A Report on the R&D of the
e-Bubble Collaboration
Colin Beal
Virginia Polytechnic Institute and State University
R.M. Wilson
Saint Louis University
Advisors
Dr. Jeremy Dodd, Dr. Raphael Galea & Dr. Bill Willis
Nevis Labs, Columbia University
REU 2005
Outline







Some Neutrino Physics
Some Holes in Neutrino Physics
Goals of the e-Bubble Detector
Physics of the e-Bubble Detector
Test Chamber
Experimental Results
Simulation Results

n  pe ?
Wolfgang Pauli, 1930
•Neutral charge
•Spin ½
•Massless
Cowan & Reines, 1956

Enrico Fermi
“neutrino”
-First experimental
evidence of neutrino
(Italian for “little
neutral one”)
Using reactor source,
  p ne
Neutrinos
Weak interactors by
the exchange of the
W and Z bosons
http://www-numi.fnal.gov/public/images/standardmodel.gif
e-
p
W
t
e
n
e
e-
W
e-
& Interactions with Matter
n  e  e   p
p  e  e   n
e
e
e-
Neutrinos
e   e  e   e
Z
e-
e
n,p
e
Z
n,p
 e  n, p  e  n, p
e
x
Neutrinos
… more interactions
t
e
e
e
eW
Z
e-
e
e-
e+
e
e-
e
e
W
W
e-
e+
e-
e
x
Neutrinos
& the Sun
Neutrinos
The Solar flux
E  10MeV
 pp ~ 500keV
 pp flux  85%
Neutrinos
The First Solar Neutrino Detector
Homestake
•Built at BNL in 1965
•615 tons tetrachloroethylene
•Observed the following solar
neutrino reaction…
 e  Cl  Ar  e
37
37

•Saw deficit in solar neutrino flux…
http://www.its.caltech.edu/~sciwrite/journal03/A-L2/greissl.html
Neutrinos
The Solar Neutrino Problem
The Solar Standard Model (SSM) is tested…
Super-Kamiokande
•H2O Cherenkov Detector, 500 metric tons
•Minimum ~3 MeV neutrinos
•Detects Cherenkov light from scattered electrons
•Reported 1/3 expected solar neutrino flux
http://ale.physics.sunysb.edu/nngroup/superk/pic/sk-half-filled.jpg
The missing neutrinos can be compensated for if a model
incorporating new physics is taken into account…
Neutrinos
They Oscillate
Assuming that neutrinos do have some mass, and that their masses are a mixture of
the neutrino (say e and m) flavor eigenstates…
Then the probability that an e will be detected as an e a distance L (km) away from
its origin is given by…
constant
Energy of Neutrino (eV)
Mass difference
Neutrinos
The Solar Neutrino Solution
Sensitive to electron, muon and tau neutrinos…
SNO
•D2O Cherenkov Detector, 1000 metric tons
•Minimum ~3 MeV neutrinos
•Detects Cherenkov light from scattered electrons
•Reported expected solar neutrino flux
So what else is there to know?
http://www.pparc.ac.uk/Nw/Press/sudburysalt.asp
Neutrinos
There is so much more…
What can we learn from low-energy neutrino experiments? …
Most of the Suns power lies at energies well below the threshold of
current real-time neutrino detection experiments.
•Our models tell us that high energy neutrino oscillations (governed by the MSW
effect) behaves much differently than low energy neutrino oscillations.
•Is nuclear fusion the primary source of the Suns energy, or is there something
else at work?
•The neutrino magnetic moment m is much more accessible for measurement at
low energies.
e-Bubble
The Objective
To design, build and implement a real-time
low-energy neutrino detector* using a
cryogenic liquid detection medium.
*The detector will be a tracking detector, i.e. one which utilizes the
ionization track of electrons produced in a e-e scattering event to extract
information about the incident particle, in this case, a neutrino.
e-Bubble
Performance Goals
Due to the nature of low-energy neutrinos, we’ll need a
detector with the following features…
•Excellent spatial resolution (sub-mm)
•Excellent energy resolution
•Large volume or high event-rate
•Low background
e-Bubble
Tracking Detector
2-D Detection Plane
Drifting Ionized Electrons
Incident Neutrino
-e interaction
e-e ionizations
Neutrino-Electron Interaction
Origin of the Electron Track
Bahcall, John H., Rev. Mod. Phys., 59, 2, 1987.
Neutrino-Electron Interaction
 Cross-Sections
Magnetic Moment
m
Neutrino-Electron Interaction
 Cross-Sections
Weak Interactions
e-Bubble
Tracking Detector
e-Bubble
Information from Tracks
Length of Track
Energy of Neutrino
Total Ionized Charge
Origin of Neutrino
Shape of Track
e-Bubble
The Detector Medium
LNe
LHe
• T = 2K
• r = 0.125 g/cm3
• ~5 metric tons
• Long tracks (1-7 mm,
100-300 keV)
• Good pointing
capability
• T = 27K
• Minimum ionizing
(low dE/dx)
• Pure (long drifts, low
internal background)
e-Bubbles
• r = 1.24 g/cm3
• ~1 metric ton
• Short tracks ( 700
mm,  300 keV)
• Pointing only for
highest energy pp
•Self-shielding
•Solar pp flux 6.2E10 cm-2s-1
LNe
•Expect ~674 ton-1year-1
• T = 27K
• Minimum ionizing
(low dE/dx)
• Pure (long drifts, low
internal background)
e-Bubbles
• r = 1.24 g/cm3
• ~1 metric ton
• Short tracks ( 700
mm,  300 keV)
• Pointing only for
highest energy pp
•Self-shielding
e-Bubbles
… A Social Metaphor
A Red Sox fan enters Yankee
Stadium…
Go home
And the “Red Sox Fan”-Bubble
phenomenon may be observed…
r
e-Bubbles
In LNe (or LHe)
• Equilibrium state of free electrons in Low-Z
noble liquids (LHe, LNe)
• Due to Pauli repulsion between free electron
and noble atoms
• ~1-2 nm diameter
• Displaces ~50-100 atoms of liquid
e-Bubbles
In LNe (or LHe)
Useful Properties…
Creates large “drag” in liquid
Low mobility
Slow drift velocity in electric field
Small diffusion due to thermal equilibrium
LNe
Physics of Ionization Tracks
•Two primary forms of charged particle energy loss…
1.
Radiative (Bremsstrahlung)
2.
Ionization
dE
 ne e 2 
dx
 eVcm 2 


