Transcript Can We Detect Dark Matter With Bubble Chambers?

COUPP:
Chicagoland Observatory for Underground Particle Physics
(FNAL Test Beam Program T-945)
Development of a bubble chamber technique for
dark matter detection.
J. Collar, K. Crum, D. Nakazawa, B. Odom, J. Rasmussen, M. Szydagis
Kavli Institute for Cosmological Physics
The University of Chicago
P. Cooper, M. Crisler, H. Nguyen, E. Ramberg,, A. Sonnenschein, R. Tesarek
Fermi National Accelerator Laboratory
* Engineering and technical support from PPD Technical Centers and
Mechanical Department: K. Krempetz, C.M. Lei, R. Schmitt, T. Nebel, J.
Zimmerman, C. Lindenmeyer, J. Korienek, D. Butler, J. Krider, D. Erikson, B.
Pritchard
Basic idea:
• Low energy nuclear recoils from
WIMP nucleus scattering can
produce a bubble in a superheated
liquid.
• Many liquids could be used, so
wide choice of potential target
nuclei to check signal dependence
on spin and A2.
• Under correctly chosen pressure
and temperature conditions,
background gammas and betas can
not produce bubbles.
dE/dX Discrimination in a Small Propane Chamber
Waters, Petroff, and Koski, IEEE Trans. Nuc. Sci. 16(1) 398-401 (1969)
Plot of event rate vs. “superheat pressure” (= vapor pressure - operating pressure)
electrons
protons
a plateau
(psi)
Bubble Nucleation in Cracks
• Trapped gas volumes in surface
imperfections are now known to be the
primary source of nucleation.
nucleation sites
Liquid
0.1 mm
• Most (all?) construction materials have
rough surfaces at scales below 1 mm, but
some materials much better than others.
• Historically, problem was overcome for
high energy physics experiments by rapid
cycling of chamber in sync with a pulsed
beam. Bubbling at walls was tolerated
because of finite speed of bubble growth.
• A few small “clean chambers” (~10 ml)
were built in the 50’s and 60’s, with sensitive
times ~1 minute.
Solid
Ways to preserve superheated state:
• Elimination of porous surfaces in
contact with superheated liquid.
• Precision cleaning to eliminate
particulates.
• Vacuum degassing.
•… a few other tricks borrowed from
chemical engineers
Design Concept for Large Chambers
• Central design issue is how to avoid metal contact with superheated liquid.
• Fabrication of large quartz or glass pressure vessels is not practical, but industrial
capability exists for thin-walled vessels up to ~ 1 m3 in volume.
PISTON
Pressure balancing bellows
Buffer fluid
Steel pressure vessel
SUPERHEATED
LIQUID
Thin- walled quartz bell jar
HEAT EXCHANGE FLUI D
VIEWPORT
VIEWPORT
VIEWPORT
Target Liquids Tested in Small Chamber
Mass Fractions
CF3Br
8% C (Z=6)
38% F (Z=9)
54% Br (Z=35)
CF3I
6% C
29% F
65% I (Z=53)
Xe
Density
Boiling
Point
@ 1 atm
Comments
1.5 g/cc -58 C
Good for spin-dependent
and spin- independent
couplings.
2.1 g/cc -23 C
Spin-dependent and
spin-independent
Non- ozone depleting
100% Xe (Z=54) 3.0 g/cc -108 C
Possible use in hybrid
scintillating bubble
chamber.
at Fermilab
test site
~300 m.w.e.
Installation of 1 Liter Chamber At Fermilab NuMi Tunnel
• Prototypes design features required for chambers up to 1000 liters
160 msec of Video Buffer (20 msec/frame)
Muon Track @ 160 psi Vapor Pressure
Data From December 1st ‘05 to March 27 ‘06
115 days in run
45k expansions
82 seconds mean
expansion time
42.8 live days
= 37% of real time
15k bubbles counted
145 GB in Enstore
Adjustment of trigger
thresholds and compression
schedule
Video trigger
turned on
Spatial Distribution of Single Bubbles, First 42 Days
• Duty cycle of chamber allowed 33% live time (14 live days out of 42 real days)
Radial Distribution
Multiple Bubble Analysis
• Statistics of multiple bubble events suggest ~10% of bulk singles (2.8 /day) are due to neutrons.
Counts
• These should be coincident with cosmic rays-- we’ll see when Muon veto is installed.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
PRELIMINARY
Number of Bubbles
Neutron Shielding and Muon Veto
• Detector is surrounded by 30 cm of polyethylene neutron moderator.
Very effective for low energy neutrons coming from (a,n) radioactivity in rock.
Simulations show reduction in rate to < 1/month.
• Active muon veto: 150 plastic scintillator counters from KTEV.
• Goal: < 0.1 m-induced neutrons/ day in 1 liter chamber, requires >98% efficiency.
To be installed underground
summer ‘06.
What Will Our Dominant Background Be?
• Alpha decay produces monoenergetic, low energy nuclear recoils.
For example, consider 210Po->206Pb:
ER= 101 keV
Ea = 5.407 MeV
a
206Pb
• The recoiling nucleus will nucleate a bubble in any chamber that is sensitive to
the lower energy (~10 keV) recoils expected from WIMP scattering.
• The 238U and 232Th decay series include many alpha emitters, including radon
(222Rn) and its daughters.
• Radon is highly soluble in bubble chamber liquids.
• Solar neutrino experiments (Borexino, Kamland, SNO) have demonstrated
feasibility of reduction to ~1 event per day in scintillator and water-about 2 orders of magnitude lower rates than seen in current-generation dark
matter experiments.
Potential Sensitivity Of 1-Liter Chamber at Fermilab Site
Goal for this phase: reduce background to <1 event per liter per day
Spin-independent
Spin-dependent
To Prove Viability of Bubble Chamber Technique,
have demonstrated:
We
 We have a bubble chamber which can remain expanded
and sensitive for a significant fraction of the time.
 Efficient detection of single bubble events from low
energy nuclear recoils.
Work at U. Chicago using radioactive sources.
• Low backgrounds from radioactivity & cosmic rays.
Installing new radio-pure inner vessel assembly
Muon veto system (KTEV plastic scintillator)
Goal: < 1 background bubble per day
Underground Lab Requirements- A Guess
1)
Which Henderson DUSEL campus will be most suitable for your experiment?
We now have prototyping space at 100 m.w.e., appropriate for 2 kg chamber.
We are looking for a deeper site in 2007 (Soudan?). ~100 kg chambers.
Need to study potential sensitivity at central campus for 2010 (tons)
Deep campus preferred, but not available until 2014.
2) What is the required space?
More than most other dark matter experiments, because cost per unit target
mass likely 1-2 orders of magnitude lower.
E.g. 100 x 1 ton modules = ~ 1000 m2 of floor space
(similar to 1 ton TPC in infrastructure matrix)
3)
What are the special needs: clean room, cryogenic facility, etc.?
Clean rooms? Water shielding? Emergency vent path for gas?