Transcript EXO-GAS Detector Status report for the SNOLAB EAC August 2007

EXO-GAS Detector
Status report for the SNOLAB
EAC August 2007
EXO Canada Team
• Laurentian
– J. Farine, D. Hallman, C. Virtue, U. Wichoski
– Adam Blais (Summer Student)
• Carleton
– M. Dixit, K. Graham, C. Hargrove, D. Sinclair
– C. Green, E. Rollin (Grad. Students)
– K. McFarlane (Engineer) L. Anselmo
(Chemist)
Heidelberg-Moscow Results for Ge
double beta decay
57 kg years of 76Ge data
Apply single site criterion
Normal and Inverted Mass
Hierarchies
Inverted
Normal
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
•
•
•
•
•
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
• 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
Liquid Detector EXO 200
• Objectives
– Prove the liquid detection concept
– Measure bb2n decay rate for Xe
– Test the HM claim for observation of bb0n
• Under construction at Stanford for deployment at
WIPP
• Major engineering support from Vance Strickland
Status of 2ν mode in
136Xe
2νββ decay has never been observed in 136Xe.
Some of the lower limits on its half life are close to (and in
one case below) the theoretical expectation.
T1/2 (yr)
evts/year in the
200kg prototype
(no efficiency applied)
Leuscher et al
>3.6·1020
<1.3 M
Gavriljuk et al
>8.1·1020
<0.6 M
Bernabei et al
>1.0·1022
<48 k
QRPA (Staudt et al) [T1/2max]
=2.1·1022
=23 k
QRPA (Vogel et al)
=8.4·1020
=0.58 M
NSM (Caurier et al)
(=2.1·1021)
(=0.23 M)
Experimental limit
Theoretical prediction
The 200kg EXO prototype
should definitely resolve this issue
EXO neutrino effective mass sensitivity
Assumptions:
1) 80% enrichment in 136
2) Intrinsic low background + Ba tagging eliminate all radioactive background
3) Energy res only used to separate the 0ν from 2ν modes:
Select 0ν events in a ±2σ interval centered around the 2.481MeV endpoint
4) Use for 2νββ T1/2>1·1022yr (Bernabei et al. measurement)
Case
Mass
(ton)
Eff.
(%)
Run
Time
(yr)
σE/E @
2νββ
2.5MeV Background
(%)
(events)
Conserva
tive
1
70
5
1.6*
Aggressi
ve
10
70
10
1†
0.5 (use 1)
T1/20ν
(yr,
90%CL)
2*1027
0.7 (use 1) 4.1*1028
Majorana mass
(meV)
QRPA‡ (NSM)#
33
(95)
7.3
(21)
s(E)/E = 1.6% obtained in EXO R&D, Conti et al Phys Rev B 68 (2003) 054201
† s(E)/E = 1.0% considered as an aggressive but realistic guess with large light
collection area
‡ QRPA: A.Staudt et al. Europhys. Lett.13 (1990) 31; Phys. Lett. B268 (1991) 312
# NSM: E.Caurier et al. Phys Rev Lett 77 (1996) 1954
*
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
. . . . . . . .
. . . . . . . .
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
. . . . . . . .
. . . . . . . .
For 200 kg, 10 bar, box is 1.5 m on a side
PMT
Triggers
• Level 1
– Light => event in fiducial volume
– Light => energy = Q +- 10%
• Level 2
– Ionization => energy = Q +- 3%
– 2 Bragg peaks
– Single site event
• Determine Ba location
• Start search for Ba
Gas Option for EXO
• Need to demonstrate good energy
resolution (<1% to completely exclude
bb2n ) tracking,
• Need to demonstrate Ba tagging
– Deal with pressure broadening
– Ba ion lifetime
– Ba++ -> Ba+ conversion
– Can we cope with background of scattered
light
Tasks to design gas EXO
• 1) Gas Choice
– Measure Energy resolution for chosen gas
– (Should be 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
• 2) TPC Design
– 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
• 3) Ba Tagging
– Demonstrate single ion counting
– Understand pressure broadening/shift
– Understand backgrounds
– Fix concept
Tasks to design EXO Gas
• 4) Overall Detector concept
– Fix shielding requirements and concepts
– Design pressure containment
– Fix overall layout
Gas Properties
• Possible gas – Xe + iso-butane + TEA
• Iso-butane to keep electrons cold, stabilize
micromegas/GEM
• TEA
– Converts Ba++ -> Ba+
• Q for TEA + Ba++->TEA+ + Ba+* ~ 0
– Converts 172 nm -> 280 nm?
– ? Does it trap electrons?
– ?Does it trap Ba+?
Measuring Gas properties
• Gridded ion chamber being used to
measure resolution, drift of electrons using
alpha source
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
Energy Spectrum for Xe + CH4
(5%)
Corrected Energy Spectrum (10 cm)
200
Counts
150
100
50
0
450
460
470
480
490
500
510
Energy (arb. units)
520
530
540
550
Amplitude vs risetime
Amplitude (Arb. units)
0.3
0.2
0.1
0
20
40
60
80
100
rise time (16 ns/bin)
120
140
Amplitude and resolution vs source distance
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
0.4
resolution
Peak Amplitude
0
5
10
15
Source distance (cm)
Xe + 5% CH4
20
Xe + Isobutane Peak vs Drift Distance
Peak amplitude
550
547
540
539
530
520
518
510
500
497
490
0
5
10
15
Drift (cm)
Note:
(1) peak width was constant at ~0.6% over the range
(2) Gas was not purified but was spec’d at 99.9%
20
Current status on energy resolution
• Ionization in gaseous Xe gives adequate
energy resolution, even for alpha particles.
