Transcript PP - Nevis Laboratories
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
?
•Neutral charge •Spin ½ •Massless Wolfgang Pauli, 1930 Cowan & Reines, 1956 Using reactor source,
p
n
e
-First experimental evidence of neutrino
Enrico Fermi
“neutrino”
(Italian for “little neutral one”)
Neutrinos Weak interactors by the exchange of the
W
and
Z
bosons
http://www-numi.fnal.gov/public/images/standardmodel.gif
t
p n
e
e e e n,p n,p
W
e
e
e -
W Z Z
e e
e
e
e
Neutrinos
& Interactions with Matter
n
e p
e
e
p
e
n e
e
e
e
e
n
,
p
e
n
,
p
x
t
e
e
e
e
W Z
e
Neutrinos
… more interactions
e
e -
W
e
e e +
e
e
e
W
e + e
e
x
Neutrinos
& the Sun
Neutrinos
The Solar flux
E
10
MeV
pp
pp
~
flux
500
keV
85 %
Homestake Neutrinos
The First Solar Neutrino Detector •Built at BNL in 1965 •615 tons tetrachloroethylene •Observed the following solar neutrino reaction…
e
37
Cl
37
Ar
e
http://www.its.caltech.edu/~sciwrite/journal03/A-L2/greissl.html
•Saw deficit in solar neutrino flux…
Neutrinos
The Solar Neutrino Problem The Solar Standard Model (SSM) is tested…
Super-Kamiokande
http://ale.physics.sunysb.edu/nngroup/superk/pic/sk-half-filled.jpg
•H 2 O Cherenkov Detector, 500 metric tons •Minimum ~3 MeV neutrinos •Detects Cherenkov light from scattered electrons •Reported 1/3 expected solar neutrino flux 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 its origin is given by… will be detected as an e a distance L (km) away from constant Mass difference Energy of Neutrino (eV)
Neutrinos
The Solar Neutrino Solution Sensitive to electron, muon and tau neutrinos…
SNO
•D 2 O 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
2-D Detection Plane
e-Bubble
Tracking Detector 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
Length of Track
e-Bubble
Information from Tracks Energy of Neutrino Total Ionized Charge Origin of Neutrino Shape of Track
e-Bubble
The Detector Medium
LHe LNe
• T = 2K • r = 0.125 g/cm 3 • ~5 metric tons • Long tracks (1-7 mm, 100-300 keV) • Good pointing capability • Minimum ionizing (low dE/dx) • Pure (long drifts, low internal background) e-Bubbles • T = 27K • r = 1.24 g/cm 3 • ~1 metric ton • Short tracks ( m m, 300 keV) 700 • Pointing only for highest energy pp •Self-shielding
•Solar pp flux 6.2
E 10 cm -2 s -1 •Expect ~674 ton -1 year -1
LNe
• Minimum ionizing (low dE/dx) • Pure (long drifts, low internal background) e-Bubbles • T = 27K • r = 1.24 g/cm 3 • ~1 metric ton • Short tracks ( m m, 300 keV) 700 • Pointing only for highest energy pp •Self-shielding
e-Bubbles
… A Social Metaphor A Red Sox fan enters Yankee Stadium… Go home r And the “Red Sox Fan”-Bubble phenomenon may be observed…
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.
2.
Radiative (Bremsstrahlung) Ionization
dE dx
n e e
2 eVcm 2 g
dE
n e e
2
dx
LNe
Physics of Ionization Tracks
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 150 keV Recoil Electron Tracks
(Single ionizations, parameterized angular distribution)
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 Length to Drift Diffusion (1 kV/cm) Ratio of Track Length to Drift Diffusion (5 kV/cm) 100 0.15
49 0.70
1.58
200 0.42
300 (highest energy pp ) 0.72
24 12 1.95
3.35
4.42
7.57
Liquid Surface s
LNe
e-Bubble Drifts Einstein-Nernst Equation for Thermal Diffusion Path of e-Bubble Drift Ionization Location
LNe
e-Bubble Drifts
Predicted Mobility…
m 1 .
6 cm 2 Vs
Drift Velocity… v d
1 .
6 cm s
v d
8 cm s E = 1000 V/cm E = 5000 V/cm
Liquid Surface
LNe
e-Bubble Drifts What happens at the liquid surface?
Why does it matter?
LNe
Trapping e-Bubbles at the Liquid-Vapor Interface •Dielectric discontinuity at the interface ( e l > e v ) •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
E d T
2-D Detection
Ejecting Charge from Liquid Surface •Method needs to be conducive to maintaining resolution (energy and spatial)
1.
2.
Local high-field pulsing at surface 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…
•Commercial CCD Cameras to read out light emission •Pixelated anode •No method for in-liquid detection found effective
GEMs
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
t d
Theoretical
d d Ed
Drift time is 78 ms @ 4 kV/cm Using µ = 1.6E-3 (cm 2 /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 1.66E 3 < µ < 1.9E-3 (cm 2 /Vs) The derived mobility was consistent with previously determined electron bubble mobility in LNe (Storchak, Brewer and Morris).
C (cm 2 /V) is a constant to compensate for omitting the emission and anode regions E-Field (kV/cm) E-Field (kV/cm)
Experiment Run:
Drift Velocity
The electron bubble drift velocity can be determined using:
V= µE For µ=1.6E-3 ( cm 2 /Vs ) and E=4 kV/cm; V = 6.64 cm/s.
Experiment Run:
Tip Charge Emission
The total charge deposited is calculated using
Q
q A a
T
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 (cm 2 /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 cm 2 /Vs)
t d
d e E e
d d Ed
d a E a
76.85 ms
Simulations:
Diffusion
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=1E-5 (cm/cmdrift) 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?