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
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
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
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.