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Radioactive Background Evaluation by Atom Counting. C. Orzel Union College Dept. of Physics and Astronomy. D. N. McKinsey Yale University Dept. of Physics. R. McMartin M. Lockwood J. Smith E. Greenwood M. Martin M. Mulligan J. Anderson C. Fletcher. S. Maleki J. Sheehan. - PowerPoint PPT Presentation
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Chad OrzelUnion College Physics
Radioactive Background Evaluationby
Atom Counting
C. Orzel Union College Dept. of Physics and Astronomy
D. N. McKinsey Yale University Dept. of Physics
R. McMartinM. LockwoodJ. SmithE. GreenwoodM. MartinM. MulliganJ. AndersonC. Fletcher
S. MalekiJ. Sheehan
$$: Research Corporation
Chad OrzelUnion College Physics Summary
What it isn’t:
Not a method for purifying gases
Complementary to purification efforts
Atom Trap Trace Analysis (ATTA)
What it is:
Method for measuring Kr contamination
High sensitivity: 10-14 level
Fast measurement: ≤ 3 hrs integration
Use atomic physics techniques
Detect single impurity atoms
Independent of production
What it might be: An answer to yesterday’s question:
What’s the best way to measure Kr in Xe?
Chad OrzelUnion College Physics
Very small velocity change84Kr=811 nmv=5.8 mm/s
Lots of photons (1015 per second)
Laser Cooling
Use light forces to slow and trap atoms
Photons carry momentum
Transfer to atoms on absorption
p
p
Use Doppler shift to selectively cool
Red-detuned laser (o)
Only counter-propagating atoms absorb
Slow, cool beams of atoms
Slow, cool atoms in 3-D microkelvin temperatures
Chad OrzelUnion College Physics Atom Trapping
Add spatially varying magnetic fields: confine atoms
Magneto-Optical Trap (MOT)
Collect up to 109 atoms, T ~ 100 K
(Na MOT at NIST)
Trapping due to light forces
Constantly scattering photons
Chad OrzelUnion College Physics Apparatus
Table-top physics
Diode lasers for light source
Standard UHV components
Undergraduate student projects
Relatively inexpensive
(m.w.e ~ 1)
Chad OrzelUnion College Physics 85Kr Contamination
85Kr:
1/2 = 10.76 yr -decay
abundance: 2.5 × 10-11 in natural Kr
activity: 1.5 Bq/m3 in air (1.1 ppm Kr)
Kr contamination major source of background counts for liquid noble gas particle detectors
Commercial gases: Kr 20 ppb
Need: Kr/Xe: 150 ppt or less (XENON) Kr/Ne: 4 × 10-15 or less (CLEAN)
Difficult to purify to this level
Difficult to measure Kr content at this level
Use laser cooling and trapping to measure Kr/Xe or Kr/Ne
Chad OrzelUnion College Physics Metastable Krypton
electron impact
Electron impact excitation
RF, DC plasma discharge sourcesLow efficiency (10-3 10-4)
5s[3/2]1
5p[5/2]2
124 nm Krlamp
819 nmlaser
Optical excitation (L. Young et al.)
Two-photon process (1 UV lamp, 1 IR laser)Excites only Kr*Potentially higher efficiency
laser cooling
5p[5/2]3
5s[3/2]2
811nm
~10 eV
Kr energy levels:
Can’t laser cool in ground state
Use metastable state ~ 30 s
Effective ground state
Chad OrzelUnion College Physics ATTA
Atom Source
Zeeman SlowerMOT
Excite Kr atoms to 5s[3/2]2 metastable state
Trap in beam-loaded MOT
Basic technique:
Atom Trap Trace Analysis (Z.-T. Lu et al., Argonne)
Single-atom detection of laser-cooled Kr*
Used to measure 85Kr abundance in natural Kr
APD
Detect single atoms by trap laser fluorescence
Count trapped atoms to determine abundance
(data from Lu group)
Chad OrzelUnion College Physics
Proposal: Use ATTA technique to measure Kr in Xe, Ne
Load source with Xe or Ne
Compare to sample with known Kr abundance
Trap, count 84Kr (57% abundance)
ATTA and Kr
Sensitivity:
Source consumption: 7 × 1016 atoms/s
MOT capture efficiency: ~10-7
Kr* sensitivity (3hrs integration): 3 × 10-14
Assumptions:
1) Same Kr* excitation, capture efficiency
2) Metastable fraction of 10-3-10-4 in beam
May be modified by interspecies collisions
May be improved with different excitation method
Typical for discharge source
Not expected to be a problem
Chad OrzelUnion College Physics Selectivity
Trapping depends on resonant photon scattering
More than 100,000 photons to trap atoms
Essentially no off-resonant background
No signal from other elements
(Figure from Lu group at ANL)
Chad OrzelUnion College Physics Contamination
laser cooling
5p[5/2]3
5s[3/2]2
811nm
~10 eV
Low sensitivity to background
Only metastables detected
10 eV internal energy
Only contamination in source matters
1) Outgassing:
Minimize with bakeout
~ 10-16 level (estimated)
2) Cross-contamination:
Discharge source embeds ions in wall
“Memory Effect” in comparing samples
Eliminate by using optical excitation
Knocked out by later impacts
[0) Sample Handling: avoid contamination]
Chad OrzelUnion College Physics Future Prospects
1) Other species
Same technique works for other noble gases
39Ar background evaluation
Ar*, Kr* wavelengths <1nm apart
Use same lasers, optics
2) Continuous monitoring?
~3hrs integration for 10-14 sensitivity
Faster for lower sensitivity: minutes
Use ATTA system to monitor purity during production?
Check for leaks during operation?
3) …? (Rn? 39Ar/Ar? Other systems?)
Chad OrzelUnion College Physics Conclusions
Atom Trap Trace Analysis can be used to measure Krypton levels in other rare gases by detecting and counting single Kr atoms in a magneto-optical trap.
High sensitivity: ~ 10-14
ATTA offers:
Low background
Fast measurement (continuous monitoring?)
Independent measurement technique
Complementary to techniques used for production of high purity gases
(see also: astro-ph/0406526)
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