IDM2004, September 6-10, 2004 J.I. Collar Some Desirable Features in a Next Generation WIMP Detector...

J.I. Collar IDM2004, September 6-10, 2004

Some Desirable Features in a Next Generation WIMP Detector

Background rejection as close to 100% as possible

Large, safe and inexpensive target mass: room temperature operation can go a long way to help here.

Simple target replacement to help ascertain a WIMP signal (e.g. neutron background and WIMP signal do not scale the same way in different target materials).

Simultaneous sensitivity to spin-dependent and -independent neutralino cross sections, maximal on both (fluorine then a must for the first, a heavy nucleus for 2nd).

Lowest possible energy threshold to maximize acceptance of signal.

Excellent neutron background rejection (the ultimate nemesis).

Is there such a thing?

An old precept:Attack on both fronts


Enter superheated liquids:

Two ongoing experiments (SIMPLE, PICASSO)

exploit Superheated Droplet technique (SDD)

Sexy heavy-liquid targets such as CF3Br and CF3I

hard to impossible to introduce in SDDs try bulk (=Bubble Chambers, a much trickier endeavor). These liquids are safe and non-toxic (used in fire extinguishers).

Total insensitivity to MIPs, yet sensitive to low-E

nuclear recoils (tunable dE/dx and E thresholds)

~$40/kg, room T… first one tonne WIMP detector?

QuickTime™ and aMPEG-4 Video decompressor

are needed to see this picture.


Recirculating chiller (-10 degrees)

Glass dewar

Cameras

Safety shield box

Superheated CF3Br

First prototypes:~20 ml active volumePressure: 0-150 psiTemp: -80 to + 40 degrees C

Stereo photography of bubbles

Three triggers: acoustic, pressure and video

Acoustic sensor

pressuresensor

filter

gas

glycol

gas

Exhaust to Room

Compressed Air @ 140 psi

air

liquid

glycol

Quartz pressure vessel

Glass dewar with heat-exchange fluid

Piston

Camera(1 of 2)

Propylene glycol bufferliquid prevents evaporation of superheated liquid.

3-way valve


A Typical Scattering Event with Am-Be Neutron Source (bubble expansion ~1 mm/ms)

X Camera Y Camera


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


Background Counting Rate at ~ 6 m.w.e.• Mean survival time for superheated state is ~ 10 m for 20 c.c. CF3Br. • 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).• Gamma rejection factor (from absence of excess nucleation rate in presence of 3 mCi 88Y 1.3E6 interactions / s) is 1E9 (14C not a concern even at the multiton level)

Ź Ź Ź Ź

Ź Ź Neutron Flux (n/cm2/s)

Ź EnergyGroundLevel

6mweLab

50 mwePit

Ź <.5 eV 0.00394 4.65E-04 4.37E-05Ź .5eV-100keV 0.00384 5.61E-04 1.60E-04Ź 100keV-10MeV 0.0015 6.87E-04 1.48E-04Ź 10 MeV-50MeV 0.00454 0.00E+00 0.00E+00Ź Total 0.01382 1.71E-03 3.52E-04

Bonner sphere(s) n flux measurement in LASR underground lab


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 ( 14C not a concern even at the multiton level)

Ź Ź Ź Ź

Ź Ź Neutron Flux (n/cm2/s)

Ź EnergyGroundLevel

6mweLab

50 mwePit

Ź <.5 eV 0.00394 4.65E-04 4.37E-05Ź .5eV-100keV 0.00384 5.61E-04 1.60E-04Ź 100keV-10MeV 0.0015 6.87E-04 1.48E-04Ź 10 MeV-50MeV 0.00454 0.00E+00 0.00E+00Ź Total 0.01382 1.71E-03 3.52E-04


Well-defined low energy threshold:

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 planned (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 modelprediction for top boundary of data point distribution(onset of sensitivity during decompression)

CF3Br data(CF3I in progress and looking good)


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 automaticinspection algorithm using NI Vision Development Module (2 kg chamber)


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 refrigerantscontaining F only (C3F8) and F+I (CF3I); much more favorable situation than Ge/Si to verify a WIMP signal.


Neutron Background Rejection Potential (bis)

•Larger chambers are “self-shielding” (innermost fiducial volume will have good rejection of energetic neutrons able to penetrate moderator)

This might reduce sensitivity to dreaded high energy “punch through” neutrons down to the ~1 count/tonne/month range allowing for exhaustive exploration of supersymmetric WIMP models.

MCNP-POLIMI simulations


Meet COUPP (2 kg CF3I target, installation at 300 m.w.e. (FNAL) Oct. 2004)

• Central design issue is how to avoid metal contact with superheated liquid.

• Fabrication of large quartz or glass pressure vessels is not practical, butindustrial capability exists for thin-walled vessels up to ~ 1 m3 in volume.





recent NIST study points at excellent CF3I stability during >5 y storage (at up to 100°C !)as long as certain metallic alloys are avoided





QuickTime™ and aMPEG-4 Video decompressor

are needed to see this picture.


COUPP (Chicago Observatory for Underground Particle Physics)Where we might be by end of 2004 (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/m2 expected if some precautions taken - The inner chamber is a closed system to Rn penetration)

~latest CDMS


The near future

2 kg CF3I chamber stable and seemingly dominated

by environmental neutrons at 6 m.w.e. Still learning some of the interesting peculiarities of this new system. Systematic study of effect of recompression time, decompression speed, “direction” of heating, etc.

Complete studies of CF3I and C3F8 response to neutron recoils (down to ~1 keV recoil energy with 124Sb/Be source)

Idem for sharpness of threshold, efficiency and

separation of I vs F recoils.

Installation in FNAL Minos-near gallery (~300 m.w.e.) during Oct. 2004. Replacement of inner chamber elements with attention paid to Rn release (electron welding of bellows, Ni o-rings). Eventual move to Soudan (~1,800 m.w.e.)

Measure concentration of U,Th in target liquids. At what level will purification be necessary?

A bit further in the horizon

Ongoing conceptual design for larger chambers (collaboration with FNAL)

Recently funded by DOE/NNSA to explore applications to low-level neutron detection (Natl. Security, collab. with PNNL)


The near future

1

2

3

4

5

6

7

8

-10 0 10 20 30 40 50 60 70

CF3I ("Harper" factor not optimized)

pres

sure

at n

ucle

atio

n (a

tm)

operating temperature (°C)

Seitz F recoilsSeitz C recoilsSeitz I recoilsReduced superheat

Am/Be

88Y/Be

88Y












The near future












The near future











adapted from C. Chung et al., NIM A301(1991)328


UoC Low-Background Detector Group

KICP Postdoctoral Fellows:Andrew Sonnenschein (bubble chambers for WIMP detection)

Graduate Students:Phil Barbeau (coherent neutrino detection)Dante Nakazawa (bubble chambers for WIMP detection)Joaquin Vieira (CAST)

Undergraduate Students:Taylor Aune (coherent neutrino detection)Jason Hall (bubble chambers for WIMP detection)David Miller (CAST)Kevin O`Sullivan (bubble chambers for WIMP detection)Aza Raskin (bubble chambers for WIMP detection)

Waterboy:Juan Collar

FNAL GroupM. CrislerD. Holmgren R.PlunkettE.Ramberg

PNNL Group (Natl. Security applications)L.BondJ. ElyM. FlakeD. Jordan

IDM2004, September 6-10, 2004 J.I. Collar Some Desirable Features in a Next Generation WIMP Detector...

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Transcript of IDM2004, September 6-10, 2004 J.I. Collar Some Desirable Features in a Next Generation WIMP Detector...