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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
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What We’re Doing: Using Atom Trap Trace Analysis 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
Why Are We Doing This, Anyway?
Summary
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Who Cares About Krypton?
Astrophysicists!
Next Generation of Neutrino Detectors:
Liquid Rare Gas Scintillation
85Kr is a source of background noise:
Eliminate all krypton
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Neutrinos
Fundamental particles
Incredibly numerous: ~300/cm3 from Big Bang
~40,000,000,000/cm2/s from the Sun
Very small mass: Electron neutrino: me < 3eV/c2
Tau neutrino: m < 15 MeV/c2
(electron mass: ~500 keV/c2)
Weak interactions:
Interact only through weak nuclear force
Neutral particles
Extremely Difficult to Detect
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Neutrino Detection
Radiochemical: e + 37Cl 37Ar + e-
e + 71Ga 71Ge + e-
Neutrino interaction converts neutron to proton
Change element Ray DavisNobel Prize 2002
Problem: Very slow readout (every few months)
No real-time information
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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 KoshibaNobel Prize 2002
Detect light with phototubes
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Sudbury Neutrino Observatory
Top-of-the-Line Scintillation Detector:
1000 tons heavy water (D2O)
9600 Photomultiplier Tubes (PMT’s)
Detect Cerenkov light
Location, Location, Location:
Creighton Mine, Sudbury, Ontario
2070 m (6800 ft) underground
(Screen out background radiation)
http://www.sno.phy.queensu.ca/
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Solar NeutrinosHow do detectors stack up? Gallium Chlorine
Radiochemical: Ga/Cl
Low threshold
No time resolution
Water
Scintillation: H2O/D2O
Time, energy resolution
High threshold
Need a better detector…
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Neutrino Detection: The Next Generation
Use some other substance as scintillator
Want: Time resolution
Low threshold
CLEAN: Cryogenic Low EnergyAstrophysics with Noble gases
http://mckinseygroup.physics.yale.edu/CLEAN.html
(astro-ph/0402007)
~100 tons of liquid neon
XMASS: ~ 20 tons of liquid xenon
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CLEAN
http://mckinseygroup.physics.yale.edu/CLEAN.html
(astro-ph/0402007)
Advantages of liquid rare gases:
1) High yield
Ne: = 80nm, 15,000 photons/ MeV
2) Self-shieldingDense liquid, absorbs radiation
3) Little or no intrinsic radioactivity
Scintillation detection with low threshold
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CLEAN SensitivityGallium Chlorine Water
0.01 0.1
CL
E
A
N
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Krypton Contamination
Problem: Krypton Contamination
85Kr: ½ = 10.76 yr
-decay at 687 keV
Looks like detection event in energy range of interest…
40 ppb
Rare isotope: 2.5 × 10-11
Major source of background
Need to remove all Kr from detector
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Krypton Removal
Need extremely high purity
Kr/Ne ~ 4 × 10-15 (any isotope)85Kr 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
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Atom Trap Trace Analysis
Technique developed by Z.-T. Lu and colleagues at Argonne National Laboratory
Used to measure 85Kr 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 × 1016 atoms/s in 3× 10-14 abundance in 3 hrs (1 atom detected)
Load source with ultra-pure Ne, Xe
Detect single Kr atoms
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Laser Cooling and Trapping
Use light forces to slow and trap atoms
Photons carry momentump
Transfer to atoms on absorptionp
Very small velocity change84Kr=811 nmv=5.8 mm/s
Lots of photons (1015 per second)
Room-temperature velocity ~ 300 m/s
100,000 photons to decelerate
Use scattering force to slow thermal motion
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Doppler Cooling
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
Exploit Doppler effect to selectively cool atoms
Use single laser beam to slow and stop beams of atoms
Use pairs of beams to cool sample
Reach microkelvin temperatures (v~10 cm/s)
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Magneto-Optical Trap
Add spatially varying magnetic fields
Confine atoms to small volume
Trapping due to photon scattering
108 photons/s per atom
(Na MOT at NIST)
Detect trapped atoms using fluorescence
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ATTA
Count trapped atoms to determine abundance
APD
Detect single atoms by trap laser fluorescence
(data from Lu group)
Atom Source
Zeeman SlowerMOTATTA Technique
Prepare Kr* atoms in metastable state
Slow beam
Trap atoms in MOT
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Selectivity
(Figure from Lu group at ANL)
Extremely selective technique
Need to scatter 105 photons
No off-resonant background
85Kr ~ 10-11
81Kr ~ 10-13
83Kr ~ 0.11
Trap only one isotope
Trap over ~ 30 MHz
Out of 370 THz
Only Kr atoms detected
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Background
laser cooling
5p[5/2]3
5s[3/2]2
811nm
~10 eV
Kr atoms trapped in metastable state
~10 eV above ground state, ~30 s
Ground-state Kr not trapped
not detected
Atoms only excited in source
Only contamination in source matters
0) Sample Handling
1) Outgassing: Keep Kr out of system. Background ~10-16 level
2) Cross-contamination: Kr from calibration samples embedded in source
Eliminate with optical excitation
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Sensitivity
Procedure:
1) Load system with Ne or Xe
2) Set lasers to trap 84Kr (57% abundance)
3) Count atoms, compare to input flux
Typical source consumption:
7 × 1016 atoms/s
Trapping efficiency:
10-8
One atom in three hours: 3 × 10-14 abundance
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Apparatus
Metastable Source
145 MHz RFPlasma discharge
Zeeman SlowerTwo-stage magnet
Decelerates beam
Trapping Chamber
Undergraduate student for scale:
Ryan McMartin ‘05
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Optical Excitation
5p[5/2]3
5s[3/2]2
811nm
~10 eV
Metastable excitation methods
1) Electron impact: RF plasma discharge
Simple, robust
Low efficiency (10-4 – 10-3)
“Memory Effect” cross-contamination
5s[3/2]1
5p[5/2]2
124 nm
Krlamp
819 nmlaser
2) Two-photon optical excitation
124 nm lamp, 819 nm laser
Excite only Kr*
Potentially higher efficiency (10-2) Improved sensitivity
Eliminate cross-contamination Lower background
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Optical Excitation
124 nm lamp
Kr inlet
819 nm laser
Mike Mastroianni ‘07
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Future Prospects
1) Other Species
Same technique works for other rare gases.
39Ar 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?
3He/4He?
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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))