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The NPDGamma Experiment at the SNS FnPB

The NPDGamma Experiment at the SNS FnPB. Christopher Crawford University of Kentucky for the NPDGamma Collaboration DNP Fall Meeting 2007-10-12. Outline. Modifications for Phase II run at the SNS: Cryogenic H 2 target improvements Magnetic fields and shielding FnPB chopper design

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The NPDGamma Experiment at the SNS FnPB

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  1. The NPDGamma Experimentat the SNS FnPB Christopher Crawford University of Kentucky for the NPDGamma Collaboration DNP Fall Meeting 2007-10-12

  2. Outline Modifications for Phase II run at the SNS: • Cryogenic H2 target improvements • Magnetic fields and shielding • FnPB chopper design • FnPB supermirror polarizer design • Expected sensitivity to A at the SNS

  3. Layout of experimental setup at the FnPB Supermirror polarizer CsI Detector Array Liquid H2 Target H2 Vent Line H2 Manifold Enclosure Magnetic Shielding FNPB guide Magnetic Field Coils Beam Stop

  4. LH2 target gas manifold and vent line

  5. LH2 Target Improvements • reduce backgrounds: thinner Al entrance window

  6. Magnetic and radiological shielding • integrated shielding: • 9”-18” concrete walls • 0.25”–0.75” 1010 steel • open design for LH2 safety,access to experiment • external field B < 50 mG • shield npd from B-field of other experiments • flux return for uniform magnetic field:Stern-Gerlach steering

  7. Magnetic Field • B-field gradients must be < 10 mG/cm • prevent Stern-Gerlach steering of neutrons • prevent depolarization of 3He in spin filter • B-field modeled in OPERA3D (S. Balascuta) • Flux return / shieldingon ceiling,floor,sides • extra coil neededto compensatehigher ceilingflux return

  8. Stray magnetic fields FP 14 side FP 12 side 788.72 348 30.5 A 1 Concrete wall 133 • Coils 152.2 E F 90.8 Z 182.68 303.83 331.65 225 X Concrete wall Magnetic shield B 359.2 2 30.5 440.72 • Facility requirements call for magnetic field to be less than 50 mGauss at the boundary of adjacent beamlines

  9. SNS 60 Hz pulses with tail: wrap-around neutron spectrum choppers placed along guide to cut out most of slow neutrons opening angle tuned to window of good neutrons Neutron beam chopper design: opening angles Figure of merit: P2N

  10. Chopper optimization – McStas simulation • based on McStas simulation of FnPB (Huffman) • active components simulated in McStas(guide, bender, windows) • passive components analyzed from MC data (choppers, collimators, RFSF, LH2 target) • ROOT integration: McStas ntuple • rapid optimization of chopper phase, angle; RFSF phase • example: investigation of counter-rotating choppers

  11. Design of supermirror polarizer • two methods of neutron polarization • spin-dependent n-3He absorption cross section • magnetized SM coating selectively absorbs 1 spin state • supermirror polarizer • spin-dependent reflection from magnetized supermirror coating • high polarization possible • requirements:at least 1 reflectionpreserve phase space

  12. Design of supermirror polarizer • McStas optimization of polarizer for NPDGammaas a function of (bender length, bend radius, #channels) • 96% polarization, 30% transmission ) 2.6£1010 n/s • 4x improvement in P2N

  13. Sensitivity of NPDG to A at SNS • Gain in the figure of merit at the SNS: • 12.0 x brighter at the end of the SNS guide • 4.1 x gain by new SM polarizer • 6.5 x longer running time •  A ~ 1.1£10-8 in 107 s at the SNS • Higher duty factor at SNS • Commission NPDGamma: Summer 2008

  14. Conclusions • NPDGamma is ready to “plug” into the SNS FnPB • a few modifications are necessary for new site • plus modifications to improve “figure of merit” (FOM) • we project to measure A=10-8 in 1 year

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