"Counting Atoms for Astrophysics"
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"Counting Atoms for Astrophysics"

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Research talk given at Amherst College in 2006

Research talk given at Amherst College in 2006

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"Counting Atoms for Astrophysics" Presentation Transcript

  • 1. Counting Atoms for Astrophysics: Atom Traps, Neutrino Detectors, and Radioactive Background Measurements Chad Orzel Union College Dept. of Physics and Astronomy D. N. McKinsey Yale University Dept. of Physics Students: M. Mastroianni R. McMartin M. Lockwood J. Smith E. Greenwood M. Martin M. Mulligan J. Anderson C. Fletcher $$: Research Corporation NSF
  • 2. Summary Why Are We Doing This, Anyway? What We’re Doing: Using A tom T rap T race A nalysis for Radioactive Background Evaluation Measure krypton contamination in other rare gases Fast measurement: Kr/Rg ~ 10 -14 in only 3 hours What We’re Not Doing: NOT a Purification Method Complementary to purification efforts
  • 3. Who Cares About Krypton? Astrophysicists! Next Generation of Neutrino Detectors: Liquid Rare Gas Scintillation 85 Kr is a source of background noise: Eliminate all krypton
  • 4. Neutrinos Fundamental particles Incredibly numerous: ~300/cm 3 from Big Bang ~40,000,000,000/cm 2 /s from the Sun Very small mass: Electron neutrino: m  e < 3eV/c 2 Tau neutrino: m  < 15 MeV/c 2 (electron mass: ~500 keV/c 2 ) Weak interactions: Interact only through weak nuclear force Neutral particles  Extremely Difficult to Detect
  • 5. Neutrino Detection Radiochemical:  e + 37 Cl  37 Ar + e -  e + 71 Ga  71 Ge + e - Neutrino interaction converts neutron to proton  Change element Ray Davis Nobel Prize 2002 Problem: Very slow readout (every few months) No real-time information
  • 6. Neutrino Detection 2 Scintillation Detectors: Neutrino collision produces light flash Electron: Nucleus: Allows real-time detection, energy measurement Problem: High energy threshold (5-8 MeV) Masatoshi Koshiba Nobel Prize 2002 Detect light with phototubes
  • 7. Sudbury Neutrino Observatory Top-of-the-Line Scintillation Detector: http://www.sno.phy.queensu.ca/ 1000 tons heavy water (D 2 O) 9600 Photomultiplier Tubes (PMT’s) Detect Cerenkov light Location, Location, Location: Creighton Mine, Sudbury, Ontario 2070 m (6800 ft) underground (Screen out background radiation)
  • 8. Solar Neutrinos How do detectors stack up? Need a better detector… Gallium Chlorine Radiochemical: Ga/Cl Low threshold No time resolution Water Scintillation: H 2 O/D 2 O Time, energy resolution High threshold
  • 9. Neutrino Detection: The Next Generation Use some other substance as scintillator Want: Time resolution Low threshold XMASS: ~ 20 tons of liquid xenon CLEAN: C ryogenic L ow E nergy A strophysics with N oble gases http://mckinseygroup.physics.yale.edu/CLEAN.html (astro-ph/0402007) ~100 tons of liquid neon
  • 10. CLEAN http://mckinseygroup.physics.yale.edu/CLEAN.html (astro-ph/0402007) Advantages of liquid rare gases: 3) Little or no intrinsic radioactivity Scintillation detection with low threshold 1) High yield Ne:  = 80nm, 15,000 photons/ MeV 2) Self-shielding Dense liquid, absorbs radiation
  • 11. CLEAN Sensitivity Gallium Chlorine Water 0.01 0.1 C L E A N
  • 12. Krypton Contamination Problem: Krypton Contamination 85 Kr:  ½ = 10.76 yr  -decay at 687 keV Looks like detection event in energy range of interest… Need to remove all Kr from detector 40 ppb Rare isotope: 2.5 × 10 -11 Major source of background
  • 13. Krypton Removal Need extremely high purity Kr/Ne ~ 4 × 10 -15 (any isotope) 85 Kr much lower ~100,000 atoms in full CLEAN Difficult to purify gas to this level Kr chemically inert Distillation, Charcoal Filter Xe distillation, Takeuchi et al. ~3.3 ppt Kr Difficult to measure purity Gas chromatography Accelerator mass spectrometry Days or weeks to measure
