate 

0 5 10 

t [µs] 

15 

b b a 

1p.e. 

1 event 

20 

Events / 7.5 p.e. 

source 

50 

40 

30 

20 

10 

0 

74 x 78 mm 2 

5.3 MeV a-source 

0 200 400 600 800 1000 

Number of photoelectrons 


TPB 

PM PM 

b 

(Univ. Zurich at CERN) 

Gas

Light yield depends strongly on argon purity 

0.1 ppm 

in pure gas : t2 = 3.140 ± 0.067 µs 

Sensitive to impurities 

light output 

t2 can be used to measure the purity 


Charge from e, g 

(E>0) 

Charge from 

recoil nuclei 

(heavily ionizing) 


e/g - background suppression: 1st experimental trick 

600 e 750 g 

3 e 

(strong recombination) 

@ 30 keV @ 30 keV 

5 kV/cm 

Energy deposit [MeV] 

Charge/light ratio: 1e/1g from e, g 

Light yield 

(E>0) 

250 g 

1e/100 g from WIMPS 

1/3 

WIMPS produce few charges 

15

Recoil nuclei (WIMPs, n) 

populate the fast decaying 1 S, 

min. ionizing particles the slow 3 S. 

Use to suppress background 

Component ratio (CR) 

fast (

500 kV, 210 stages 


Feasibility study: 1 ton of LAr: ArDM 

0kV 

-4kV 

-500kV 

-1kV 

Operation on surface (CERN) 

Goal: 30 keV threshold 

120 cm drift length 

Gas 

500kV Generator Cockcroft Walton circuit 

CHARGE + LIGHT 

Wave Length Shifter Re�ectors 

Liquid 

CHARGE READ OUT 

WIMP 

Direct VUV 

photons 

Drifting 

electrons 

E �eld 

4kV/cm 

128nm 

Conversion of 

indirect VUV 

photons 

Photomultipliers 

Large electron multiplier (LEM) 

Wavelength shifter 

128nm -> 430 nm 

TPB on Tetratex foils 

14 x 8” 

Hamamatsu R5912-02MOD, Pt-underlay, evaporated, low radiation 

17

Large scale evaporator 

UV illumination 

Detector insertion 

8’’ PMT 

under UV 

illumination 

Exp. area at CERN 

Top flange 


Spraying: granularity 

Wavelength shifters 

Optimum: 1 mg/cm 2 

Best: evaporation on Tetratex (glued on 3M 

foil as mechanical support) 


WLS on PMT (direct light detection): 


Best

To the cartridge 


Liquid surface 

PT0 

First filling of 1t detector in May 2009 

PT0 immerseddetector 

is full! 

[ns] 

2 

250 mm 

1500 

1000 

500 

0 

Top flange 

≈200 K while filling 

275 mm 

PT0 

PT1 

PT2 

PT3 

PT4 

τ 2~1.5µs for >30days with bulk 

LAr w/o purification circuit 

30 days 

(no charge detection) 

Fit function: p0 * t + p1 

/ ndf 

200 300 400 500 600 

Literature τ2=(1.6±0.1)μs Time [h] 

2 

p0 

p1 

35.7 / 138 

0.0025 � 0.022 

1540 � 8.57 

21


Light yield for 1 ton LAr 

• External 22 Na (2 x 511 keV and 1275 keV) 

Triggered on external NaI crystal 

• Only half (7) of the PMTs were installed 

22

Na 22 

Log 

3 

10 

2 

10 

10 

1 

Data 

Simulation 

Ext. trigger on 1275 keV Na 

511 keV (ext. trig) 

511 keV 

662 keV 

1275 keV 

0 0 

0 200 400 600 800 1000 12000 

100 0 200 200 300400 400600500 800600 1000 1200 

511 662 1275 keV 


Arbitrarily scaled 

Ext. trigger on 511 keV Na 

B D 

F G 

Int. trigger Cs 

Top 

180 

160 

140 

120 

100 

80 

60 

40 

20 

Lin 

350 

300 

250 

200 

150 

100 

50 

≈ 50 keV 

511 

Int. trigger 

Na 

511 keV 

Npe Nr of photoelectrons 

662 

Zoom 

511 keV data 

≈0.8 pe/keV ee 

(assuming 14 PMTs) 

1275 keV 

Npe 

about 0.2 p.e. / keV WIMP, 

hence 6 p.e. for 30 keV 

Goal achieved 

Npe

Polyethylene 

NaI(Tl) 

370 MBq Am-Be 

241 Am (T1/2 = 430 a) 


Neutron background 

Neutrons e.g. from the rocks (spallation from cosmic rays) 

can simulate WIMPs! s(n A) ~ 10 18 s(WIMP-A) !! 

