Computer simulations of the n- 4 He parity-violating spin-rotation experiment at NIST Bret Crawford...
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Transcript of Computer simulations of the n- 4 He parity-violating spin-rotation experiment at NIST Bret Crawford...
Computer simulations of the n-4He parity-violating spin-rotation
experiment at NIST
Bret CrawfordGettysburg College
DNPOct. 28, 2006
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
Ambient Magnetic Field Rotations
• Rotation angle
• Magnetic field suppression, longitudinal B<100G
B=100G, L=1m, =5Ang
Compare with experimental goal of
• Subtracting Data from Upstream and Downstream targets cancels non-target related effects
v
BLt NL
8.3
radA
35
104.2
radPV7103
4He Diamagnetism
• Reduces ambient external field B in target region
• Neutrons in target cell precess slightly less than neutrons in empty cell
6101 B
B
radA
95
102
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
• 100G field in 1 meter
oute
foutin n
n
1
6105.2 fe nn
radA
95
106
Small-angle scattering
• 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)
cmLL 003.0~10~ 5
Target positions Wave guides detector
radA
65
107
E
Eo 2
rad
A45
51010
Small-angle scattering
• 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 With Up-Down subtraction non-PV rotations are in the few xthe few x1010-8-8radrad range range
Target positions Wave guides detector
Neutron Transport Simulation*
• Random trajectories within critical angle of guide• If wall angle < critical angle, bounce; otherwise
absorbed.• wave guide (c=1 mrad/Ang)• input coil (c=1 mrad/Ang)• target cell (empty LU, full RD)s• pi-coil between target• output coil• ASM (apertures only; c=3 mrad/Ang)
*Murad Sarsour, Mike Snow, Bret Crawford
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 curveCalculate cross section from q and EDetermine if scatters within targetFollow new trajectory to target
,
0
22
qSk
kb
dEd
d
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
221
0
44 qr
SqS
d
d
c
bb
rotation angle for entire beam line – 477cm (no pi-coil)
Rotation angle for Bz=100G
(all neutrons) (scattered only)
rotation angle for entire beam line (pi-coil reverses rotation between targets)
Rotation angle for Bz=100G
Up Target Down Target
Large rotation values
Up Target Down Target
rotation angle for entire beam line (pi-coil reverses rotation between targets)
Rotation angle for Bz=100G
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) – 100G
*initial angles chosen to be within critical angle of guide
initialfinal EE
radsim8108
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
Neutron Transport
• 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]
• Supermirrorx[-2.85,2.85] y[-2.25,2.25]
• Gaps [9.0, 6.8, 12.0]
guide Input coil targets Output coil ASM
116cm 89cm 41.6cm 41.6cm 108cm 28cm
x
zy
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