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W. M. SnowPhysics Department
Indiana University/IUCFEDM collab meeting
Monitoring the Cold Neutron Beam
(During Experiment)
What to measure (fluence [=n/sec], polarization)
How to measure (some ideas)
Where to measure (upstream, before cryo/cell entrance, after cell, I hope not inside the cryostat!)
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Parameters
~5x106 n/A/cm2/s of 9A neutrons
Polarization: >95%
Upstream polarizer/splitter
Separate beams into ~8cm x 10 cm EDM cells
Phase space @apparatus ~ 3* c
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Can we monitor the beam downstream of cryostat?
6.4 m
2.5 mEven if beam/polarization can get out,9A beams will strongly overlap in space by end of apparatus without internal guide-> at most one
could measure fluence
of sum of beams
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Measure Between end of Guide and cryo vacuum? [space, magnetic constraints]
Collimator
Be Foil 4°K ShieldBe Foil 50°K Shield
Be/Teflon Multi-Foil Window at LHe/Vacuum Interface
Measurement Cell
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Beam Monitor Example (Various Possibilities)
n+ 3He p + t + 765 keV ionizes gas mixture [3He +4He(~.5 atm) +N2(~.5 atm)]
Works in current mode for NPDGamma
Other (nonmetallic, nonmagnetic) possibility:6Li scintillator
PANIC’05 BL
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How can neutrons be polarized/analyzed?
B
B gradients (Stern-Gerlach,sextupole magnets)electromagneticF=()B
Reflection from magneticmirror: electromagnetic+strongf=a(strong) +/- a(EM) with | a(strong)|=| a(EM)|f+=2a, f-=0
B
Transmission throughpolarized nuclei: strong≠ - T ≠ TSpin Filter:T=exp[-L]
L
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Polarized 3He Spin Filter
Dielectric mirror on 1mm thick fused silica
Polarizer cube
Collimating lens
mirrormirror
mirror
3He Cell
diode laser
fiber-optic cable
Dielectric mirror on 1mm thick fused silica
quarter-waveplatequarter-wave
plate
3He cell in magnetically shielded solenoid Apparatus for on-line optical pumping
For 9A neutrons, cell~few cm thick, P~1 atm reasonable parameters(spin exchange): on-line pumping, oven, optics,etc. or circulation (metastability exchange): flow-through system <1E-3/cm B/B uniformity required
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• Neutrons are polarized through
• spin-dependent scattering from
• magnetized mirrors
• Polarization: ~98%
• transmission: ~35%
28 cm
White NeutronBeam
Magnet Box
Plate CurvatureRadius ~ 10m
polarized NeutronBeam
“Supermirror” Neutron Polarizer/Analyzer
Permanent magnet box
B
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More Compact Possibility for Polarization Analyzer?
1) Long wavelength cut-off filter: stack of Ni/Ti supermirrors deposited on~100 Si wafers. c = 3* c(Ni); c(Ni) = 21 mrad/nm and oriented at a small angle wrt beam
Reflected beam
Transmitted beam [kill this with absorber]
Incident beam
Supermirrors
In our case: eat one spin state with absorber on rear of mirror surface, use remanent polarizing SM as for incident beam polarization, but will it be nonmagnetic enough
to not interfere with B requirements?
~few cm thick
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Remanent Polarizing Supermirrors Exist
• Retains its magnetization in a weak opposite field.
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Monitoring Fluence and Polarization Between Guide/Cryostat Vacuum (Do we want to do this?)
Li glass
SM Analyzer
Li
glass
Cryo
Vacuum
Beam 1
Beam 2
Move IC1, IC2, SM analyzer in and out of beams horizontally
Nonmagnetic enough? Reliability/Access? But measures just before entrance and no guide cut
Adiabatic Spin Flipper Upstream
~20 cm?
Not shown: n spin transport into vac
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Adiabatic flipper with B gradient upstream
x neutronsy
zI
Bz
B
x
Precession in effective field in rotating frame: B2+BRF2
Rotation frequency of effective field seen by neutron: v/L
B
Adiabatic Condition: Easy to meet for cold neutrons
L1/peaksN
sin21+k2]
Adiabatic parameterk=LB/v>>1
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Monitoring Fluence and Polarization Further Upstream
Similar ideas will work
BUT need to cut the guide->flux loss, moving objects in vacuum,…
Insert a crystal to diffract 9A beam for analysis?
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Ferromagnetic Shield/RF: do we have a problem with metal objects messing with RF fields?
Inner Dressing CoilOuter Dressing Coil
50°K Shield
4°K Shield
Superconducting Lead Shield
Ferromagnetic Shield
B0 cosθ Magnet
Gradient Coil
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EDM Experiment Horiz. Section View 2
Light Guide
Measurement CellGround ElectrodeElectric Field Return
HV Generator
HV Electrode Support