New developments for wavelength- shifting fiber (WLSF

New
developments for
wavelengthshifting fiber
(WLSF) neutron
detectors at SNS
Cai-Lin Wang, Kenneth W. Herwig, and
Richard Riedel
Instrument and Source Division
IEEE NSS
Oct.30, 2013
Wavelength-Shifting Fiber Neutron
Scintillator Detector Status
• Characteristics
– Scintillator is ZnS:Ag/6LiF microparticle
– 154 x 7 pixels of 5 x 50 mm2 each
– 0.3 m2 area
• 30 units installed in two SNS diffractometers,
POWGEN and VULCAN
• V3 modules reduce the number of missassigned neutron positions (“ghosting”) to
about 0.1% by changing the fiber encoding
and improving the position algorithm.
Eight modules installed on
POWGEN stands
– Cost is ~30% in counting efficiency
– 10 were installed in POWGEN this summer
• Technology transfer to PartTec Ltd.,
Bloomington, Indiana
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Module in fabrication.
Fiber encoding schemes
V2
G-PMTs
X-PMTs
V3
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Issues
• WLSF detectors cannot have ghosting-free imaging and high detection efficiency at
the same time.
-C.L. Wang et al., RSI 84, 013308 (2013).
• Non-uniform efficiency (variance 30-40%) and spatial resolution (variance~20%)
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Photon diffusion: light cone
measurements
• Scintillation photons generated by thermal
neutrons diffuse 4-10 mm.
• Photon-diffusion increases the neutron
positioning uncertainty.
• Non-uniform spatial resolution is due to the
non-uniform photon-diffusion.
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Detection efficiency
• Efficiency is proportional to the light intensity (photon numbers)
collected by an X-PMT.
• Non-uniformity in efficiency is due to:
- Non-uniform photon transport efficiency
from the scintillator to PMTs.
- Non-uniform photon diffusion
- Shot noise (photon-number fluctuation)
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Centroid Algorithm (CEA)
• CEA: neutron position is determined by the photon-number averaged position.
• Maximum-photon algorithm (MPA) was used traditionally .
Diamond powder, BL7
CEA
Ghosting
MP
A
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Correlation Algorithm (CA)
Diamond, BL7
CEA
• Neutron position is determined
by the maximum correlation
coefficient between the photon
numbers {ni} and LRF.
• The efficiency is 86% of MPA.
MPA
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Efficiency uniformity
• CEA can improve uniformity efficiency by 10-20%.
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Future Work
• Version 4 with more photo-multiplier tube channels
• Reduced ghosting without efficiency loss
• Higher spatial resolution (5 x 10 mm2) required for low- and high-angle use
• Control cost using multi-anode PMT’s
• Search for brighter scintillators (everything gets easier)
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Higher detection efficiency
Reduced -sensitivity
Better spatial resolution
Improved uniformity
• Positioning algorithm development: achieve more uniform spatial
resolution and efficiency, and reduce gamma-sensitivity.
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