Presentation on theme: "W. M. Snow Physics Department Indiana University/IUCF EDM collab meeting Monitoring the Cold Neutron Beam (During Experiment) What to measure (fluence."— Presentation transcript:
W. M. Snow Physics Department Indiana University/IUCF EDM 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!)
Parameters ~5x10 6 n/A/cm 2 /s of 9A neutrons Polarization: >95% Upstream polarizer/splitter Separate beams into ~8cm x 10 cm EDM cells Phase space @apparatus ~ 3* c
Can we monitor the beam downstream of cryostat? 6.4 m 2.5 m Even 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
Measure Between end of Guide and cryo vacuum? [space, magnetic constraints] Collimator Be Foil 4°K Shield Be Foil 50°K Shield Be/Teflon Multi- Foil Window at LHe/Vacuum Interface Measurement Cell
Beam Monitor Example (Various Possibilities) n+ 3 He p + t + 765 keV ionizes gas mixture [ 3 He + 4 He(~.5 atm) +N 2 (~.5 atm)] Works in current mode for NPDGamma Other (nonmetallic, nonmagnetic) possibility: 6Li scintillator PANIC’05 BL
How can neutrons be polarized/analyzed? B B gradients (Stern-Gerlach, sextupole magnets) electromagnetic F=( )B Reflection from magnetic mirror: electromagnetic+ strong f =a(strong) +/- a(EM) with | a(strong)|=| a(EM)| f+=2a, f-=0 B Transmission through polarized nuclei: strong ≠ - T ≠ T Spin Filter:T =exp[- L] L
Polarized 3 He Spin Filter Dielectric mirror on 1mm thick fused silica Polarizer cube Collimating lens mirror 3 He Cell diode laser fiber-optic cable Dielectric mirror on 1mm thick fused silica quarter-wave plate quarter-wave plate 3 He 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
Neutrons are polarized through spin-dependent scattering from magnetized mirrors Polarization: ~98% transmission: ~35% 28 cm White Neutron Beam Magnet Box Plate Curvature Radius ~ 10m polarized Neutron Beam “Supermirror” Neutron Polarizer/Analyzer Permanent magnet box B
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
Remanent Polarizing Supermirrors Exist Retains its magnetization in a weak opposite field.
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
Adiabatic flipper with B gradient upstream x neutrons y zI BzBz B x Precession in effective field in rotating frame: B 2 +B RF 2 Rotation frequency of effective field seen by neutron: v/L BB Adiabatic Condition: Easy to meet for cold neutrons L sin 2 1+k 2 ] Adiabatic parameter k= L B/ v>>1
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?
Ferromagnetic Shield/RF: do we have a problem with metal objects messing with RF fields? Inner Dressing Coil Outer Dressing Coil 50°K Shield 4°K Shield Superconducting Lead Shield Ferromagnetic Shield B0 cos θ Magnet Gradient Coil
EDM Experiment Horiz. Section View 2 Light Guide Measurement Cell Ground Electrode Electric Field Return HV Generator HV Electrode Support