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 H2 target improvements Magnetic fields and shielding FnPB chopper design FnPB supermirror polarizer design Expected sensitivity to A at the SNS

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

LH2 target gas manifold and vent line

LH2 Target Improvements reduce backgrounds: thinner Al entrance window

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

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 / shielding on ceiling,floor,sides extra coil needed to compensate higher ceiling flux return The NPDGamma magnetic field is low (10 Gauss) but occupies a large volume. The shielding enclosure is lined with 0.25”-0.75” of 1010 steel to act as a flux return. This also serves to provide magnetic shielding and to confine stray magnetic fields. We used commercial magnetic FEA software (OPERA3D) to model our magnetic field arrangement.

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

Neutron beam chopper design: opening angles Figure of merit: P2N 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

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

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 reflection preserve phase space

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

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

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