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)