Transcript EXO Gas - snolab 2008

EXO Gas
Progress and Plans
October, 2008
David Sinclair
EXO Collaboration
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Canada
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USA
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Alabama, Caltech, Colorado State, UC irvine,
Maryland, Massachusetts, SLAC, Stanford
Switzerland
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Carleton, Laurentian
Bern
Russia
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ITEP
EXO People
Canadian Team
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Laurentian
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Carleton
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Jacques Farine, Doug Hallman, Ubi Wichoski
Madhu Dixit, Kevin Graham, Cliff Hargrove, David
Sinclair
Christina Hagemann (RA Arrives 2 weeks)
Etienne Rollin (PhD Student)
Chad Greene, James Lacey (MSc students)
Currently 3 undergraduate thesis/project
students
New effort for the gas phase
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NSF grants to Stanford and Alabama for RA,s
students to work on gas EXO
New EXO collaborators at ITEP who have
just completed a Xe TPC project
Possible collaboration with Spanish group
Heidelberg-Moscow Results for Ge
double beta decay
57 kg years of 76Ge data
Apply single site criterion
We need to develop
new strategies to
eliminate
backgrounds to probe
the allowed space
Barium tagging may
offer a way forward
EXO – Enriched Xenon Observatory
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Look for neutrino-less double beta decay in Xe
136Xe --- 136Ba + e- + eAttempt to detect ionization and the Ba daughter
Ba is produced as ++ ion
Ba+ has 1 electron outside Xe closed shell so has
simple ‘hydrogenic’ states
Ba++ can (?) be converted to Ba+ with suitable
additive
Advantages of Xe
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Like most noble gases/liquids it can be made
extremely pure
No long lived radioactive isotopes
High Q value gives favourable rates
Readily made into a detector
Possible barium tagging to eliminate
backgrounds
Liquid or Gas
Liquid
Gas
Compact detector
No pressure vessel
Small shield -> lower purity reqd.
Energy resolution
Tracking & multi-site rejection
In-situ Ba tagging
Large Cryostat
Poorer energy, tracking resolution
Ex-situ Ba tagging
Large detector
Needs very large shield
Pressure vessel is massive
EXO 200
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A 200 kg liquid xenon detector is nearing
completion at WIPP
We play a major role in this project and there
is on-going activity at SNOLAB supporting
this project
This talk will focus on the gas counter as this
is a potential candidate for a SNOLAB project
Xe offers a qualitatively new tool against background:
136Xe
136Ba++ e- e- final state can be identified
using optical spectroscopy (M.Moe PRC44 (1991) 931)
Ba+ system best studied
(Neuhauser, Hohenstatt,
Toshek, Dehmelt 1980)
Very specific signature
“shelving”
Single ions can be detected
from a photon rate of 107/s
•Important additional
constraint
•Huge background
reduction
2P
1/2
650nm
493nm
4D
3/2
metastable 80s
2S
1/2
Possible concept for a gas double beta counter
Anode Pads
Micro-megas
WLS Bar
Electrode
Xe Gas
Lasers
Grids
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For 200 kg, 10 bar, box is 1.5 m on a side
PMT
Possible concept for a gas double beta counter
Anode Pads
Micro-megas
Electrode
Xe Gas
Isobutane
TEA
WLS Bar
Lasers
Grids
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For 200 kg, 10 bar, box is 1.5 m on a side
PMT
Program as stated last year
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Need to demonstrate good energy resolution
(<1% to completely exclude bb2n ) tracking,
Need to demonstrate Ba tagging
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Deal with pressure broadening
Ba ion lifetime
Ba++ -> Ba+ conversion
Can we cope with background of scattered light
Tasks to design gas EXO
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1) Gas Choice
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Measure Energy resolution for chosen gas
(Should be almost as good as Ge but this has
never been achieved)
Measure gain for chosen gas
Measure electron attachment for chosen gas
Understand optical properties
Measure Ba++ -> Ba+ conversion
Measure Ba+ lifetime
Tasks to design EXO Gas
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2) TPC Design
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What pressure to use
What anode geometry to use
What chamber geometry to use
What gain mechanism to use
Develop MC for the detector
Design electronics/DAQ
Tasks to design EXO Gas
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3) Ba Tagging
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Demonstrate single ion counting
Understand pressure broadening/shift
Understand backgrounds
Fix concept
Tasks to design EXO Gas
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4) Overall Detector concept
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Fix shielding requirements and concepts
Design pressure containment
Fix overall layout
Gas Properties
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Possible gas – Xe + iso-butane + TEA
Iso-butane to keep electrons cold, stabilize
micromegas/GEM
TEA
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Converts Ba++ -> Ba+
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Q for TEA + Ba++->TEA+ + Ba+* ~ 0
Converts 172 nm -> 280 nm?
