PICASSO

STATUS
PROGRESS
OUTLOOK

SNOLAB Opening Workshop, May 16, 2012, V. Zacek Université de Montréal

Université de Montréal Indiana University Czech Technical Saha Institute for
Queen’s University South Bend University In Prague Nuclear Physics, Kolkata
University of Alberta
Laurentian University
SNOLab, Montréal, June 2011
Bubble Technology I.
 Active Target: C4F10
2
 MMA 
WIMP interaction with matter:  A  4G F 
2
 CA

 M  MA 
Enhancement factor

Spin independent interaction (CA  A2 )‫‏‬

Depending on the type of target nucleus
and WIMP composition
Spin dependent interaction

Isotope Spin Unpaired 2

CA = (8/)(ap<Sp>+ an<Sn>)2(J+1)/J
7Li 3/2 p 0.11

19F 1/2
2
p 0.863
23Na 3/2 p 0.011
29Si 1/2 n 0.084
19FMost favorable nucleus for SD
73Ge 9/2 n 0.0026
interactions
127I 5/2 p 0.0026
131Xe 3/2 n 0.0147
XENON 1t DEAP3600 XENON100
CDMS

most of the activity in SI sector !

 • no channel favored
• largely uncorrelated
• desctructive interference
possible in SI sector

XENON 1t DEAP3600 XENON100

CDMS

most of the activity in SI sector !

 PICASSO ‘12

PICASSO 0.5 t • no channel favored

• largely uncorrelated
• desctructive interference
possible in SI sector
XENON100
XENON 1t DEAP3600 CDMS

Need to explore both sectors!

 Particle Detection with Superheated Liquids

Liquid
A bubble forms if : Rmin
Pvap
p = Superheat • a particle creates a heat spike
Pext
Vapor • with enough energy Emin Pvap
Tb Top • deposited within Rmin Pext
« proto-bubble »

dE
Edep   Rmin  Emin
dx

2 (T ) 16  3 (T )
Rmin  Emin 
P(T ) 3  (P(T )) 2

 (T) = Surface tension

 = critical length factor
 = energy convers. efficiency

F. Seitz, Phys. Fluids I (1) (1958) 2

  150 m droplets of C4F10 dispersed in polymerised gel *
 Droplets superheated at ambient T & P (Tb= - 1.7 oC )
 Radiation triggers phase transition
 Events recorded by piezo-electric transducers
 Operating temperature determines energy threshold

Main attractive features:

 low threshold 45OC  Eth = 2 keV
 inexpensive! 0.19 k$/kg (C4F10)
 insensitive to  - background

* Inspired by personal neutron dosimeters

@ Bubble Technology Industries, ON
Ho to make PICASSO Detectors

R&D in collaboration with BTI, Chalk River

- Start with monomer matrix solution

- add active gas in liquid form

- magnetic stirring to disperse droplets

- polymerize matrix

- all done in clean room to avoid U/Th, Rn

Droplet Ø fct(, v,t)‫‏‬

Stirring time 200mØ

Purification & Fabrication (J. Farine, X. Dai, M.C. Piro)
 Progress in Detector Production
 Swiched to “saltless” receipt (polyethylene glycol) with substantially reduced background
 Replace gradually all detectors with higher background

a) b)

c) d)
a) 0.2 m filter added for monomer

b) top polymer blanc added

c) outgassing time increased

d) x 4 purification of TEMED
 Calibration of Energy Threshold

Mono-energetic n-test beam

@ Montréal tandem accelerator

C4F10
 threshold function:
P( E, Eth )  1  exp a(1  E )
Eth
 a varies with Eth
Just about to
fill the gap!
 2.5 < a < 10

 19F triggers first EFRec = 0.18 x En

Erec = 0.86 keV

210Po - recoils
Detector Response
from 226Ra ‫‏‬spike
E/E =15% FWHM

Precise description of threshold resolution

 Detector Response

- dE/dxBragg
241Am spike

E/E = 20%

Full efficiency above threshold!

 Detector Response

Neutrons from
AcBe source
(data + MC)‫‏‬

Good agreement with MC- simulations

 Detector Response
Recoil nuclei from
50 GeV /c2 WIMP

 - emitters are dominant background source!

