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Radioactive Background Evaluation by Atom Counting

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.

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Radioactive Background Evaluation by Atom Counting

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  1. 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

  2. Summary What it isn’t: Not a method for purifying gases What it might be: An answer to yesterday’s question: Complementary to purification efforts What’s the best way to measure Kr in Xe? 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

  3. Laser Cooling Use light forces to slow and trap atoms Photons carry momentum Transfer to atoms on absorption p p Very small velocity change 84Kr l=811 nm Dv=5.8 mm/s 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 Lots of photons (1015 per second)

  4. 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

  5. Apparatus Table-top physics Diode lasers for light source Standard UHV components Undergraduate student projects Relatively inexpensive (m.w.e ~ 1)

  6. 85Kr: abundance: 2.5 × 10-11 in natural Kr t1/2 = 10.76 yr b-decay activity: 1.5 Bq/m3 in air (1.1 ppm Kr) Kr contamination major source of background counts for liquid noble gas particle detectors Difficult to purify to this level Difficult to measure Kr content at this level 85Kr Contamination Commercial gases: Kr  20 ppb Need: Kr/Xe: 150 ppt or less (XENON) Kr/Ne: 4 × 10-15 or less (CLEAN) Use laser cooling and trapping to measure Kr/Xe or Kr/Ne

  7. Kr energy levels: 5p[5/2]2 5p[5/2]3 819 nm laser laser cooling 811nm 5s[3/2]1 5s[3/2]2 electron impact Kr lamp ~10 eV 124 nm 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 Metastable Krypton

  8. Atom Trap Trace Analysis (Z.-T. Lu et al., Argonne) Zeeman Slower MOT Single-atom detection of laser-cooled Kr* Used to measure 85Kr abundance in natural Kr 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 ATTA

  9. 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 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 Kr* sensitivity (3hrs integration): 3 × 10-14

  10. 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)

  11. Contamination Low sensitivity to background 5p[5/2]3 laser cooling 811nm Only metastables detected 5s[3/2]2 10 eV internal energy ~10 eV 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  Only contamination in source matters [0) Sample Handling: avoid contamination]

  12. 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?)

  13. 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)

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