  • 14. Atom Trap Trace Analysis Technique developed by Z.-T. Lu and colleagues at Argonne National Laboratory Used to measure 85 Kr abundance Used for radioisotope dating Trap, detect single atoms of rare isotopes Determine abundance by counting Proposal: Use ATTA to measure Kr in Ne or Xe 7 × 10 16 atoms/s in  3× 10 -14 abundance in 3 hrs (1 atom detected) Load source with ultra-pure Ne, Xe Detect single Kr atoms
  • 15. Laser Cooling and Trapping Use light forces to slow and trap atoms Photons carry momentum p Transfer to atoms on absorption p Very small velocity change 84 Kr  =811 nm  v=5.8 mm/s Lots of photons (10 15 per second) Room-temperature velocity ~ 300 m/s  100,000 photons to decelerate Use scattering force to slow thermal motion
  • 16. Doppler Cooling Exploit Doppler effect to selectively cool atoms Use single laser beam to slow and stop beams of atoms   o Tune laser to lower frequency (red)  <  o |e> |g> Stationary atoms do not absorb Atoms moving toward laser see blue shift Absorb photons, slow down Use pairs of beams to cool sample Reach microkelvin temperatures (v~10 cm/s)
  • 17. Magneto-Optical Trap Add spatially varying magnetic fields Confine atoms to small volume Trapping due to photon scattering 10 8 photons/s per atom (Na MOT at NIST) Detect trapped atoms using fluorescence
  • 18. ATTA Count trapped atoms to determine abundance APD Detect single atoms by trap laser fluorescence (data from Lu group) Atom Source Zeeman Slower MOT ATTA Technique Prepare Kr* atoms in metastable state Slow beam Trap atoms in MOT
  • 19. Selectivity (Figure from Lu group at ANL) 85 Kr ~ 10 -11 81 Kr ~ 10 -13 83 Kr ~ 0.11 Only Kr atoms detected Extremely selective technique Need to scatter 10 5 photons No off-resonant background Trap only one isotope Trap over ~ 30 MHz Out of 370 THz
  • 20. Background Kr atoms trapped in metastable state ~10 eV above ground state,  ~30 s Ground-state Kr not trapped not detected 0) Sample Handling 1) Outgassing: Keep Kr out of system. Background ~10 -16 level laser cooling 5p[5/2] 3 5s[3/2] 2 811nm ~10 eV Atoms only excited in source  Only contamination in source matters 2) Cross-contamination : Kr from calibration samples embedded in source Eliminate with optical excitation
  • 21. Sensitivity Procedure: 1) Load system with Ne or Xe 2) Set lasers to trap 84 Kr (57% abundance) 3) Count atoms, compare to input flux One atom in three hours: 3 × 10 -14 abundance Typical source consumption: 7 × 10 16 atoms/s Trapping efficiency: 10 -8
  • 22. Apparatus Metastable Source 145 MHz RF Plasma discharge Zeeman Slower Two-stage magnet Decelerates beam Trapping Chamber Undergraduate student for scale: Ryan McMartin ‘05
  • 23. Optical Excitation Metastable excitation methods 1) Electron impact: RF plasma discharge Simple, robust Potentially higher efficiency (10 -2 )  Improved sensitivity Eliminate cross-contamination  Lower background 5p[5/2] 3 5s[3/2] 2 811nm ~10 eV Low efficiency (10 -4 – 10 -3 ) “ Memory Effect” cross-contamination 5s[3/2] 1 5p[5/2] 2 124 nm Kr lamp 819 nm laser 2) Two-photon optical excitation 124 nm lamp, 819 nm laser Excite only Kr*
  • 24. Optical Excitation 124 nm lamp Kr inlet 819 nm laser Mike Mastroianni ‘07
  • 25. Future Prospects 1) Other Species Same technique works for other rare gases. 39 Ar evaluation  Ar*, Kr* < 1nm apart: use same optical system 2) Continuous monitoring 3hrs for 10 -14 level Less time for lower sensitivity (XENON): continuous purity check? 3) Other systems? 3 He/ 4 He?
  • 26. Conclusions Next generation of neutrino detectors will require ultra-pure rare gases Can use Atom Trap Trace Analysis to measure Kr contamination High sensitivity, low background Independent of purification method Fast measurement (3 hrs for 3 ×10 -14 ) Complement to experimental efforts to purify gases (see also: astro-ph/0406526, Nucl. Instr. Meth. A 545 , 524 (2005))