 g 
LNe
Physics of Ionization Tracks
dE
2
 ne e 
dx
LNe
Physics of Ionization Tracks
LNe
Physics of Ionization Tracks
LNe
Physics of Ionization Tracks
LNe
Physics of Ionization Tracks
250 keV Recoil Electron Tracks
(Single ionizations, parameterized angular distribution)
150 keV Recoil Electron Tracks
LNe
Pointing Capability
How well can we determine the origin of the incident neutrino?
• Angular diffusion of the ionization track
• Length of ionization track
• Diffusion over drift in detector
LNe
Pointing Capability
Energy of
Ionized
Electron (keV)
Average
Track Length
(mm)
Average
Angular
Diffusion
(degrees)
Ratio of Track Ratio of Track
Length to Drift Length to Drift
Diffusion (1
Diffusion (5
kV/cm)
kV/cm)
100
0.15
49
0.70
1.58
200
0.42
24
1.95
4.42
300 (highest
energy pp)
0.72
12
3.35
7.57
LNe
e-Bubble Drifts
Liquid Surface
Einstein-Nernst Equation
for Thermal Diffusion
s
Path of e-Bubble Drift
Ionization Location
LNe
e-Bubble Drifts
Predicted Mobility…
 cm 2 

m  1.6 
 Vs 
Drift Velocity…
cm
v d  1 .6
s
E = 1000 V/cm
cm
vd  8
s
E = 5000 V/cm
LNe
e-Bubble Drifts
Liquid Surface
What happens at the liquid
surface?
Why does it matter?
LNe
Trapping e-Bubbles at the Liquid-Vapor Interface
•Dielectric discontinuity at the
interface (el > ev)
•Potential well just beneath
surface
•e-Bubble has some
probability of tunneling
through potential barrier in
time
Schoepe, W. and G.W. Rayfield, Phys. Rev. A, 7, 6,
1973.
LNe
Trapping e-Bubbles at the Liquid-Vapor Interface
Barrier Height
A

Ed T
2-D Detection
Ejecting Charge from Liquid Surface
•Method needs to be conducive to maintaining
resolution (energy and spatial)
1.
Local high-field pulsing at surface
2.
Photo-emission
Due to their large size, e-Bubbles are
highly sensitive to photo-excitation.
Effective, but noisy
2-D Detection
Charge Amplification
Due to low ionized charge, a method of amplification is required…
GEMs
•High localized fields
•Charge amplification and light
emission (~1000x amplification)
2-D Detection
Charge Amplification
Due to low ionized charge, a method of amplification is required…
GEMs
•Commercial CCD Cameras to
read out light emission
•Pixelated anode
•No method for in-liquid
detection found effective
Garfield simulation of charge
amplification and drifts
In the mean time…
some proof of principle.
•Experimental verification of LNe
physics
• Simulated LNe drifts
All essential in constructing a large
scale detector
Research and Results

