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!)

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

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

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

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

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

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

• 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

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 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

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 CoilOuter Dressing Coil

50°K Shield

4°K Shield

Superconducting Lead Shield

Ferromagnetic Shield

B0 cosθ Magnet

Gradient Coil

EDM Experiment Horiz. Section View 2

Light Guide

Measurement CellGround ElectrodeElectric Field Return

HV Generator

HV Electrode Support