Lifetime Measurements from
the Metastable He* State
Sean Hodgman, Lesa Byron, Rob Dall,
Ken Baldwin, and Andrew Truscott
ACQAO and RSPhysSE, ANU, Canberra, Australia
Outline
• Motivation
• He* experiment
• 23P1 lifetime
• 23S1 lifetime
• 23P2 lifetime
• Conclusion
Motivation
Energy
23P0
1
2
2.3 GHz
Cooling
and
Trapping
Laser
~5.7ms
23S1
~3s
58.4nm
~8000s
58.4nm
62.56nm
11S0
•Transition lifetimes of He
important tests of QED
•Many not measured
experimentally
•23S1 metastable state
especially important
•Longest lifetime of any
atomic excited state
•Main aim to measure
23S1 metastable lifetime
Metastable Lifetime
• Transition doubly
forbidden
• Decays via M1
radiation
• Emits 62.6 nm
(19.8 eV) XUV
photon
• 30% uncertainty in
only previous
Significant improvement needed! measurement
13000
12000
Theory
Lifetime (s)
11000
[11
]
10000
9000
Experimental
Krause
Feinberg
& [7]
Sucher
Woodworth
&[9]
Moos
Johnson
et.[2]
al.
8000
7000
Drake
[6]
Johnson
[8]
& Lin
6000
Lach &
Pachucki
[10
Lin
et. al.
]
5000
1970
[3]
1975
1980
1985
1990
Year
1995
2000
2005
2010
Ultracold He* Experiment
Using Cold He* to Measure
Metastable Lifetime - Easy?
• As simple as trapping He* atoms and
monitoring XUV photon emissions?
• Not quite…
• 3 major issues:
–Trap lifetime only ~10s
–Require precise knowledge of detector
efficiency, atom number, volume etc
–Must ensure no stray counts on detector
• These are challenging!
Relative Measurement
Energy
23P0
1
2
P1 laser
Cooling
and
Trapping
Laser
~5.7ms
23S1
~3s
58.4nm
~8000s
58.4nm
62.56nm
11S0
• Measure 23S1 transition
rate relative to fast
transition
• 23P1 → 11S0 transition
rate ~ 200Hz
• No experimental
measurement
• First measure 23P1
transition rate accurately
23P1 Method
R. G. Dall et. al., PRL 100, 023001 (2008)
• Load He* atoms into
MOT via DC discharge
source, collimator and
Zeeman slower
• P1 Laser pulsed on to
populate 23P1 state
• MOT beams recapture
atoms
• Repeated as atom
number decays
• Monitor
ions
to
produce decay curve
23P1 Method
P1 off
P1 on
R. G. Dall et. al., PRL 100, 023001 (2008)
• 23P1 population
measured with
InGaAs PD
• 23P1 – 11S0 decay
dominant trap loss
mechanism
• Loss due to
background on much
larger time scale
23P1 Results
23P1 measured
transition rate
= 177 + 8 Hz
R. G. Dall et. al., PRL 100, 023001 (2008)
• Experimental
uncertainty = 4.4%
• Mainly shot-to-shot
variations in MOT
population
and
laser intensity
23P1 Result vs Theory
190
23P1 measured
transition rate
= 177 + 8 Hz
Theory
Experimental
Transition Rate (Hz)
185
180
Lach & Packucki
Drake
175
Johnson et. al.
