Moving the NPDGamma Experiment to the SNS FnPB Christopher Crawford University of Kentucky overview of NPDGamma transition to the SNS expected results MadisonSpencer

Overview of NPDG experiment  3 He polarizer  beam monitors  RF spin flipper  LH 2 target  CsI detectors f  ¼ 5£10 -7 A  = f  ¼ 5 £ N / 1 + P n A  cos  + P n A PC sin   A = 10% for N ¼ 5 £ events

3 He neutron polarizer  n + 3 He  3 H + p cross section is highly spin-dependent  J=0 = 5333 b / 0  J=1 ¼ 0  10 G holding field determines the polarization angle rG < 1 mG/cm to avoid Stern-Gerlach steering Steps to polarize neutrons: 1.Optically pump Rb vapor with circular polarized laser 2.Polarize 3 He atoms via spin-exchange collisions 3.Polarize 3 He nuclei via the hyperfine interaction 4.Polarize neutrons by spin- dependent transmission n n + n n p p p p n n p p n n + p p P 3 = 57 %

16L liquid para-hydrogen target 15 meV ortho para capture E n (meV)  (b)  30 cm long  1 interaction length  99.97% para  1% depolarization  super-cooled to reduce bubbles  SAFETY !! p p p p para-H 2 p p p p ortho-H 2  E = 15 meV

CsI(Tl) Detector Array  4 rings of 12 detectors each 15 x 15 x 15 cm 3 each  VPD’s insensitive to B field  detection efficiency: 95%  current-mode operation 5 x 10 7 gammas/pulse counting statistics limited

NPDGamma setup at LANSCE (LANL)

Spallation Neutron Source (SNS) spallation sources: LANL, SNS pulsed -> TOF -> energy LH2 moderator: cold neutrons thermal equilibrium in ~30 interactions Oak Ridge National Laboratory, Tennessee

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

Supermirror polarizer FNPB guide CsI Detector Array Liquid H 2 Target H 2 Vent Line Beam Stop Magnetic Field Coils Magnetic Shielding H 2 Manifold Enclosure Layout of experimental setup at the FnPB

LH 2 target gas manifold and vent line

LH 2 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 LH 2 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 3 He 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

Stray magnetic fields E F CoilsCoils Magnetic shield Z Concrete wall 1 X A B FP 12 side FP 14 side  Facility requirements call for magnetic field to be less than 50 mGauss at the boundary of adjacent beamlines

Neutron beam chopper design: opening angles

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- 3 He 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£10 10 n/s  4x improvement in P 2 N

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 10 7 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