Improved Determination of the
Neutron Lifetime
M. Scott Dewey
PSI2013
Physics of Fundamental Symmetries and
Interactions
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Cold neutron beam tn measurement
Neutron beam
Beam fluence
measurement
(
Decay proton counting
volume
)
(
)
ABSOLUTE MEASUREMENT of beam fluence and decay rate
Requires known proton trapping/detection efficiency and known
neutron detection efficiency
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The NIST beam lifetime experiment
a,t detector
Precision aperture
B = 4.6 T
p detector
n
6LiF
deposit
Proton trap
+800 V
Neutron monitor
• Proton trap electrostatically traps decay protons and directs them to
detector via B field
• Neutron monitor measures incident neutron rate by counting n + 6Li
reaction
(a + t)
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2013
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Alpha-Gamma
Determining
tn
Proton rate measured for varying trap lengths
Proton detection efficiency
n + 6Li reaction product counting
Neutron flux monitor efficiency for
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Definition of fluence monitor (FM)
6Li
Detected a + t (
Neutron beam (
deposit
)
)
wavelength
Absorbed neutrons
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Neutron absorption
probability
a, t detection probability5
Neutron
detection
efficiency
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Figure courtesy J. Nico et al.
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The Alpha-Gamma device
HPGe detector
Totally absorbing
10B target foil
Neutron monitor
PIPS detector
with aperture
Alpha-Gamma
device
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HPGe detector
7
l measurement
device
Alpha-Gamma
device
Neutron
monitor
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Thick target
1/ “g/FM”
239Pu
counting
“RPu”
Thin target
1/ “a/g”
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Wavelength
“Rl”
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Uncertainty budget
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Final result
To update lifetime, temporal stability of neutron monitor
(measurements of
and
) must be assessed.
•
- new metrology of lifetime aperture fixture
•
- Bayesian analysis of past and present measurements
of neutron-induced activity from three 6LiF foils
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Our measurements show that both quantities are zero, and thus:
Can also update the
based lifetime result with ENDF-B/VII and corrected
:
Independent methods for determining the neutron
monitor efficiency give the same lifetime.
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To improve lifetime fluence determination, we must check the
temporal stability of neutron monitor.
(measurements of
and
)
•
- new metrology of lifetime aperture fixture
•
- Bayesian analysis of past and present measurements
of neutron-induced activity from three 6LiF foils
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Determining
- comparison of metrology
Origin to aperture distance (mm)
Fractional difference in distance squared
Aperture diameter (mm)
Fractional difference in aperture area
Aperture tilt (deg)
Discrepancy driven by aperture diameter
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Determining
- IRMM measurements of
~20 mg/cm2 foil
a.k.a. “20”
“radiometer” foil
a.k.a. “30”
tn foil
a.k.a. “40”
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The data:
Modeled with 3 “true” charged particle rates (
),
a conversion constant ( ), and a mass loss term ( ) that is forced to be
negative
No loss
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Lifetime foil loss
“handling”
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All loss
“evaporation”
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results
• OpenBUGS - Bayesian analysis shows a strong preference for the noloss model. In those instances where a loss model is preferred, the
loss is on average ~0.1% of the lifetime deposit areal density.
• Simple c2 minimization routine - strong preference for no-loss model
• c2 nearly the same for each model, the extra degree of freedom
makes the no-loss the best fit
• Need to decide the appropriate
to use
• Use
based on strong preference for no-loss model
and ~0.1% uncertainty from OpenBUGS result
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Including this change…
Updated
(2013)
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Thank you for your attention!
• A preprint is available on the archive:
http://arxiv.org/abs/1309.2623
• Thanks to:
– Andrew Yue, David Gilliam, Geoff Greene, Sasha
Laptev, Jeff Nico, Mike Snow, and Fred Wietfeldt
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