Counting Atoms for Astrophysics: Atom Traps, Neutrino Detectors,

and Radioactive Background Measurements

Chad Orzel Union College Dept. of Physics and Astronomy

D. N. McKinsey Yale University Dept. of Physics

Students: M. Mastroianni R. McMartin M. Lockwood J. Smith E. Greenwood M. Martin M. Mulligan J. Anderson C. Fletcher

$$: Research Corporation NSF

What We’re Doing: Using Atom Trap Trace Analysis for Radioactive Background Evaluation

Measure krypton contamination in other rare gases

Fast measurement: Kr/Rg ~ 10-14 in only 3 hours

What We’re Not Doing: NOT a Purification Method

Complementary to purification efforts

Why Are We Doing This, Anyway?

Summary

Who Cares About Krypton?

Astrophysicists!

Next Generation of Neutrino Detectors:

Liquid Rare Gas Scintillation

85Kr is a source of background noise:

Eliminate all krypton

Neutrinos

Fundamental particles

Incredibly numerous: ~300/cm3 from Big Bang

~40,000,000,000/cm2/s from the Sun

Very small mass: Electron neutrino: me < 3eV/c2

Tau neutrino: m < 15 MeV/c2

(electron mass: ~500 keV/c2)

Weak interactions:

Interact only through weak nuclear force

Neutral particles

Extremely Difficult to Detect

Neutrino Detection

Radiochemical: e + 37Cl 37Ar + e-

e + 71Ga 71Ge + e-

Neutrino interaction converts neutron to proton

Change element Ray DavisNobel Prize 2002

Problem: Very slow readout (every few months)

No real-time information

Neutrino Detection 2

Scintillation Detectors:

Neutrino collision produces light flash

Electron: Nucleus:

Allows real-time detection, energy measurement

Problem: High energy threshold (5-8 MeV)

Masatoshi KoshibaNobel Prize 2002

Detect light with phototubes

Sudbury Neutrino Observatory

Top-of-the-Line Scintillation Detector:

1000 tons heavy water (D2O)

9600 Photomultiplier Tubes (PMT’s)

Detect Cerenkov light

Location, Location, Location:

Creighton Mine, Sudbury, Ontario

2070 m (6800 ft) underground

(Screen out background radiation)

http://www.sno.phy.queensu.ca/

Solar NeutrinosHow do detectors stack up? Gallium Chlorine

Radiochemical: Ga/Cl

Low threshold

No time resolution

Water

Scintillation: H2O/D2O

Time, energy resolution

High threshold

Need a better detector…

Neutrino Detection: The Next Generation

Use some other substance as scintillator

Want: Time resolution

Low threshold

CLEAN: Cryogenic Low EnergyAstrophysics with Noble gases

http://mckinseygroup.physics.yale.edu/CLEAN.html

(astro-ph/0402007)

~100 tons of liquid neon

XMASS: ~ 20 tons of liquid xenon

CLEAN

http://mckinseygroup.physics.yale.edu/CLEAN.html

(astro-ph/0402007)

Advantages of liquid rare gases:

1) High yield

Ne: = 80nm, 15,000 photons/ MeV

2) Self-shieldingDense liquid, absorbs radiation

3) Little or no intrinsic radioactivity

Scintillation detection with low threshold

CLEAN SensitivityGallium Chlorine Water

0.01 0.1

CL

E

A

N

Krypton Contamination

Problem: Krypton Contamination

85Kr: ½ = 10.76 yr

-decay at 687 keV

Looks like detection event in energy range of interest…

40 ppb

Rare isotope: 2.5 × 10-11

Major source of background

Need to remove all Kr from detector

Krypton Removal

Need extremely high purity

Kr/Ne ~ 4 × 10-15 (any isotope)85Kr much lower

~100,000 atoms in full CLEAN

Difficult to purify gas to this level

Kr chemically inert

Distillation, Charcoal Filter

Xe distillation, Takeuchi et al.

