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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
−80
−70
−60
−50
−40
−30
−20
−10
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
−60
−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)
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