1. 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

2. Chad Orzel Union 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?

3. Chad Orzel Union College Physics Very small velocity change 84 Kr =811 nm  v=5.8 mm/s Lots of photons (10 15 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

4. Chad Orzel Union College Physics Atom Trapping Add spatially varying magnetic fields: confine atoms Magneto-Optical Trap (MOT) Collect up to 10 9 atoms, T ~ 100  K (Na MOT at NIST) Trapping due to light forces Constantly scattering photons

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

6. Chad Orzel Union College Physics 85 Kr Contamination 85 Kr:  1/2 = 10.76 yr  -decay abundance: 2.5 × 10 -11 in natural Kr activity: 1.5 Bq/m 3 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

7. Chad Orzel Union College Physics Metastable Krypton electron impact Electron impact excitation RF, DC plasma discharge sources Low efficiency (10 -3  10 -4 ) 5s[3/2] 1 5p[5/2] 2 124 nm Kr lamp 819 nm laser 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

8. Chad Orzel Union College Physics ATTA Atom Source Zeeman Slower MOT 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 85 Kr abundance in natural Kr APD Detect single atoms by trap laser fluorescence Count trapped atoms to determine abundance (data from Lu group)

9. Chad Orzel Union 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 84 Kr (57% abundance) ATTA and Kr Sensitivity: Source consumption: 7 × 10 16 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

10. 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)

11. Chad Orzel Union 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]

12. Chad Orzel Union College Physics Future Prospects 1) Other species Same technique works for other noble gases 39 Ar 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? 39 Ar/Ar? Other systems?)

13. 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. 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)