 Detector Response

MIP's & 's from 22Na, 60Co..

Gamma & MIP rejection better 108 above Erec= 10 keV!

 Extraction of WIMP Signals / Limits

Flat  - backg.
19F

p = 1pb
MW = 10, 30 100 GeV /c2

Fit WIMP response & flat  background  determine - limits

…or‫‏‬better:‫‏‬no‫‏‬ - backg !!!

2.6 kg PHASE

PICASSO

• Since fall ‘08 : 32 detectors  2.6 kg of C4F10

• 288 acoustic R/O channels
• 3D event localization (start to be used)
• Control via Internet by shift operator
 2.6 kg  Move to Ladder Lab

• Move Sept. 2010 - Dec. 2010

• Improved UPW shielding (30 cm  50 cm)
• Easy access & one extra > m3 for det. R&D
• Improved n-shield  amb. n-flux / factor 400 !
 PICASSO

Temperature & Pressure Control

System (TPCS)‫‏‬
- 8 TPCS units
- temp. precision: ± 0.10C
- 4 detectors /TPCS
- 40 h data taking
- 15 h re-compression
 Treatment of Signal Waveforms

• Filter events (high pass >20 kHz)‫‏‬

• Integrate amplitude
•  Variable  to signal energy “evar”

Background n-signal
evar

• Time frequency analysis

• Fourier transform variable
• (10-30 kHz) / (45-60 kHz) = “fvar”‫‏‬
Raw >18 KHz
 evar vs. fvar  Background Discrimination!

Particle induced

Acoustic noise
Mine blasts

Electronic noise Fractures

evar
Black: n- calibration run
Red: background run
 Background Discrimination in keV Range

• > 40o C ( < 5 keV) : new class of backg. with WIMP-like acoustic energy
• Risetime and signal energy content <25 s : “rvar”‫‏‬
(Risetime prameter)

n-recoils

500C (0.8 keV)

(Signal energy)
 2012 Physics Analysis
« Golden » detectors

• Analysis period Nov. 2008 –Sept. 2010

• 32 detectors operated
Tank repairs
• 10 golden detectors used (+ > 10 soon) Move
• “Golden exposure”: 114 kgd Calibrations

• best detector 20 cts/kg/d

• fit WIMPS & flat -response

10 « Golden »detectors
6 Calibrations 11/ ’08 -08/11

• n-calibrations about every 3 months

• Determination of cuts
AmBe-
source • Verification of active mass
•  long term detector stability ok!
 2012 Physics Analysis

10 « golden detectors » combined with 140 kgd exposure

 19F & low threshold (1.7 keV) improved sensitivity for MW < 15 GeV/c2 !

Mw = 7 Ge/c2
SI = 1.2 x 10-4 pb
SD = 5.0 x 10-3 pb
 2012 Physics Results (March 27, Phys. Lett. B)
« Constraints On Low Mass WIMP interactions on 19F from PICASSO » arxiv: 1202.1260

SD- sector:

Effect of theshold
uncertainty

• Results factor 5 better than 2009

• DAMA « channeling » now ruled out
 2012 Physics Results (March 27, Phys. Lett. B)
« Constraints On Low Mass WIMP interactions on 19F from PICASSO » arxiv: 1202.1260

SD- sector:

Effect of theshold
uncertainty

COUPP ‘12
SIMPLE’12

• Results factor 5 better than 2009

• DAMA « channeling » now ruled out
 2012 Physics Results (March 27, Phys. Lett. B)
« Constraints On Low Mass WIMP interactions on 19F from PICASSO » arxiv: 1202.1260

SI-sector:

Effect of theshold
uncertainty

• Most of DAMA /LIBRA excluded

• Approaching CoGeNT & CRESST
• Interesting: Isospin viol. DM !
R&D‫‏‬ISSUES….
 Dynamics of Bubble Growth & Signal Formation
NJP 13(2011)103017
Bubble growth in superheated liquids occurs in five stages*:

Stage 1: t = 10-16 s:
Interaction of charged particle;
energy transfer to electrons

Stage 2: t = 10-13 s:
Transfer of energy from e- to Acoustic emission: but very small
kin. energy of atoms; heating (UHE -physics: Antares, IceCube)
around track, electroacoustical
effect

