Barium tagging for EXO

CAP meeting, QuebecJune 10, 2008Etienne Rollin

Enriched Xenon Observatory

EXO is designed to measure the effective neutrino mass <m

ββ> by measuring the neutrinoless double beta decay

(0νββ) rate for 136Xe.

Source: arXiv:0708.1033

Limit of the half-life is > 1.9x1025 years,which corresponds to m

ββ < 0.35 eV.

see Kevin Graham's talk on Wednesday morning for more details on the experiment, or http://www-project.slac.stanford.edu/exo/

file:///home/erollin/Physics/Exo/.eqe/library/eqe-latex-72986.png

Barium Tagging Principles

D3/2

P1/2

S1/2

649.8693nm30%

493.5454nm70%

Laser

About 108 photons per second are emited from one barium ion when we saturate the S

1/2 to the P

1/2 with a laser.

The P1/2

state can decay into the D3/2

state which stops the fluorescence. A second laser deshelves the ion.

Barium ion spectroscopy

Ba++

e-

e-

Xe

0νββ

Tagging in high pressure gas

● Performed in the active volume.

● “Trapping” is done by collisions with xenon.

● Linewidth increases with pressure.~100 mW to saturate the transition at 1 atm.

● Rayleigh scattering is very bad at atmospheric pressure.~1012 photons/s/cm scattered at 100 mW.

Barium tagging protocol

D3/2

P1/2

S1/2

The blue laser is continuously on while a red sensitive PMT counts red photons. The D state decays to the ground state.

The whole cycle might last ~30 ns at atmospheric pressure, so 30,000,000 red photons per second per ion are emitted.

The blue light from the laser is filtered out.

“Your final protocol seems reasonable.” -Bill Baylis

“Your estimate of a 1 ns lifetime sounds right.” -Warren Nagourney

“I would not be surprised if your quenching rate is in the ns regime given the high pressures you are working with.” -Alan Madej

SNOLABTwo Coherent 899 dye ring lasers

Sabre UV laserVerdi laser

~150 mW of blue light~300 mW of red light

SNOLAB

Sparking apparatus

Barium metal tipsCopper electrodes

(gap of 6 mm between them)

Sparks in low pressure argon

With ~0.03 atm of argon

Sparks in low pressure argon

Barium + ArgonArgon

P1/2

to S1/2

P1/2

to D3/2

Intensity vs. Wavelength measured with a spectrometer

Resonance achieved

Fluorescence inducedby the laser beam

` D3/2

P1/2

S1/2

649.8693nm30%

493.5454nm70%

Laser

Blue fluorescence from the red laser

FluorescenceRayleighscattering

D3/2

P1/2

S1/2

Laser

Linewidth

Linewidth of the 493.5454 nm resonance

σ(650/614)=0.0006±0.0002 nmσ(650/553)=0.0006±0.0002 nmσ(650/455)=0.0006±0.0002 nm

At ~ 0.03 atm

Linewidth

Natural linewidthΔEΔt ~ ħ/2lifetime of the P

1/2 state ~10 ns

ΔE ~ 2x10-7eVE ~ hc/λ ~ 2 eVΔE/E = Δλ/λ ~ 1x10-7

Doppler broadeningΔλ/λ = √kT/mc2 ~ √0.025/(137x938,000,000) ~ 4x10-7

Pressure broadening18.6 MHz/torr, at 0.03 atm, Δν ~ 5x108 HzΔν/ν = Δλ/λ ~ 5x108/5x1014 ~ 1x10-6

Laser width0.00015nm

σ(493nm)*calculated

~ 0.0006 nm

σ(493nm) measured

~ 0.0006 nm

* Δλ => FWHM => 2.35 σ

Branching Ratio

Using 128 spectra, we measured the branching ratio of the P

1/2 to decay

to the S1/2

state.

BRmeasured

= 74% ± 4%

BR*calculated

= 73% ± 2%

* A. Gallagher, Phys. Rev., 157(1), 24-30 (1967)

D3/2

P1/2

S1/2BR

New Apparatus

Baffles

Spark gap

Laserwindow

PMTwindow

Field rings

10 cm

New Apparatus

Future measurements

● Lifetime of the ion vs. pressure in different quencher gas species.

● Lifetime of the D3/2

and P1/2

state vs. pressure.

● Linewidth and position vs. pressure.

● Branching ratio vs. pressure.

● Sensitivity studies (single ion?).

Conclusion

● Barium tagging would eliminate natural radioactivity background, removing any doubt about a positive measurement of neutrinoless double beta decay.

● Fluorescence at a fraction of an atmosphere has been observed for an ion cloud.

● The apparatus has been modified to produce ions at higher pressure (up to 1 atm) and drift them far away from the spark light.

EXO Collaboration