Double Beta Decay in High Pressure Xenon (EXO)

1
Double Beta Decay in High Pressure Xenon (EXO)

• David Sinclair
• TPC Symposium
• Paris 2008

2
Context for the talk

• Chronologically, this talk should follow that of
  Jean-Luc and Razvan
• Xe is a promising medium for the search for
  neutrino-less double beta decay
• Can be made into a counter
• Can be made very pure
• Prospect for major background reduction and
  confidence building through Barium tagging
• Data exist on the performance of a Xe gas TPC
• There are advantages of a liquid detector and EXO
  is about to commission a 200 kg liquid Xe TPC

3
Why Investigate a Gas Counter Further?

• Gas counters will give some tracking information
  for the events
• Location of event
• 2 Bragg peaks
• Single site criterion
• ? Angular correlation?
• Problems of barium tagging are different and all
  avenues should be pursued to find a working
  technique
• Energy resolution may be better

4
How to look for neutrino-less decay

• Measure the spectrum of the electrons

5
Heidelberg-Moscow Results for Ge double beta
decay
Results from 56 kg-years Of data taking with 76Ge
6
Possible neutrino Mass solutions
7
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
2P1/2
650nm
493nm
4D3/2

• Important additional
• constraint
• Huge background
• reduction

metastable 80s
2S1/2
Default Ba tagging scheme
8
Default concept for a gas double beta counter
Anode Pads
Micro-megas
WLS Bar
Xe Gas Isobutane/CF4 TEA
Electrode
Lasers
. . . . . . . .
. . . . . . . .
Grids
PMT
For 200 kg, 10 bar, box is 1.5 m on a side
9
Some problems with the default design

• What gas can we use?
• Operation of micromegas is much easier with
  quench (although might be possible to use pure
  Xe)
• Any quench is likely to kill the scintillation
  light by de-exciting the Xe dimers
• Any hydrocarbon is likely to react with the Ba
• Any additive will make the gas purification
  difficult
• We need to convert Ba to Ba for tagging but
  any additive will probably eat the Ba

10
New Concept for Barium Tagging

• Barium is formed as a 2 ion in the double beta
  decay (higher charge states possible due to shake
  off etc but likely relax to 2 in Xe)
• Laser tagging requires 1
• Transformation requires an additive in Xe but
  additives will likely interfere with the
  scintillation process (needed for time 0 signal)
  and with the Ba lifetime

11
Can we measure Ba Directly?

• Extract the ion from the high pressure into a
  vacuum
• Measure mass and charge directly
• A mass 136, ion is a unique signature of Ba.
  (Assumption is Xe cannot survive long enough to
  be a problem)
• This has been done for Ba in Ar gas

12
Experimental evidence that Ba ions survive in
Ar and can be extracted
From Marius Facina PhD thesis
13
Barium ions are guided towards the exit orifice
and focused using an asymmetric field technique.
The second chamber is maintained at a pressure of
10-30 mb Using a cryopump and is lined with an
RF carpet. An RF funnel guides the ions Towards
the RF quadrupole which is at high vacuum. The
ion is identified using TOF and magnetic rigidity
14
RF Carpets RF Funnels
15
Riken Ion Source
Gas cell length is 1 m Gas is He at 100 torr RF
is 150 V at 10 MHz
16
RF Carpet operating at low pressure (10s of mb)
MSU Source
17
Ion path near the orifice
18
Problems with RF carpets

• These devices work best with low pressure, light
  gases
• Solution
• Make the carpet very porus, surround with
  cryopump at 20K
• Keep a He pressure but pump away Xe as it enters
• Discussions with MSU started to get simulations
  going

19
SPIG QPIG

• Multipole to be determined
• Seems to be an established procedure
• Specific to e/m so should reject any singly
  charged Xe that reaches it
• Most Radioactive beam facility SPIGs are limited
  by space charge. We just have 1 ion.
• Help from Bob Moore (McGill)
• Cannot measure inefficiency
• Possible help from Jens Dilling (TRIUMF)

20
Final measurement of Q,M

• A linear trap can be unloaded in ns
• Accelerate ions and bend with magnet
• Measure Time of Flight
• This is all conventional
• Gives a mass spectrum for further confidence in
  data

21
Testing

• Need to have a calibrated source of Ba
• One concept
• Use a Ba beam from an accelerator
• Scatter from target to achieve low rate
• Pass through thin window into Xe
• Ba will be stripped to hi Q but relax to 2
• Measure ion arrival using scintillation
• Look for Ba detection vs scintillation

22
Optimal detector with pure noble gas

• Gas gain may prove problematic in pure noble gas
• With micromegas maximum size of devices may be
  limitation (How to measure energy when track
  crosses boundary?)
• Possible to use electroluminescence as suggested
  by Nygren
• EL is preferred technique for getting energy
  resolution at low energies

23
Top EL/Scint Detector (Tracking)
EL Grid
Field Cage
Ba Channel
Cathode Grids
Bottom EL/Scint Detector (Energy)
24
Concept for an electroluminescence readout
Design copied from Fermilab RICH counter
25
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
• No-one has demonstrated good energy resolution in
  MeV range
• We propose to construct a detector to establish
  performance of EL for this application

26
Pressure vessel for 10 bar And UHV 400 2x2 cm
readout pads At each end Use electron sources up
to 1 MeV Filling pure Xe or Xe/Ne mix
27
Should we use a mixed noble gas?

• Electron tracks in Xe look really ugly
• Electron tracks in He look OK
• Tracks in Ne and Ar are progressively worse

28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
Mixed Gas

• We can consider working with a mixed noble gas
• We need a lot of the lighter gas to make a
  difference
• It puts the pressure up
• Straightens the tracks
• Shortening the tracks means that more of the
  tracks are contained better use of precious Xe
• El spectrum of mixed gas same as Xe for gt1 Xe

33
Gas Pressure

• If we want to do Ba tag in situ then there is
  incentive to use low pressure to reduce pressure
  broadening
• With the Ba extraction there is an incentive to
  go to high pressure
• If the best tracking we can do just identifies
  the region of ionization, then this volume goes
  as P-3 and this is the volume we need to assay
• Pressure dependence of extraction not yet known

34
Criteria for a Xe Gas counter

• Energy resolution
• Target is 1 (s)
• Set by need to separate 2n from 0n processes
• This is conservative but good resolution is also
  key to gaining confidence in a measurement

35
Criteria for a Gas detector

• Tracking
• Low energy electron tracks in Xe are ugly!
• Need to be able to define contained events and a
  fiducial volume
• Want to see 2 Bragg peaks
• Want to identify multisite events
• Ba tagging improves with better tracking
• Ideally would measure the bb angular correlation

36
Where are we going?

• We will have first data from EXO-200 next year
• We should have established the gas feasibility at
  that time
• If liquid detector is background free then next
  step is a bigger liquid detector
• If gas looks feasible and backgrounds observed in
  EXO-200 then go for a big gas counter
• Detector could be located at SNOlab in either
  Cryopit (16m diameter, 16m high) in a water
  tank for shielding.

37
Space is available at SNOLAB very soon!!