~3.3 ppt Kr

Difficult to measure purity

Gas chromatography

Accelerator mass spectrometry

Days or weeks to measure

Atom Trap Trace Analysis

Technique developed by Z.-T. Lu and colleagues at Argonne National Laboratory

Used to measure 85Kr abundance

Used for radioisotope dating

Trap, detect single atoms of rare isotopes

Determine abundance by counting

Proposal: Use ATTA to measure Kr in Ne or Xe

7 × 1016 atoms/s in 3× 10-14 abundance in 3 hrs (1 atom detected)

Load source with ultra-pure Ne, Xe

Detect single Kr atoms

Laser Cooling and Trapping

Use light forces to slow and trap atoms

Photons carry momentump

Transfer to atoms on absorptionp

Very small velocity change84Kr=811 nmv=5.8 mm/s

Lots of photons (1015 per second)

Room-temperature velocity ~ 300 m/s

100,000 photons to decelerate

Use scattering force to slow thermal motion

Doppler Cooling

o

Tune laser to lower frequency (red) < o|e>

|g>

Stationary atoms do not absorb

Atoms moving toward laser see blue shift

Absorb photons, slow down

Exploit Doppler effect to selectively cool atoms

Use single laser beam to slow and stop beams of atoms

Use pairs of beams to cool sample

Reach microkelvin temperatures (v~10 cm/s)

Magneto-Optical Trap

Add spatially varying magnetic fields

Confine atoms to small volume

Trapping due to photon scattering

108 photons/s per atom

(Na MOT at NIST)

Detect trapped atoms using fluorescence

ATTA

Count trapped atoms to determine abundance

APD

Detect single atoms by trap laser fluorescence

(data from Lu group)

Atom Source

Zeeman SlowerMOTATTA Technique

Prepare Kr* atoms in metastable state

Slow beam

Trap atoms in MOT

Selectivity

(Figure from Lu group at ANL)

Extremely selective technique

Need to scatter 105 photons

No off-resonant background

85Kr ~ 10-11

81Kr ~ 10-13

83Kr ~ 0.11

Trap only one isotope

Trap over ~ 30 MHz

Out of 370 THz

Only Kr atoms detected

Background

laser cooling

5p[5/2]3

5s[3/2]2

811nm

~10 eV

Kr atoms trapped in metastable state

~10 eV above ground state, ~30 s

Ground-state Kr not trapped

not detected

Atoms only excited in source

Only contamination in source matters

0) Sample Handling

1) Outgassing: Keep Kr out of system. Background ~10-16 level

2) Cross-contamination: Kr from calibration samples embedded in source

Eliminate with optical excitation

Sensitivity

Procedure:

1) Load system with Ne or Xe

2) Set lasers to trap 84Kr (57% abundance)

3) Count atoms, compare to input flux

Typical source consumption:

7 × 1016 atoms/s

Trapping efficiency:

10-8

One atom in three hours: 3 × 10-14 abundance

Apparatus

Metastable Source

145 MHz RFPlasma discharge

Zeeman SlowerTwo-stage magnet

Decelerates beam

Trapping Chamber

Undergraduate student for scale:

Ryan McMartin ‘05

Optical Excitation

5p[5/2]3

5s[3/2]2

811nm

~10 eV

Metastable excitation methods

1) Electron impact: RF plasma discharge

Simple, robust

Low efficiency (10-4 – 10-3)

“Memory Effect” cross-contamination

5s[3/2]1

5p[5/2]2

124 nm

Krlamp

819 nmlaser

2) Two-photon optical excitation

124 nm lamp, 819 nm laser

Excite only Kr*

Potentially higher efficiency (10-2) Improved sensitivity

Eliminate cross-contamination Lower background

Optical Excitation

124 nm lamp

Kr inlet

819 nm laser

Mike Mastroianni ‘07

Future Prospects

1) Other Species

Same technique works for other rare gases.

39Ar evaluation Ar*, Kr* < 1nm apart: use same optical system

2) Continuous monitoring

3hrs for 10-14 level

Less time for lower sensitivity (XENON): continuous purity check?

3) Other systems?

3He/4He?

Conclusions

Next generation of neutrino detectors will require ultra-pure rare gases

Can use Atom Trap Trace Analysis to measure Kr contamination

High sensitivity, low background

Independent of purification method

Fast measurement (3 hrs for 3×10-14)

Complement to experimental efforts to purify gases

(see also: astro-ph/0406526, Nucl. Instr. Meth. A 545, 524 (2005))