1. PICASSO, COUPP and PICO - Search for Dark Matter with Bubble Chambers
Robert Filgas
(on behalf of the PICO Collaboration)
Institute of Experimental and Applied Physics (IEAP),
Czech Technical University in Prague
International Conference on New Frontiers in Physics, July 31 – August , Kolymvari, Greece

2. C. Amole, M. Besnier, G. Caria, G. Giroux, A. Kamaha, A. Noble
I. Lawson, E. Vázquez Jáuregui
M. Ardid, M. Bou-Cabo, I. Felis
D.M. Asner, J. Hall
D. Baxter, C.E. Dahl, M. Jin,
J. Zhang
S. Gagnebin, C. Krauss, D. Marlisov, P. Mitra
P. Bhattacharjee.
M. Das, S. Seth
R. Filgas, S. Pospisil,
I. Stekl
E. Behnke, H. Borsodi, O. Harris, I. Levine, E. Mann, J. Wells
S.J. Brice, D. Broemmelsiek,
P.S. Cooper, M. Crisler, W.H. Lippincott, M.K. Ruschman, A. Sonnenschein
K. Clark
F. Debris, M. Fines-Neuschild, C.M. Jackson, M. Lafrenière, M. Laurin, L. Lessard,
J.-P. Martin, M.-C. Piro, A. Plante, O. Scallon, N. Starinski, V. Zacek
J.I. Collar,
R. Neilson,
A.E. Robinson
N. Dhungana, J. Farine, R. Podviyanuk, U. Wichoski
D. Maurya, S. Priya

3. Superheated liquids
Detection of elastic scattering of WIMPs on target nucleus
Critical energy deposition within small volume induces bubble nucleation
Advantages of bubble chambers:
Insensitive to electron and gamma background
Easily scalable to large sizes at reasonable cost
Interchangeable liquids, excellent SD sensitivity
Drawbacks:
Threshold detectors – lack of direct energy measurement
Sensitive to alpha radiation

4. PICASSO Acrylic container ø14 × 40 cm 4.5 liters 200 µm droplets
200 µm droplets of C4F10 dispersed in polymerised gel
Droplets superheated at ambient temperature and pressure
Operating temperature determines energy threshold (20 °C – 50 °C)
Events recorded by 9 piezo-electric transducers
Bubbles recompressed back to droplets by pressurization (6 bar)
Low threshold Eth (45 °C) = 2 keV
4.5 liters
200 µm droplets
of C4F10
Stainless steel frame
9 piezo-sensors

5. PICASSO 32 modules housed in 8 temperature and pressure control units
Radiopure acrylic/steel vessels constructed at IEAP
Total of 2.6 kg of active mass
40-50 hour runs of data-taking followed by 11 hours of recompression
Located at SNOlab, Canada
2 km of rock and 50 cm of UPW shield against neutrons

6. PICASSO response
At operating temperatures (< 50 °C) insensitive to electrons and gamma radiation
Alpha particles are dominant background
The acoustic signal from bubble formation is recorded and used for analysis
Neutrons
Alphas
50 GeV WIMPs
Gammas, electrons

7. Neutron – alpha discrimination
PICASSO pioneered the use of acoustic sensors to reject alphas
Events initiated by alpha-decays include both long alpha tracks (~10 microns) and recoiling daughter nuclei - acoustic pulses are several times louder than those emitted by single nuclear recoil events
> 99.4% alpha discrimination!
Confirmed by COUPP detector

8. COUPP
Superheated fluid contained within single pressure-controlled vessel
Uses CF3I – probes both SD and SI sectors
Cameras – excellent position reconstruction - able to identify multiple scattering events characteristic of neutrons
Located at SNOLAB
COUPP 4 kg -> PICO 2L
2013 – COUPP 60 kg

9. COUPP Energy threshold determined by pressure rather than temperature
Rapid repressurization after each nucleation
The detector consists of a quartz (synthetic fused silica) bell jar, a stainless steel diaphragm/bellows, seals, and highly purified fluids.

10. PICASSO + COUPP = PICO
PICO combines operational principle of COUPP bubble chamber and fluid handling expertise from PICASSO
Uses C3F8
better SD sensitivity due to higher fluorine concentration
higher efficiency for detecting fluorine recoils
lower threshold without gamma and electron background

11. PICO
PICO 2L (former COUPP 4kg) running at SNOLAB Collaboration goal is to build PICO 250L - C3F8 or CF3I target fluid
Background needs to be less than 1 event/year
IEAP part of the material assay group to test U/Th contamination of all materials (dominant neutron source) and to devise Radon-cleaning facility (dominant alpha source)

12. PICO 2L VIDEO

13. Results Spin dependent Spin independent Results to be published soon:
PICASSO run ended Dec 2013
COUPP 60 kg 1st run ended May 2014
PICO 2L 1st run ended May 2014

14. Extra Slides

15. SD and SI
Reaction classified as spin-dependent (SD), when the sign of the scattering amplitude depends on the relative orientation of particle spin