Neutrons 

40 n/s 

Cooling LAr 

Reflector/WLS 

Pure LAr 

210 Pb source 

•Neutron energy up to 10 MeV 

•4.4 MeV g (coincidence) 0 6 12 

MeV 

PMT 

74 x 78 mm 2 

Hamamatsu 

R6091-01 mod 

J. Marsh et al., NIM A 366 (1995) 340 

24

Component Ratio 

= fast (

NSD fusion source 

(60’000 €) 

Shielding: 1600 kg water extended 

polyester mith boron absorber 

(Colemanite) 

Plasma discharge, heated getter 

disks with adsorbed D2 

Specs: 10 7 n/s in 4p 


2010 

10 

HV (kV) 

specs 

data

Generator 

LAr cell 

X-monitor 

n-Monitor 

Radiation level below 1 µSv/h at places of human access for 10 7 n/s 


Energy calibrations 

1) of neutron counters: liquid scintillator (LSC) 

Na 

EJ301 SCIONIX, 2, 3, 5 “ 

C6H4(CH3)2 65% n-scattering on protons 

L = 5.5 cm 

Also measurement of the 

energy resolution 

! 

Cannot be calibrated only with g’s 

due to non linearity, quenching) 


3 “ 

5 “

[MeV] 

Recoil energy [MeV] Recoil energy 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 

4.4 MeV g 

True proton recoil energy vs � t for neutrons [MeV] 

AmBe-source AmBe 

Neutrons n 

�160 �140 �120 �100 �80 �60 �40 �20 0 20 

Time of �ight vs. TOF tagged vs tagged photon Photon [ns] 

2) neutron energy 

from generator: 

�� 

[ns] 

max recoil energy for central collisions np 

Calibration of recoil energy 

Good agreement at low energies 

n-energy 

3” 

2.5 MeV 

Tn [MeV] 


5” 

collimator 

3”

TOF-> Tn 

Recoil spectrum 

from generator 

(ideally should be flat) 

fit 

Better: unfolding response spectrum of LSC 

Energy deposit in LSC 

Repeat for all Tn 

Tn 

[MeV] 


0 

16% 

Collimator 

scattering 

2.5

First measurements with LAr 

(2.5 MeV) 


Essential to go below 30 keV: 


Measurement of the quenching factor as a function of recoil energy 

1.2 p.e /keV ee 

poor gas purity 

(t2 = 300 ns!) 

10 keV recoil reachable 

60 keV Am 

Preliminary 25% at 69 keV (Q = 65 o ) 

? 

40 keV Kr 

33

Conclusions 


• Current upper limits for s (WIMP-N) : 3.4 x 10 -44 cm 2 

Goal for the future: sensitivity around 10 -47 cm 2 , 

large detectors e.g. noble liquids 

• DARWIN consortium (> 2016), LXe, LAr 

• LAr technology for DM searches is still in its infancy, intensive R&D 

• For the first time ArDM (1t) was operated (on the surface): 

Light yield is consistent with expectations (0.8 photoelectrons / keV ee) 

Goal of 30 keV threshold should be reachable 

First successful detection of 50 keV energy in 1t LAr detector 

• More R&D needed, e.g. 

- Quenching factors vs. energy 

- Charge readout 

- Evaluation of background (e.g. Ar 39 contamination, purity of components) 

• Foreseen location for ArDM: Canfranc Laboratory (Spanish Pyrenees) 

34


Canfranc underground laboratory (2450 mwe)

© 1994 Deneba Systems, Inc. 



Y. Allkofer, C. Amsler, W. Creus, A. Ferella, C. Regenfus, L. Scotto-Lavina, M. Walter

C. Amsler Physik-Institut der Universität Zürich

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