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					EXO Gas

  Progress and Plans
  October, 2008
  David Sinclair
EXO Collaboration

   Canada
       Carleton, Laurentian
   USA
       Alabama, Caltech, Colorado State, UC irvine,
       Maryland, Massachusetts, SLAC, Stanford
   Switzerland
       Bern
   Russia
       ITEP
EXO People
Canadian Team

   Laurentian
       Jacques Farine, Doug Hallman, Ubi Wichoski
   Carleton
       Madhu Dixit, Kevin Graham, Cliff Hargrove, David
       Christina Hagemann (RA Arrives 2 weeks)
       Etienne Rollin (PhD Student)
       Chad Greene, James Lacey (MSc students)
   Currently 3 undergraduate thesis/project
New effort for the gas phase

   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
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- + e-

   Attempt 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
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
Liquid or Gas
Liquid                                Gas

Compact detector                      Energy resolution
No pressure vessel                    Tracking & multi-site rejection
Small shield -> lower purity reqd.    In-situ Ba tagging

 Large Cryostat                        Large detector
 Poorer energy, tracking resolution    Needs very large shield
 Ex-situ Ba tagging
                                       Pressure vessel is massive
EXO 200

   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,                2P
   Toshek, Dehmelt 1980)                   1/2
   Very specific signature                           650nm
Single ions can be detected
from a photon rate of 107/s
•Important additional                              metastable 80s
•Huge background              2S
Possible concept for a gas double beta counter
                                                   Anode Pads
                                                    WLS Bar

                       Xe Gas


              . . . . . . . .
              . . . . . . . .                          PMT

      For 200 kg, 10 bar, box is 1.5 m on a side
Possible concept for a gas double beta counter
                                                   Anode Pads
                                                    WLS Bar
  Electrode     Xe Gas


              . . . . . . . .
              . . . . . . . .                          PMT

      For 200 kg, 10 bar, box is 1.5 m on a side
Program as stated last year

   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 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

   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
   TEA
       Converts Ba++ -> Ba+
           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



 Gridded Ion Chamber
Progress on energy resolution – Pure
Xe, 2 Bar
                              Xe Energy Spectrum 3cm 2b 5992


                                                            s = 0.6%



           500   510    520     530    540       550        560   570   580   590   600
                                             Energy (MeV)

                       Alpha spectrum at 2 b pressure.
Program with Gridded Ion Chamber

   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

   Grid and anode of chamber replaced by
   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

   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
   Will attempt again with the T2K style
Progress on Detector Simulations

   Double beta events being simulated in Xe
    gas using GEANT and EGS
   Tracks are ugly!
Containment of tracks
Case for mixed gas

   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
Measuring the scintillation light signal
Energy and position response for
scintillation light
Light from gas mixtures

   (this slide intentionally left blank)
Measuring scintillation light in Xe gas
   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                1/2
Excite D state                                 650nm
Then pulse 650 nm               493nm
Laser to un-shelf D
state                                      4D

                                             metastable 80s

 Does not work!         2S
New Concept for Laser Tagging in High
   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

   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

   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
   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

   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
   Use electroluminescence in place of gas
    electron gain
Concept for an electroluminescence

    Design copied from Fermilab RICH counter
Electroluminescence Demonstration

   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
   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

   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
   Transport that ‘volume’ to the edge of the
   Stretch and squeeze it using field gradient
    into a long pipe
Barium Identification (Cont)

   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

   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

   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

   Field Asymmetric Ion Mass
FAIMS Operation

   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

   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

   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
Only data found to date in doubly charged
ion mobility
                     Mobilities for Xe+, Xe++ in Ne


             5                                        Xe+
             3                                        Xe++
                 0        50         100        150
Where Might This Lead

   We are aiming at a detector design at 200 kg
   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
Requirements for the Detector

   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

   Overwhelming endorsement for the ongoing
    R&D program
   Continued SNOLAB support
       Part time technician to operate and maintain the
       Engineering support
   Note that a request for a large detector
    underground is likely next year – candidate
    for the Cryopit

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