HYSPEC IDT Options for polarization analysis with 3 He for the Hybrid Spectrometer (HYSPEC) L. Passell, V.J. Ghosh, I. Zaliznyak, S. Shapiro, W. Leonhardt, (BNL), M. Hagen, W.T. Lee (SNS), and T.R. Gentile (NIST) We acknowledge helpful discussions with L.D. Cooley, A.K. Ghosh. W. Sampson, R. Gupta and B. Parker

HYSPEC IDT Polarized 3 He Neutron Polarization Analyzer Attractive Features  Efficient thermal neutron polarization analyzer at 75 % 3 He polarization. Promise of even better efficiency if 3 He polarization can be pushed into the 80-90% range.  Operates over a neutron energy range extending from sub-thermal to epi-thermal.  Analyzer efficiency can be optimized at any given neutron energy by changing the 3He gas pressure.  Does not require a highly collimated incident neutron beam.  Potential to cover the entire active area of a wide- angular-acceptance TOF detector.

HYSPEC IDT T = 12cm; p(He 3 )=0.75; p n > 0.9 E n (meV) P(atm)

HYSPEC IDT HYSPEC ANALYZER Sample axis 70 cm window Detector bank Horizontal acc: 60deg. Vertical acc: ± 7.5 deg.

HYSPEC IDT Design Constraints  Cell lifetime sensitive to holding magnetic field uniformity. Necessary to screen stray magnetic fields and provide field uniformity  H/H<10-4/cm over cell volume. Also sensitive to power supply and field coil vibration-induced fluctuations  Design issues relating to influence of guide and holding field configurations on incident neutron beam polarization  Wide angular acceptance applications require substantial volumes of polarized 3He gas that will have to be supplied by a central optical pumping facility  Wide angular acceptance applications require the fabrication of large-volume, kidney- shaped gas cells which must operate as both vacuum and pressure vessels  Design and fabrication issues relating to influence of cell wall material on cell lifetimes  Design issues relating to the possibility of in-situ optical pumping and to the choice of optical pumping method  Design issues relating to quick, easy and convenient interchange of gas cells

HYSPEC IDT Low Field PASTIS-type 3 He polarization analyzer Guide field coils Sample radiation shield Radial collimator 3 He Cell 3 He cell volume < 2 litres Dimensions: Beam direction 12cm Vertical ~ 10cm Horizontal, normal to beam direction ~ 24cm

HYSPEC IDT Medium field 3 He polarization analyzer Sample radiation shield Compensated sample magnet coils Vertical-axis solenoid 3 He cell Radial collimator

HYSPEC IDT Medium field 3 He polarization analyzer Compensated sample magnet coils Sample radiation shield 3 He cell Radial collimator Horizontal-axis solenoid

HYSPEC IDT Conventional magnetic shielding Material Mechanical properties Field that can be shielded (Gauss) High grade μ metal annealed ~10 Fe0.2Ni0.8 susceptible to heat and stress Mid-grade μ metal partly-annealed or not annealed ~100 FexNi(1-x) Fe0.97Si0.03 ~1000

HYSPEC IDT Oxford 15 Tesla superconducting sample magnet

HYSPEC IDT High-field CRYOPAD-type 3 He polarization analyzer Passive, persistent-mode Superconducting magnetic shield Room-temperature cavity 4K dewar 3 He cell

HYSPEC IDT Passive, persistent-mode, superconducting magnetic shielding Superconductor Density propertiesField that can be shielded by (g/cm3) 1 mm thick foil (Tesla) Nb 8.5 good strength and rigidity ~0.2 electron beam weldable Al clad Nb0.37Ti good strength and rigidity ~1.0 Al clad Nb 3 Sn 8.9very brittle ?

HYSPEC IDT Polarized proton filter guide Polarized proton filter E n > 100meV 5 < E n < 100meV E n = 5meV

HYSPEC IDT Design of magnetic fields OPERA – software for electromagnetic design used for the design of RHIC magnets the SNS accumulator ring the design of MRI magnets Using coils instead of permanent magnets – versatility Magnetic field simulations of Race-track coils required to generate guide fields Helmholtz coils required for the low-field PASTIS-type geometry Solenoids required to generate the 3 He cell magnetic field for the medium field option A superconducting coil solenoid to check what magnetic fields can be shielded by a1or 2mm thick NbTi sheet.

HYSPEC IDT Simulation of a vertical magnetic field at the 3 He cell Racetrack coils to generate guide field Neutrons move left to right along X axis Guide field and 3He cell holding field are both vertical (Y axis) 3 He cell placed at the coil center Large sample Pair of Helmholtz coils: Radius = separation = 52cm Jc=6.5Amp/mm 2 3 He cell volume < 2 litres Dimensions: Beam direction 12cm Vertical ~ 10cm Horizontal, normal to beam direction ~ 24cm

HYSPEC IDT Magnetic fields at the center of the 3 He cell T. Gentiles magic formula for the 3he cell lifetime: T = 58 x * P/ (gradB/B)**2 where P is the pressure in bars T = 58 hours if (gradB/B)**2 =10 -6 Cell dimension in the beam direction is 12cm

HYSPEC IDT Magnetic field at the center of the 3 He cell 3 He cell dimensions: Beam direction 12cm Vertical ~ 10cm Horizontal, normal to beam direction ~ 24cm Cell lifetime > 58 hours vertical Horizontal direction normal to the beam

HYSPEC IDT Simulation of a horizontal magnetic field at the 3 He cell perpendicular to the direction of incident neutrons Scattering angle is 90 degrees Racetrack coils to generate guide field Pair of coils: Radius = separation = 52cm Jc=6.5Amp/mm2 sample 3He cell Guide field is vertical 3 He cell holding field is horizontal, along axis normal to the beam direction (Z axis)

HYSPEC IDT Simulation of a horizontal magnetic field at the 3 He cell perpendicular to the direction of incident neutrons Scattering angle is 90 or 120 degrees For particular holding field directions at the 3 He cell the field direction and magnitude along the beam path will change substantially. The time required by the neutron to travel this distance will depend on its energy. Both these factors will influence the spin flip probability. The partial depolarization of the neutron beam is possible under certain conditions.

HYSPEC IDT Future Plans 1.The partial depolarization of the neutron beam is possible under certain conditions even for the low-field ‘PASTIS-type’ layout. 2.Exploring the medium field option. When the sample field is generated by 3-4 Tesla magnet the holding field at the 3 He cell will have to be generated by a solenoid. Simulation of the sample magnet and the holding fields. Can conventional magnetic shielding like mu metal be used to expel the fringe field due to the sample magnet? The depolarization of the neutron beam will probably be a concern in this case. Both these issues can be explored further with computer simulations.