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 (10’s 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

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 >1% 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!!