23P1
170
165
160
1970
1980
1990
Year
R. G. Dall et. al., PRL 100, 023001 (2008)
2000
2010
• Confirms leading
theoretical
predictions
• Need uncertainty
~1% to
distinguish
theory
calculations
Metastable Lifetime
Energy
23P0
1
2
P1 laser
Cooling
and
Trapping
Laser
~5.7ms
23S1
~3s
58.4nm
~8000s
58.4nm
62.56nm
11S0
• Can now use 23P1 – 11S0
transition rate to
measure metastable
lifetime
• Direct measurement of
photon flux requires
detector
• This needs shielding to
remove background
counts
Shielding Detector
• Enclosed channeltron
in metal shielding
• 150nm thick Al foil
allows transmission of
XUV photons
• Stray atoms, ions and
photons blocked
• 2 foils necessary to
ensure no pinholes
• Background
count
rate < 0.02 Hz
Metastable Lifetime Method
Shielded
channeltron
detector
150nm
Al foil
XUV
photons
Trapped
He* atoms
• Load He* atoms
into MOT
• Transfer ~ 108
He* atoms at 200
µK into magnetic
quadrupole trap
• Measure XUV
photons emitted
from 23S1
transition for 0.5s
using channeltron
Metastable Lifetime Method
Shielded
channeltron
detector
150nm
Al foil
XUV
photons
Trapped
He* atoms
•Pulse on laser to
saturate 23P1 transition
•23S1 lifetime found
from:
– Relative UV photon
flux from 2
transitions
– Previously
determined 23P1
transition rate
– Saturation of 23P1
transition splits
population 50/50
between levels
Measured UV photon rate (Hz)
56,000 Runs Later…
•Statistical error 1.7%
•23P1 lifetime 4.4%
•Systematic error 4.7%
including contributions from:
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
0.1
0.2
0.3
t (s)
0.4
0.5
0.6
– Transmission and QE
differences for
different wavelength
photons (3.1%)
– Error in saturation of
23S1 laser (2.3%)
– Population not
constant over 50ms
time bin (2.1%)
•Total uncertainty =
6.4 %
•23S1 transition rate = 1.26(8) ×10-4 Hz
•23S1 lifetime = 7920 + 510 s
Metastable Lifetime Results
•Result agrees with
leading theoretical
calculations
13000
12000
Theory
Lifetime (s)
11000
Experimental
Krause
10000
9000
Feinberg
& Sucher
8000
Drake
7000
Johnson
et. al.
Johnson
& Lin
6000
5000
1970
•Improves by factor
of 5 on previous
measurement
Woodworth
& Moos
Lach &
Pachucki
Lin et. al.
1975
1980
1985
1990
Year
1995
2000
2005
2010
•Again need error
~1% to distinguish
23P2 Method
Energy
23P0
1
2
P1 laser
Cooling
and
Trapping
Laser
~5.7ms
23S1
~3s
58.4nm
~8000s
58.4nm
62.56nm
11S0
• Load He* in MOT
• Measure XUV photons
emitted from 23P2 to ground
state for 50ms
• Compare channeltron signal
with PD signal of IR
emissions
• At end of cycle pulse laser
on to saturate 23P1 transition
• Data averaged from 16,000
experimental runs
23P2 Results
20
18
Fitted transition rate= 0.322 Hz
Stat error = 1.3%
P1 error = 4.4%
Total error = 4.6%
Count Rate (Hz)
16
14
12
10
8
6
4
2
0
0
0.01
0.02
0.03
0.04
0.05
0.06
t (s)
• 23P2 transition rate = 0.322 + .015 Hz
•
•
•
•
Statistical error 1.3%
Error in 23P1 population 4.4%
Systematic error significantly lower
Total uncertainty = 4.6%
23P2 Results
0.45
Theory
Transition Rate (Hz)
0.40
Lin et. al.
Experimental
Kundu
et. al.
0.35
0.30
Johnson
et. al.
Drake
Lach &
Packucki
0.25
0.20
Johnson
& Lin
0.15
0.10
1965
Krause
1975
1985
1995
2005
2015
Year
• Again, agrees with leading theory
calculations
23P0 Forbidden Transition
Energy
23P0
1
2
P0 laser
Cooling
and
Trapping
Laser
~5.7ms
23S1
~3s
58.4nm
~8000s
58.4nm
62.56nm
11S0
• Theoretically 0
• Locked laser to 23P0
line and repeated
experimental
method
• Placed upper bound
on transition rate of
~0.01 Hz
• Limited by dark
count rate
Conclusion
Energy
•3 transition rates
measured
accurately
for
1
2
first time
•Upper bound placed on a
P1 laser
4th
Cooling
and
Trapping
•Transition rates span 6
Laser
orders of magnitude
5.66ms 23S1
3.11s
•All confirm leading
58.4nm
7920s theoretical calculations
58.4nm
23P0
2.3 GHz
62.56nm
11S0
ANU He* BEC group
Lesa Byron (doesn’t
like photos)
Rob Dall (also doesn’t like photos)
Ken Baldwin (…doesn't push
himself up when he does
push-ups, he pushes the
world down)
Andrew Truscott (likes
finding tenuous lookalikes)