Transcript Chad Orzel Union College Physics Radioactive Background Evaluation by Atom Counting C. Orzel Union College Dept.

Chad Orzel
Union College Physics
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
$$: Research Corporation
Chad Orzel
Union College Physics
Summary
Atom Trap Trace Analysis (ATTA)
What it is:
Method for measuring Kr contamination
High sensitivity: 10-14 level
Fast measurement: ≤ 3 hrs integration
Independent of production
Use atomic physics techniques
Detect single impurity atoms
What it isn’t:
Not a method for purifying gases
Complementary to purification efforts
What it might be: An answer to yesterday’s question:
What’s the best way to measure Kr in Xe?
Chad Orzel
Union College Physics
Laser Cooling
Use light forces to slow and trap atoms
Photons carry momentum
p
Transfer to atoms on absorption
p
Very small velocity change
84Kr
l=811 nm
Dv=5.8 mm/s
Lots of photons (1015 per second)
Use Doppler shift to selectively cool
Red-detuned laser (w < wo)
Only counter-propagating atoms absorb
Slow, cool beams of atoms
Slow, cool atoms in 3-D  microkelvin temperatures
Chad Orzel
Union College Physics
Atom Trapping
Add spatially varying magnetic fields: confine atoms
Magneto-Optical Trap (MOT)
Collect up to 109 atoms, T ~ 100 mK
(Na MOT at NIST)
Trapping due to light forces
Constantly scattering photons
Chad Orzel
Union College Physics
Apparatus
(m.w.e ~ 1)
Table-top physics
Diode lasers for light source
Standard UHV components
Undergraduate student projects
Relatively inexpensive
Chad Orzel
Union College Physics
85Kr:
85Kr
Contamination
abundance: 2.5 × 10-11 in natural Kr
t1/2 = 10.76
b-decay
yr
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 Orzel
Union College Physics
Metastable Krypton
Kr energy levels:
5p[5/2]2
819 nm
laser
5s[3/2]1
5p[5/2]3
laser
811nm
cooling
5s[3/2]2
electron
impact
~10 eV
124 nm
Kr
lamp
Can’t laser cool in ground state
Use metastable state  t ~ 30 s
Effective ground state
Electron impact excitation
RF, DC plasma discharge sources
Low efficiency (10-3 - 10-4)
Optical excitation (L. Young et al.)
Two-photon process (1 UV lamp, 1 IR laser)
Excites only Kr*
Potentially higher efficiency
Chad Orzel
Union College Physics
ATTA
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
Zeeman Slower
MOT
Atom Source
APD
Basic technique:
Excite Kr atoms to 5s[3/2]2 metastable state
Trap in beam-loaded MOT
(data from Lu group)
Detect single atoms by trap
laser fluorescence
Count trapped atoms to
determine abundance
Chad Orzel
Union College Physics
ATTA and Kr
Proposal: Use ATTA technique to measure Kr in Xe, Ne
Load source with Xe or Ne
Trap, count 84Kr (57% abundance)
Compare to sample with known Kr abundance
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
May be modified by interspecies collisions
Not expected to be a problem
2) Metastable fraction of 10-3-10-4 in beam
Typical for discharge source
May be improved with different excitation method
Chad Orzel
Union 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 Orzel
Union College Physics
Contamination
Low sensitivity to background
Only metastables detected
10 eV internal energy
5p[5/2]3
laser
811nm
cooling
5s[3/2]2
 Only contamination in source matters
[0) Sample Handling: avoid contamination]
1) Outgassing:
Minimize with bakeout
~ 10-16 level (estimated)
2) Cross-contamination:
Discharge source embeds ions in wall
Knocked out by later impacts
“Memory Effect” in comparing samples
Eliminate by using optical excitation
~10 eV
Chad Orzel
Union 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 Orzel
Union 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.
ATTA offers:
High sensitivity: ~ 10-14
Low background
Independent measurement technique
Fast measurement (continuous monitoring?)
Complementary to techniques used for production of
high purity gases
(see also: astro-ph/0406526)