Stage 3: t = 10-10 – 10-9 s:

Emergence of gas phase in Acoustic emission: but very small
nucleation region 10-9 - 10-8 m

Stage 4: t > 10-9 s:

Growth of proto-bubble once Acoustic emission: large ( x 104 S2  S)
R > rc = 2 /(P - P) studied for BC-operation **

Stage 5: t > 10-3 s:

Oscillations of free bubble * Y.N.
Martynyuk and N.S. Smirnowa; Sov. Phys.Acoust. 37 (1991) 376
** D.A. Glaser and D. Rahm, Phys.Rev. 97, 474 (1955)
 Acoustic Signal Formation
 V2  : density of liquid
Intensity of emitted sound (*): I c : speed of sound
4 c
V : volume of expanding region
For Rayleigh growth: 16   16 2
IR  t
c

 Intensity increases with temperature!

T1
 16
For thermal growth: IT  2 Time (s)
T1 < T 2 !
4c t
T2
 Intensity decreases with temperature!
Time (s)

Rayleigh growth !!

Temperature oC * Landau & Lifschitz

Discrimination of Nuclear Recoils from Alpha Particles
: - events are « louder » than recoil induced events…

I : sound intensity
 V 2
: density of liquid
I c : speed of sound
4 c V : volume of expanding region
n-calibration

Recoil - particle - background

Expanding vapour bubbles

NJP 10(2008)103017 arXiv: 0807.1536

Signals carry information about very

first instants of bubble formation
NJP 13(2011)103017 arXiv: 1011.4553
 Recent Progresss in Alpha - Recoil Discrimination
 Required amplitude resolution < 30% FWHM @ 450 (2 keV)
 3D localisation  fid.vol.; solid angle correction
 1st 25 s of signal  precise gain control
 Discrimination: 20 kHz  140 kHz for 250  450
 Increase sampling frequency 400kHz  800 kHz

450 (2 keV)
800 KHz
Neutrons -decays
Recoils

-decays

Amplitude (Amplitude)2

99.34% -rejection @ 80% WIMP acceptance Similar to COUPP

 Event Localization
Important for alpha-discrimination
Determination of fudcial volume
Find t0 from wave form
t0
Reconstruction efficiency > 80%

 thi  th0   ti  t0  

2
8
    
2 thi : Calculated time from the fitted point to the ith piezo.

i
ti : Measured time of the beginning of the event on the ith channel.
i 0  

3D- distribution of - events (spiked detector)

A Curiosity: The Amazing Steepness of the -Threshold!

Localization of events
in an alpha-spiked
detector...

Hydrostatic pressure
19.50C
200C change over detector:
+15cm
p/p =4.7%

E  2 keV @ E= 140 keV !?

20.50C 21 0C
-15cm
E/E  1.6% ?
 Limit of Stability: Homogeneous Nucleation

• Limit of stability when thermal fluctuations deposit Emin within Rmin

• Theory predicts instability of liquid phase at app. 90% Tc

1 3 Ec (T )
P( s cm )  exp(  )
kT
 response C4F10
(- electrons)‫‏‬

Homogeneous nucleation

Future applications:
1 keV 0.1 keV 15 eV coherent neutrino scattering?
 Next Generation Detectors R&D

Aim for substantial increase of active mass & sensitivity

For example:

• Single Droplet Technique

• Gel matrix very clean!
• surface activity: 10-6 /d/cm2
• Tests with 20 g modules ok!
• 32 x 2.5 L modules  80 kg 19F
• 100% -discr. 100 d  SD < 10-5 pb
• COUPP  replace PZT transducers!
OUTLOOK
 Outlook & Projected Sensitivity

Standard analysis 32 detectors  slow progress

 Outlook & Projected Sensitivity

-recoil discrimination applied to acquired & new data

Outlook & Projected Sensitivity

R&D towards next gen. experiment

Conclusions
• In cruising regime with 32 detectors in improved shielding

• Calibrated detector response down to 1 keV

• Increased sensitivity to low mass WIMPs due to low threshold

• Published results with 114 kgd exposure

• SI sensitivity close to CoGeNT region

• Progress in -recoil discrimination  30% FWHM

• R&D for next generation  > 80 kg 19F