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Developing a Lithium Doped Glass Detector to Measure the Electric Dipole Moment of Ultra
Cold NeutronsLori Rebenitsch
University of WinnipegJune 17, 2014
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Overview Neutron electric dipole moment (nEDM) Experimental set-up UCN counter
o Requirementso Specifications
Please note that captions appear in top right corner of slides!
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Neutron Electric Dipole Moment
Baryogenesiso Baryon/antibaryon asymmetry in the early universeo Sakharov conditions (Sakharov, 1967)
• Baryon number violation • CP-symmetry violation• Interactions outside of thermal equilibrium
Standard model has small sources of CP-violationo CKM matrix - quarkso Electric dipole moment of fundamental particles
Extensions increase CP-violation Possible EDM in neutrons due to quark structure
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Ultra Cold Neutrons Properties
o < 3mKo ~7m/so Subject to gravityo Polarizable
Find neutron electric dipole moment (nEDM) by finding Larmor frequency
| e-cm for current experimental limit (Harris et. al) | e-cm for new physics | e-cm for CKM in Standard Model | e-cm for
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nEDM Experiment Requirements
o High UCN densityo Stable magnetic shieldso High counting efficiency
Goalo UCN/cycleo Comagnetometer ~10fTo nEDM measurement e-cm in first phase for new physics
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Neutron EDM Facility Graphic of planned facility
proto
ns
UCNdetect
or
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Detector Requirements Handle rates >1.3 MHz for periods of few seconds Reject background
o Gammaso Thermal neutrons
0.05% efficiency stability Normalize UCN density
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Lithium Doped Glass
Based on design from PSI (Ban et. al)
Dual layered scintillating glasso Optically bonded
• Top layer – lithium depleted• Bottom layer – lithium doped
o Capture full scintillation path
Right: Autodesk image of detector. Bottom: Diagram of dual layer glass and how UCN capture and scintillate.
𝐿𝑖+𝑛→𝛼(2.05𝑀𝑒𝑉 )+𝑡(2.73𝑀𝑒𝑉 )❑6
Lithium glass
Lightguides
PMTs
UCN
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Measurement
Visibility of fringes
Statistical uncertainty
Formula for determining
Ramsey resonance
The four points on the graph are used to find the Ramsey resonance frequency.
𝑁𝑢𝑝
𝜔
𝑁𝑢𝑝𝑚𝑎𝑥
𝑁𝑢𝑝𝑚𝑖𝑛
Γ
𝜔𝑟
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High Rates DPP-PSD
o Generates analysis faster than the computer
o Describes signal in few variables in place of full waveform
o Reduces computation load on DAQ DAQ
o Minimal computingo Saves
• PSD• Slow control
Analyzero Separate from DAQo Puts data into TTree
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DPP-PSDLeft: Inverted signal from Am source. Longer events are α’s and short events are γ’s. Right: Waveform with corresponding DPP-PSD gates. The long and short gates collect charge when open.
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Background Rejection
By comparing the PSD variables, gamma background can be removed from data.
Comparison of the gate values for a thermal neutron source, . Note how the gamma background has a strong 1:1 ratio while the neutrons do not.
neutrons
gammas
Comparison of PSD Charge Integration Gates
Short
Inte
gra
tion G
ate
(A
DC
)
Long Integration Gate (ADC)
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Stability of Efficiency
Environmental – long term efficiencyo Glasso Temperature o Tests in progress
Pile-upo Occurs 2-3MHz
between pulseso Pile-up events have
• Higher than average long gate value
• Average short gate value
• Can be flagged and recounted
Top right: Scope example of a double pulse generated from pulser. Bottom right: Rate of pile-up with respect to frequency of double pulse.
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Normalize UCN Density UCN are produced in
cycles Number of UCN vary per
cycle Example to normalize
UCN densityo Utilize volume below cell
for UCN density estimationo Factors
• Volume below cell is ~10x larger
• Volume presents greater probability of pile-up
• Throttle the UCN and/or account for pile-up
detector
analyzer foil
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Future Work Detector in process of being built Stability tests RCNP proposal to take data with UCN spallation
source this fall
University of Winnipeg, University of Manitoba NSERC, CFI
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Thank you