1. Mar. 21, 2005 J.I. CollarUoM HEP seminar Recirculating chiller (-10 degrees) Glass dewar Cameras Safety shield box Superheated CF 3 Br First prototypes: ~20 ml active volume Pressure: 0-150 psi Temp: -80 to + 40 degrees C Stereo photography of bubbles Three triggers: acoustic, pressure and video Acoustic sensor Quartz pressure vessel Glass dewar with heat-exchange fluid Piston Camera (1 of 2) Propylene glycol buffer liquid prevents evaporation of superheated liquid. 3-way valve

2. Mar. 21, 2005 J.I. CollarUoM HEP seminar

3. Mar. 21, 2005 J.I. CollarUoM HEP seminar A Typical Scattering Event with Am-Be Neutron Source (bubble expansion ~1 mm/ms) X CameraY Camera

4. Mar. 21, 2005 J.I. CollarUoM HEP seminar Can Bubble Chambers be made stable enough?  Old Bubble Chambers radiation-ready for only few ms at a time (coincident with beam spill)  Gas pockets in surface imperfections and motes can act as inhomogeneous nucleation centers.  A BC dedicated to WIMP searches must remain superheated indefinitely, except for radiation-induced events. Low degree of superheat helps, but is not enough.  Recent progress in neutralization of inhomogeneous nucleation sites (from work unrelated to bubble chambers!). E.g. use of liquid “lid”, outgassing in presence of buffer liquid, cleaning techniques and wetting improvement via vapor deposition. P. Reinke, Exp. Heat Transf. 10 (1997) 133 SEE ->

5. Mar. 21, 2005 J.I. CollarUoM HEP seminar Background Counting Rate at ~ 6 m.w.e. Bonner sphere(s) n flux measurement in LASR underground lab Mean survival time for superheated state varies due to periodic episodes of nucleation on chamber walls, but is usually ~ 10 minutes. Live time (due to long recompression cycle) is already 62%. Counting rate for “real events” is 4/hour (compatible with measured fast neutron flux in the lab). Intrinsic gamma rejection factor (from absence of excess nucleation rate in presence of Y-88  1.3E6  interactions / s) is  1E9 (  14 C not a concern even at the multiton level)

6. Mar. 21, 2005 J.I. CollarUoM HEP seminar Background Counting Rate at ~ 6 m.w.e. Mean survival time for superheated state varies due to periodic episodes of nucleation on chamber walls, but is usually ~ 10 minutes. Live time (due to long recompression cycle) is already 62%. Counting rate for “real events” is 4/hour (compatible with measured fast neutron flux in the lab). Intrinsic gamma rejection factor (from absence of excess nucleation rate in presence of Y-88  1.3E6  interactions / s) is  1E9 (  14 C not a concern even at the multiton level)

7. Mar. 21, 2005 J.I. CollarUoM HEP seminar Sensitivity to < 7 keV recoils demonstrated (while having no response to 3mCi  source). In agreement with models. Sensitivity to ~1 keV recoils in progress (Sb-124/Be source) Further studies in progress (efficiency, sharpness of threshold, disentanglement of I and F response). Detector is insensitive to gammas (see previous transparency) yet fully responsive to low-E recoils Lines show Seitz model prediction for top boundary of data point distribution (onset of sensitivity during decompression) CF 3 Br data (CF 3 I in progress and looking good) Well-defined low energy threshold

8. Mar. 21, 2005 J.I. CollarUoM HEP seminar Fancy: Position Reconstruction Bubble positions can be reconstructed in 3 dimensions by scanning images taken by two cameras offset by 90 degrees. Position resolution is currently 530 microns r.m.s. (approximately 1/4 bubble diameter) Uniform spatial distribution of background events, consistent with background neutrons. 163 background events (1.5 live days) intermediate step in automatic inspection algorithm using NI Vision Development Module (2 kg chamber)

9. Mar. 21, 2005 J.I. CollarUoM HEP seminar Neutron Background Rejection Potential Multiple simultaneous bubbles are present in ~4% of events in our “background” data set. Neutrons can do this, WIMPs cannot. The response to neutrons and WIMPs interacting mostly via SI is very different for refrigerants containing F only (C 3 F 8 ) and F+I (CF 3 I); more favorable situation than Ge/Si to verify a WIMP signal (generally speaking)

10. Mar. 21, 2005 J.I. CollarUoM HEP seminar Meet COUPP (2 kg CF 3 I target, installation at 300 m.w.e. (FNAL) Jan. 2005) Central design issue is how to avoid metal contact with superheated liquid. Bellows mechanism compensates pressure inside and outside of inner vessel in present design

11. Mar. 21, 2005 J.I. CollarUoM HEP seminar Central design issue is how to avoid metal contact with superheated liquid. Bellows mechanism compensates pressure inside and outside of inner vessel in present design Meet COUPP (2 kg CF 3 I target, installation at 300 m.w.e. (FNAL) Jan. 2005)

12. Mar. 21, 2005 J.I. CollarUoM HEP seminar Central design issue is how to avoid metal contact with superheated liquid. Bellows mechanism compensates pressure inside and outside of inner vessel in present design Meet COUPP (2 kg CF 3 I target, installation at 300 m.w.e. (FNAL) Jan. 2005) triple n-induced nucleations in 2 kg CF 3 I movie available from http://cfcp.uchicago.edu/~collar/triple_bubble(0.4s).mov

13. Mar. 21, 2005 J.I. CollarUoM HEP seminar

14. Mar. 21, 2005 J.I. CollarUoM HEP seminar Can be made to operate like a “regular” BC muon tracks visible at high degrees of superheat

15. Mar. 21, 2005 J.I. CollarUoM HEP seminar COUPP ( Chicagoland Observatory for Underground Particle Physics ) Where we might be by end of 2005 with 2 kg. Take these with a grain of salt) are these projections overly cautious? Better safe than sorry (e.g., alpha backgrounds from Rn emanation from steel - however only few  Bq/m 2 expected if some precautions taken - The inner chamber is a closed system to Rn penetration) ~latest CDMS

16. Mar. 21, 2005 J.I. CollarUoM HEP seminar are these projections overly cautious? Better safe than sorry (e.g., alpha backgrounds from Rn emanation from steel - however only few  Bq/m 2 expected if some precautions taken - The inner chamber is a closed system to Rn penetration) ~latest CDMS COUPP ( Chicagoland Observatory for Underground Particle Physics ) Where we might be by end of 2005 with 2 kg. Take these with a grain of salt)