Fundamental Neutron Physics at the SNS

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Technology RD for the Neutron/Anti-Neutron
Oscillation Experiment at DUSEL Geoff Greene
University of Tennessee Oak Ridge National
Laboratory Nov, 2007
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• The major technical challenges to the vertical
  
• oscillation experiment do not require advances on
  
• established engineering practices in most
  required
• technologies.
• However, considerable study will be required to
  allow
• the development of a preliminary design and
  credible
• cost estimate.
• Outstanding issues include
• Reactor and Cold Source Design
• Vacuum system
• Conventional engineering in vertical shaft

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• There are two area in which we must extend the
  state
• of the art to attain our experimental goals
• 1. Magnetic Shielding
• 2. Neutron Reflection Optics

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Magnetic Shielding

• Neutron-AntiNeutron oscillation is suppressed in
  the presence of a magnetic field.
• In order to allow a free oscillation for time t,
  we must have a magnetic field which satisfies
• Where B is the average magnetic field and µn is
  the neutron magnetic moment.
• For the proposed DUSEL experiment, this implies a
  field of Blt10 nT over the neutron flight path.
• Such fields are routinely obtained in relatively
  small (few m3) volumes, but the large size of the
  n-nbar apparatus poses a challenge.
• Fields at this level were obtained using a two
  layer shield over a flight path of 80m in the
  earlier ILL free n-nbar oscillation experiment.

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Issues

• The inclination of the Earths field at DUSEL is
  about 70, so the dominate component of the
  magnetic filed is vertical.
• An infinitely long cylindrical shield will
  saturate and provide no effective shielding.
• The vertical component of the field must canceled
  using solenoidal windings.
• A reliable method for the measurement of the
  field in situ must be incorporated into the
  experimental methodology.

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Magnetic Shielding Measurements for Neutron spin
rotation experiment at NIST
Photo courtesy W.M.Snow
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Magnetic Shielding for N EDM Experiment at
Petersburg Nuclear Physics Institute
Photo courtesy W.M.Snow
Photo courtesy A. Serebrov
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Proposed RD

• Identification of scheme for suppression of both
  magnetic fields in the presence of the µ-metal
• Identification of an assembly, annealing, and
  demagnetization strategy
• Measurement of the ambient magnetic field at the
  proposed location
• Preliminary design of mechanical design of the
  vertical shield
• Analysis of the loss of sensitivity of the
  oscillation measurement to residual magnetic
  fields
• Investigation of materials compatibility for the
  vacuum chamber.

Photo Institut Laue Langevin
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Focusing Reflector is Required to Optimize
Neutron Beam Phase Space
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Neutron Reflection from Matter
Neutrons will undergo complete external
reflection from a polished surface for most
materials Ni or 58Ni are particularly useful as
a neutron mirror material For most neutron
beams this means
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Reflectivity of Neutron Mirror

• A Simple Neutron Mirror has Nearly Unit
  Reflectivity
• Up to a Maximum Critical Angle

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n
Reflectivity
q
q
critical
Angle
Qcritical ? 2x10-3/Å for 58Ni
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A Multilayer can add Psuedo Bragg Peak
n
1
q
Reflectivity
q
?Bragg
critical
Angle
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Additional Multilayers add More Peaks
1
Reflectivity
q
critical
Angle
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The Supermirror Extends the Effective
?critical

• Commercial Supermirror Neutron Guides
• are Available With m 3 - 4

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1000 layers
Reflectivity
q
q
m
critical
critical
Angle
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Commercial Supermirrors are available with m3.6
number of layers scales as m4
QA Inspection Data for SNS Super Mirror for
Beamline 13
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Neutron Reflectors are Items of Commerce
Guide Installation
Focusing Guide
Photos Swiss Neutronics, LANL, ILL
Guide Manufacturers Catalog
Guide Array in Reactor
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Issues

• Current neutron guide coatings are optimized for
  many reflections
• High more important than high critical angle.
• Current guide construction methods are not easily
  scalable
• DUSEL experiment will require 300-800 m2 of
  super mirror guide.
• Current world wide capacity is estimated at
  500-1000 m2/year
• No commercial source in the US.
• Should we purchase guide or fabricate our own
  multi-layers?

Photo Institut Laue Langevin
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KEK has produced prototype supermirrors with m6
Useful neutron flux scales roughly as m2
Image Courtesy H. Shimizu
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Proposed RD for Mirror Reflector

• Monte Carlo simulations of neutron optics to
  determine optimal design for size, shape,
  reflectivity, and cost?
• Explore in detail possible strategies for super
  mirror fabrication
• Commercial purchase from existing vendor?
• Development of in-house capability?
• Partnership other institutions?

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End of Presentation