"Counting Atoms for Astrophysics"

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Information about "Counting Atoms for Astrophysics"

Published on September 9, 2008

Author: orzelc



Research talk given at Amherst College in 2006

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

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

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

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

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

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

Sudbury Neutrino Observatory Top-of-the-Line Scintillation Detector: 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)

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

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 (astro-ph/0402007) ~100 tons of liquid neon

CLEAN (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

CLEAN Sensitivity Gallium Chlorine Water 0.01 0.1 C L E A N

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

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

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

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

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)

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

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

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

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

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

Apparatus Metastable Source 145 MHz RF Plasma discharge Zeeman Slower Two-stage magnet Decelerates beam Trapping Chamber Undergraduate student for scale: Ryan McMartin ‘05

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*

Optical Excitation 124 nm lamp Kr inlet 819 nm laser Mike Mastroianni ‘07

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?

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))

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