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Computer simulations of the n- 4 He parity-violating spin-rotation experiment at NIST

Computer simulations of the n- 4 He parity-violating spin-rotation experiment at NIST. Bret Crawford Gettysburg College DNP Oct. 28, 2006. Outline. Systematic Effects Ambient magnetic field rotations 4 He diamagnetism Neutron slowing down in liquid He target Small angle scattering

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Computer simulations of the n- 4 He parity-violating spin-rotation experiment at NIST

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  1. Computer simulations of the n-4He parity-violating spin-rotation experiment at NIST Bret Crawford Gettysburg College DNP Oct. 28, 2006

  2. Outline • Systematic Effects • Ambient magnetic field rotations • 4He diamagnetism • Neutron slowing down in liquid He target • Small angle scattering • Simulation • Neutron transport • Modeling scattering cross section • Future Plans

  3. Ambient Magnetic Field Rotations • Rotation angle • Magnetic field suppression, longitudinal B<100mG B=100mG, L=1m, l=5Ang Compare with experimental goal of • Subtracting Data from Upstream and Downstream targets cancels non-target related effects

  4. 4He Diamagnetism • Reduces ambient external field B in target region • Neutrons in target cell precess slightly less than neutrons in empty cell

  5. Neutron slowing down in target • Difference in indices of refraction between a full and empty target • Neutron slows in target causing larger rotation in ambient field • 100mG field in 1 meter

  6. Small-angle scattering Target positions detector Wave guides • Upstream-downstream subtraction is incomplete • Lower energy for scattered neutrons (Up-target scatters travel farther at lower energy than down-target scatters) • path length of neutrons scattered in target is different for different target positions (down stream angle is larger)

  7. Small-angle scattering Target positions detector Wave guides • different detector solid angles from target positions From simulation see ~3% more scattered neutrons in Detector from Down target than Up target • Amount of scattering into detector is small but not that small ~0.2% of detected neutrons have a new angle and new energy from scattering (simulation) With Up-Down subtraction non-PV rotations are in the few x10-8rad range

  8. Neutron Transport Simulation* • Random trajectories within critical angle of guide • If wall angle < critical angle, bounce; otherwise absorbed. • wave guide (qc=1 mrad/Ang) • input coil (qc=1 mrad/Ang) • target cell (empty LU, full RD)s • pi-coil between target • output coil • ASM (apertures only; qc=3 mrad/Ang) *Murad Sarsour, Mike Snow, Bret Crawford

  9. Modeling the Scattering Cross section : n-4He • Absorption is negligible • Scattering is coherent • Detailed knowledge of scattering at low momentum transfer is a research question • Model for scattering in simulation code Choose q from S(q) Find energy from dispersion curve Calculate cross section from q and E Determine if scatters within target Follow new trajectory to target

  10. Modeling the Scattering Cross section : n-4He Dispersion curve S(q) arbitrary S(q) q(1/Ang)

  11. Scattering Cross section • q<0.56 use Tsipenyuk and May results Tsipenyuk, May (arXiv:cond-mat/0207278 v1, 2002) -- unpublished data for S(0) • q>0.56 use Sommers’ data Sommers, Dash and Goldstein (Phys Rev, 97)1954

  12. Simulation energy at detector wavelength at detector

  13. Rotation angle for Bz=100mG q (all neutrons) q (scattered only) rotation angle for entire beam line – 477cm (no pi-coil)

  14. Rotation angle for Bz=100mG Up Target q Down Target q rotation angle for entire beam line (pi-coil reverses rotation between targets)

  15. Rotation angle for Bz=100mG Large rotation values Up Target q Down Target q rotation angle for entire beam line (pi-coil reverses rotation between targets)

  16. Rotation angle for Bz=100mG (U-D)/(U+D) q (mrad)

  17. Simulation: Preliminary Results • Neutron flux along beamline (z) * • Entering target 1 (UR): 23% • Entering target 2 (DL): 19% • Into detector: 11% • Scattering Info • 26% entering target scatter • 0.2% entering detector have scattered, • Rotation after Up-Down Subtraction, averaged over all neutrons (angles, energies, positions) – 100mG *initial angles chosen to be within critical angle of guide

  18. Future • Improve scattering model • Use x-ray data for low-q region of S(q) (R. Hallock, PRA 5, 1972) • Analytic calculation of double differential cross section • Include multiple scattering • Run for test targets

  19. Neutron Transport Top View guide Input coil targets Output coil ASM x • Guide and Input Coil x[-3.05,-0.35],[0.35,305] y[-2.55,2.55] • Output Coil and targets x[-3.0,-0.35],[0.35,30] y[-2.5,2.5] • Supermirror x[-2.85,2.85] y[-2.25,2.25] • Gaps [9.0, 6.8, 12.0] z y 116cm 89cm 41.6cm 41.6cm 108cm 28cm

  20. Plots x-distribution entrance before target 1

  21. Plots x-distribution after target 1 (UL) after target 2 (DR)

  22. Plots x-distribution Before ASM detector

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