• We can now use this to explore gain
options
Studying Ba ions in high pressure
Xe gas
Thin (5 mm) Pt wire + Ba
- - - - - - - - - - - - - - - - - - Grid 1
Laser Beams __________________________
- - - - - - - - - - - - - - - - - - Grid 2
Filter
PMT
Pulse red and blue lasers out of phase with each other
Ion production in test cell (detection
using Channeltron)
Counts per 1 ms bin
Ion detection from hot Pt wire
1000000
100000
10000
1000
100
10
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time since heating pulse (s)
0.8
0.9
1
Progress on Ba tagging
Problems with Proposed technique
• It appears that the D state de-excites
through collisions on a timescale short
compared to our laser pulsing
• This would allow a different approach
• Use cw blue laser and look for red
fluorescence lines
• Red sensitive PMT on order
Si detector
228Th
Laser Beam
Lens
PMT
Concept for single ion fluorescence of Ra
Plans (Dreams)
• We are working to address the technical
issues associated with a large gas Xe
double beta decay detector
• If all goes well we will seek funding to build
a 200 kg gas detector with Ba tagging
EXO GAS DETECTOR CONCEPT
200 Kg
Crinkled Cubic Copper Liner
3,000 lb (if 0.1 inch thick)
10.2 feet each side
Plan View
Acrylic Cylindrical Shell
14.9 feet diameter,
12.2 feet high
Water Tank
28 diameter
for 2 meters H2O
Vacuum
Around acrylic
blocks ?
Water Shield
490 tonnes water
If filled without internals
Acrylic Blocks
9 tonnes
(Fills 25% of space)
H2O
(3.3 psi + 18.2 psi)
~ 21.4 psia
Xe
200 kg
at 18.2 psia
H2O
(7.7 psi + 18.2)
~ 25.9 psia
Note: Decreasing the Xe pressure
to 1 bar requires increasing the copper
tank to 11 foot sides.
Elevation
Longer term plans
• If things go really well we can consider a
ton scale detector.
• Could be either liquid or gas
• If Ba tagging works very well then
incentive to use separated isotope Xe is
weaker
• A detector of several tons could be
accomodated in either the cube hall or the
cryopit.
EXO Progress Update
Laurentian University
Jacques Farine
EXO Gas Option Simulation
First step: containment efficiencies
• Pressure and mass dependence
• Cylinder, take H=2R to minimize S/V
• Filled with 136Xe
• Cu walls
• 0nbb decay, Q = 2457.8 keV
• Differentiate e–//both crossing fid. vol.
Uncertainties obtained from 20
independent simulations.
+ Points include detailed low
energy processes, scintillation and
E=1kV/cm ( .. 30x CPU cycles).
2n / 0n differential c at edges
• Simulations for 1T
at 5 atm, equator
• 10,000 evts ea.
• Contam. of 2n in 0n
increases towards
the edge
• > Optimize fiduc.
volume and/or vary
fraction of
contamination
Next steps
• Add chemical composition / drift / attenuation /
absorption / attachment // light+charge readout
• Add backgrounds as source of singles
• Write code to detect Bragg peaks
• For single/double separation, determine:
– Contamination / sacrifice
– Effect of Bremsstrahlung
• Light collection options > E resolution
Studies related to both
L+G Options
Material screening - radon
emanation tests
•
•
•
•
•
Continued program at SNOLAB
Sensitivity 10 220Rn/day, 20 222Rn/day
Measure EXO-200 plumbing
No substantial source
Clean !
Characterize counters for Ar/Xe
• Allow for:
– Absolute emanation
measurements
– Diffusion studies in
• Absolute cross-calibration
between gases
N2 = Ar; Xe 23% lower
Radon Trap Development
1) ESC on EXO-200
• Augmented with:
–
–
–
–
–
–
CO2 trap
Rn source
Water vapour trap
Radon trap Mark I (LN2)
Heat exchanger
Recirculation pump
• Study Rn removal efficiency:
– In misc. gases Air/N2/Ar > Xe
– Rn trap Mark I
Radon trap tests at ES-III (Stanford)
 Mark I trap: 2” of SS wool at LN2, multiple passes
 efficiency too low (60% in 160 mbar N2) - sets scope
Radon Trap Development
2) At SNOLAB
•
•
•
•
•
•
•
222Rn
and 210Rn sources
development
Radon extractor board as
trap testbed
Refrigerator purchased
Cold head integration
underway
Xenon purchased
Xe plumbing assembly
initiated (w/ RCV vessels)
ESC integration underway
Xenon heat
exchanger
in construction
Diffusion of Rn in Xe
Gas at p,T
L
Reduction factor along dead legs
• Known, irreducible source term
• Want max. ingress rate at distance L
• For 220/222Rn in N2/Ar/Xe
Theory - KTG in binary, dilute mixture, calculate D12
• 1D diffusion model with decay
Experimental check
 Diffusion length for 222Rn at 1 atm:
d = 2m in Ar; 1.2m in Xe