? Does it trap electrons?
?Does it trap Ba+?
Progress This Year
Movable source holder
Contacts rings with wiper
Field Rings
Source
Grid
Anode
Gridded Ion Chamber
Progress on energy resolution – Pure
Xe, 2 Bar
Xe Energy Spectrum 3cm 2b 5992
200
s = 0.6%
Counts
150
100
50
0
500
510
520
530
540
550
560
570
Energy (MeV)
Alpha spectrum at 2 b pressure.
580
590
600
Program with Gridded Ion Chamber
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Response for many gas mixtures measured
New data on drift velocities in Xe + Methane,
isobutane, TEA
Some electron attachment measured but may
be due to gas impurities
First efforts with Micromegas
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Grid and anode of chamber replaced by
micromegas
Collaboration with Saclay and CERN to
produce micromegas
Using new ‘microbulk’ form of micromegas as
this is thought to offer best resolution
Ion density with alphas too high for this
technology – resolution ~ 1.7%
Switch to betas
Spectroscopy with micromegas
22 keV
109Cd
source
Status of Micromegas
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Energy resolution of 4% observed for 22 keV
x-ray is promising (-> 0.4% at 2 MeV)
Microbulk technology is not sufficiently robust
for this application
Xe requires high fields for gas gain and
lifetime of the micromegas is hours for these
fields
Will attempt again with the T2K style
micromegas
Progress on Detector Simulations
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Double beta events being simulated in Xe
gas using GEANT and EGS
Tracks are ugly!
Containment of tracks
Case for mixed gas
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There is incentive from previous slides to
investigate a mixed gas (Ne-Xe or He-Xe)
Tracks in the lighter are straighter
Better containment for given amount of
(expensive) xenon
Ratio of projected track to the total track
length
Measuring the scintillation light signal
Energy and position response for
scintillation light
Light from gas mixtures
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(this slide intentionally left blank)
Measuring scintillation light in Xe gas
mixtures
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It appears that any quench gas in Xe kills the
scintillation light
It appears that the mechanism is not
absorption of the photons but interaction
between Xe dimers and the additives which
de-excite the dimers.
Barium tagging
Original concept
Pulse 493 nm laser to
Excite D state
Then pulse 650 nm
Laser to un-shelf D
state
2P
1/2
650nm
493nm
4D
3/2
metastable 80s
Does not work!
2S
1/2
New Concept for Laser Tagging in High
Pressure
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The D state is quenched by gas interactions
in ns
So – use only blue laser, look for red light
Barium fluorescence Observed
Status of tagging
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A number of linewidth measurements made
with the arc source
Changing from an arc source to a laser
ablation source
We have demonstrated production of about
105 ions/pulse using an old N2 laser
We are about to modify chamber to introduce
this source
New Detector Concept
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We have some as yet unresolved issues with
the original concept
We do not get scintillation light with
quenchers but we cannot have gas gain
without
We are concerned that additives such as TEA
will give us gas purification difficulties so how
do we convert Ba++ to Ba+ and we do not
know that TEA like additives will not form
molecules or clusters with the Ba ions
New Concept
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Identify the barium production by extracting
the ion into vacuum and using conventional
techniques to identify a mass 136, ++ ion.