Outline:

e-Bubble Test Chamber Setup

Experimental Data

Computer Simulation Results
Experimental Run:
Design
e-Bubble experiment is
set up at Brookhaven
National Lab
A cryostat uses liquid
Helium (~4K) and liquid
Nitrogen (~77K) to cool
the test chamber.
Optical windows enable
“first-hand” observation
of the experimental runs
Experimental Run:
Test Chamber Setup
Electrons must be “artificially” inserted
into the test chamber
Goals:
- Test electron sources
- Make electron bubble drift measurements
Experimental Run:
Electron Sources



Photo-Cathode
High Voltage Tip
Radioactive Alpha
Source
Experimental Run:
Drift Time
Experimental
Theoretical


d 

t
1
m

  

  d 
   Ed  
 

d
Drift time is 78 ms @ 4 kV/cm
Using µ = 1.6E-3 (cm2/Vs)
Drift time is ~80 ms @ 4 kV/cm
Although the experimental drift time differs from the predicted time
by only a few ms, many approximations were used.
…stay tuned
Experimental Run:
Mobility
Using the predicted drift time
equation, mobility was fitted as
a free parameter
Drift time (ms)

1.66E-3 < µ < 1.9E-3
(cm2/Vs)
The derived mobility was
consistent with previously
determined electron bubble
mobility in LNe (Storchak,
Brewer and Morris).
Drift time (ms)

E-Field (kV/cm)
C (cm2/V) is a constant to compensate for
omitting the emission and anode regions
E-Field (kV/cm)
Experiment Run:
Drift Velocity

The electron bubble drift velocity can be
determined using:
V=µE
For µ=1.6E-3 (cm2/Vs) and E=4 kV/cm;
V = 6.64 cm/s.
Experiment Run:
Tip Charge Emission

The total charge deposited is calculated using
 A  T 
Q  q 

 a  t 
where; Q is the total charge at the anode (MeV), q is the charge injected by pulse
(MeV), A is the measured amplitude (mV), a is the calibrated pulse voltage (mV),
∆T is the measured signal FWHM (ms), and ∆t is the calibrated signal FWHM
(ms).
q = 10 MeV, a = 14:6 mV and t = 0:222 ms.
Experiment Run:
Mesh Transmission

The meshes in the test chamber will stop many
electron bubbles.
Experimental Run:
Trapping Time

The first attempt at measuring the electron bubble
trapping time at the liquid-vapor interface in LNe was
inconclusive.
Experiment Run:
Conclusions







Photo-Cathode in LNe= Bonk!
High Voltage Tip= Success!
Drift Time = 76 ms (under a 4 kV/cm drift field)
Drift Mobility = 1.66 x 10-3 (cm2/Vs)
Drift Velocity = 6.64 cm/s
Tip Charge Emission
Mesh Transmission
Simulations

Garfield:

Cell Definition

Gas Definition

Field

Drift

Signal
Simulations:
Drift Time; Mobility

Electrons were drifted through
the simulated cell by defining
mobility=1.9E-3 (µ=1.9E-3
cm2/Vs).

Recall the experimental drift
time was ~78 ms.
The predicted drift time
is 78. 5 ms (µ=1.9E-3
cm2/Vs)


d 

t
1   de  dd  da  
m   Ee   Ed   Ea  
76.85 ms
Simulations:
Diffusion
LongDiff = .001, TransDiff=1E-5 (cm/cmdrift)

Diffusion (longitudinal
and transverse) effects
the result of the
simulated drifts.

Generally, as diffusion
increases the observed
signal will widen and
exhibit a more
predominant tail
LongDiff = .001, TransDiff=1 (cm/cmdrift)
Simulations:
Diffusion

Diffusion displays a “threshold” characteristic.
Simulations:
Signal
Signal resulting from 80 electron bubble drifts
Simulation:
Conclusions

Consistent drift time results.
 Yields accepted electron bubble mobility and velocity

Diffusion “threshold” characteristic

Simulated signal for direct comparison to experimental data
What now?

Little Picture:




Trapping Time
Gas Bubbles
GEM Characteristics
Big Picture



Finish Research and Design
Ramp Up
Construction
Any Questions?