Miguel Felizardo

( for the SIMPLE Collaboration)

COSMO 2011

Department of Physics, Faculty of Science and Technology, New University of Lisbon, Portugal

Instituto Tecnológico e Nuclear, Sacavém, Portugal

The Phase II SIMPLE Dark Matter Search

Experiment / Instrumentation / Acquisition

Signal Analysis

Results

SIMPLE (Superheated Instrument for Massive Particle Experiments) is a direct search activity using superheated liquid detectors, it is operated at the 1500 mwe level of the Low Noise Underground Laboratory (LSBB)

(i) E > Ec = 4ππππrc2γ γ γ γ / 3εεεε

(ii) ∆∆∆∆E/∆∆∆∆x > Ec / Λrc

key feature : selective sensitivity

tunable (P,T) double threshold device

30-100 cm thick concrete

50-75 cm thick water (shield, pool)

20 cm thick wood

1 cm thick steel

10 cm PE

50 cm thick concrete (floor)

atmospheric radon: 20 – 1000 Bq/m3

reduction : • ventilation 10x per day

• top-circulation of waterpool (25 liter/min)

neutron :

α :

0.253 ± 0.001 ± 0.003 evt/kgd

5.72 ± 0.12 ± 0.29 evt/kgd

0 2000 4000 6000 8000 10000 12000 14000 16000−90

−80

−70

−60

−50

−40

−30

−20

−10

639.51 Hz

Frequency [Hz]

Pow

er

[dB

]

MCE-200microphone

PGA 2500adaptive amplifier

3 TB pcs (8 chl each), MatLab files of ~ 8

mineach SDD

noise: 2-3 mV ∆ττττ : 1.6 x10-2 ms ∆V: 0.3 mV ∆f: 0.01 Hz

Pressure transducer

Temperature control refrigerator PT-100

SDDs in 9ºC water pool

An acoustic system is used to

record the signals.

Monitored continuously (temperature, pressure and data signals)

� The combined results yield a contour minimum of σp = 4.2 ×10−3 pb at 35 GeV/c2 on the spin-dependent sector

of WIMP-proton interactions, the most restrictive to date from a direct search experiment and overlapping for

the first time results previously obtained only indirectly.

� In the spin-independent sector, a minimum of 3.6× 10−6 pb at 35 GeV/c2 is achieved, with the exclusion contour

challenging the recent low WIMP-mass region of current interest.

Various spin-dependent WIMP-proton exclusion contours for SIMPLE, together with the leading direct and indirect search results; shown are both previous and reanalyzed Stage I results, Stage II, and a merging of the two.

Various spin-independent contours for SIMPLE, together with those of the leading spin-independent search results; shown are both previous and reanalyzed Stage 1 results, Stage 2, and a merging of the two.

Histogram of calibration neutron and alpha amplitudes, each population consisting of several hundred events. The vertical dashed line indicates the neutron discrimination cut at A ≤100 mV, which includes all neutron calibration events.

Scatter plot of the amplitudes and frequency of the primary harmonic of each true nucleation event observed over the entire Stage 1 and 2 exposures.

Stage I: ( Oct 2009 – Feb 2010)

� 13.47 kgd ≤ 2.2 bar

� analyzed via Feldman-Cousins

⇒reconstruction of neutron shield (0.25 evt/kgd)

⇒improved radio-assays⇒new detector efficiency

⇒fine tuning analysis

Stage II: (May 2010 – July 2010)

� 6.71 kgd ≤ 2.2 bar

� zero undiscriminated events

Particle induced eventand its associated

Power Spectral Density

A Superheated Droplet Detector consists

of a dispersion of superheated liquid droplets homogeneously distributed

within a gel matrix, which may undergo a

transition to the gas phase upon energy deposition by incident radiation.

The ion recoil threshold curves similarly evolve to higher

temperatures and energies with

pressure increase, rendering the SDDs increasingly less sensitive to

the moderated neutrons.

Two nucleation criteria are thermodynamic;

so variation of either temperature or pressure modifies the recoil threshold energy and thus

SDD sensitivity.

Acoustic data is acquired in

sequential Matlab files of ~ 8

MB each at a constant rate for a period of 8 minutes, stored in

one TB pc per 8 detector

channels.

The instrumentation is

based on a PANASONIC (omni directional) high

quality electret microphone cartridge (MCE-200) with a

frequency range of 20 Hz –

16 kHz (3 dB), signal-to-noise ratio (SNR) of 58 dB

and a sensitivity of 7.9 mV/Pa at 1 kHz.

New detector capping

eliminates previous pressure

microleaks. Each cap contains feedthroughs for signal and

pressure monitoring, and a microphone encased in a latex

sheath.

SDDs pressurized separately

to 2.00±0.05 bar, and

operated in a 700 liter water pool maintained at a bath

temperature of 9.0±0.1 ºC.

Spin-dependent Spin-independent

0 500 1000 1500 2000 2500 3000 3500 4000−100

−90

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2.51 kHz

Frequency [Hz]

PS

D [

dB

]

Ventilation on

ventilation

0 500 1000 1500 2000 2500 3000 3500 4000−120

−100

−80

−60

−40

−20

0

0.00 mHz

719.85 Hz

Frequency [Hz]

PS

D [

dB

]

Knocking on cap

cap knocking

0 500 1000 1500 2000 2500 3000 3500 4000−120

−100

−80

−60

−40

−20

0Human noise in GESA

0.00 mHz

742.35 Hz

Frequency [Hz]

PS

D [

dB

]

human noise

0 500 1000 1500 2000 2500 3000 3500 4000−120

−100

−80

−60

−40

−20

0

0.00 mHz

Frequency [Hz]

PS

D [d

B]

Release SDD’s pressure

X: 1118Y: −72.71

pressure release

0 500 1000 1500 2000 2500 3000 3500 4000−120

−100

−80

−60

−40

−20

0

0.00 mHz

X: 1488Y: −51.17

Frequency [Hz]

PS

D [

dB

]

Water bubbles in the bath

water bubbles

0 500 1000 1500 2000 2500 3000 3500 4000−120

−100

−80

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−40

−20

0Fenwick − GESA door closed

0.00 mHz

Frequency [Hz]

PS

D [

dB

]

Fenwick motion

0 2000 4000 6000 8000 10000 12000 14000 16000−90

−80

−70

−60

−50

−40

−30

−20

−10

3.44 kHz

Frequency [Hz]

Po

we

r [d

B]

microleaks

0 500 1000 1500 2000−90

−80

−70

−60

−50

−40

−30

−20

−10

34.37 Hz

Frequency [Hz]

Po

we

r [d

B]

fracture

0 20 40 60 80 100−100

−95

−90

−85

−80

−75

−70

−65

−60

−55

−50

40.33 Hz

Frequency [Hz]

Po

we

r [d

B]

trapped N2 gas

Acoustic Discrimination(s)

Particle Induced Event

0 10 20 30 40 50 60 70

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

pre

ssure

(b

ars

)

time (days)

Pressure record of the 15 SDDs throughout the stage II

10 20 30 40 50 60 70

1,6

1,8

2,0

2,2

2,4

2,6

2,8

3,0

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

pre

ssure

(ba

rs)

time (days)

Pressure record of the 15 SDDs throughout the stage I

The event-by-event analysis permits isolation of the particle-induced nucleation events with an efficiency of better than 97% at 95% C.L.

450 500 550 600 650 700 750

10

100

1000

10000

Stage I

Stage I

Stage II Stage II

Stage I & II

alpha

neutron

am

plit

ud

e (

mV

)

frequency (Hz)