Expect this to be unique to Ba
Operate the detector in pure noble gas (Xe or
Xe+Ne)
Use electroluminescence in place of gas
electron gain
Concept for an electroluminescence
readout
Design copied from Fermilab RICH counter
Electroluminescence Demonstration
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EL is a well studied technique in noble gases and
mixed noble gases
EL is preferred over electron proportional counters
for gamma ray detectors
In Ne + Xe all of the light comes out at the Xe
scintillation wavelength (175 nm) for admixtures of
>1% Xe
No-one has demonstrated energy resolution in MeV
range
We propose to construct a detector to establish
performance of EL for this application
We plan a 20 x 20 array of 2 cm pads on each end
Barium Identification
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Because of the complexity of the electron
tracks in Ba, it will be hard to determine
exactly where the Ba is produced.
We have some volume within which it will be
contained.
Transport that ‘volume’ to the edge of the
detector
Stretch and squeeze it using field gradient
into a long pipe
Barium Identification (Cont)
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At end of pipe have an orifice leading to
evacuated region
Trap ions as they leave the gas using a
Sextupole Ion Trap (SPIG)
Once the ion is in vacuum, use conventional
techniques to identify it (eg Wein filter +
quadrupole MS or TOF + rigidity or ….
Critical Design Point
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What is the efficiency for getting the ion out of
the pipe and trapped by the spig?
We will start by simulations for the trap
varying trap geometry, pressures, gas mix
Possibly do tests on existing traps
Look at improving delivery of ions down pipe
using RF carpets or FAIMS
RF Carpets RF Funnels
Riken Ion Source
Gas cell length is 1 m
Gas is He at 100 torr
RF is 150 V at 10 MHz
RF Carpet operating at low pressure (10’s
of mb)
MSU Source
Ion path near the orifice
Problems with RF carpets
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These devices work best with low pressure,
light gases
We need to work with at least a substantial
fraction of Xe and we would like to work at or
above atmospheric pressure
FAIMS for EXO
Field Asymmetric Ion Mass
Spectrometer
Concept
FAIMS Operation
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Deflection during 1 cycle
d = E D (mhi - mlo)
Let m (E) = m0 + a E
Then
d = E2 D a / 2
Correction field
Ec = E2 D a / 2 [ mo 3 D ]
Ec = E2 a / 6 mo
Selecting ions based on a
FAIMS in non uniform field
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For a non-uniform E field
Say E = Eo (1 + b y)
Then there is a restoring field
Ec = Eo2 (ba/6m) y
Coaxial cylinders ion selection
Mass Spec on Hydrolyzed Yeast
Is FAIMS useful for EXO
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Would explore a different geometry with
focusing to center of pipe
Need data on mobility of Ba++ in Xe, (Ne)
The technique is used at atmospheric
pressure and tested to 2 bar
Need to explore impact of longitudinal drift
field
Only data found to date in doubly charged
ion mobility
Mobilities
Mobilities for Xe+, Xe++ in Ne
8
7
6
5
4
3
2
1
0
Xe+
Xe++
0
50
100
E/N
150
Where Might This Lead
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We are aiming at a detector design at 200 kg
scale
Would be world’s first ‘background free’
double beta decay experiment – competitive
with the best in the world for sensitivity
Would be a test of concept for a ton scale
detector
Requirements for the Detector
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Needs to be deep underground to avoid
cosmogenic production of radioactive Cs
Needs to be well shielded to cut the 2.614
MeV gamma background (136Xe bb Q value is
at the Compton edge for 2.614 MeV
gammas) – Water shield
Size depends on the pressure and gas mix
Would likely occupy much of Cryopit
What do we want from EAC
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Overwhelming endorsement for the ongoing
R&D program
Continued SNOLAB support
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Part time technician to operate and maintain the
lasers
Engineering support
Note that a request for a large detector
underground is likely next year – candidate
for the Cryopit