The NPDGamma Experiment at the SNS FnPB
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Transcript of The NPDGamma Experiment at the SNS FnPB
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The NPDGamma Experimentat the SNS FnPB
Christopher Crawford
University of Kentucky
for the NPDGamma Collaboration
DNP Fall Meeting
2007-10-12
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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
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Supermirror polarizer
FNPB guide
CsI Detector Array
Liquid H2 Target
H2 Vent Line
Beam Stop
Magnetic Field Coils
Magnetic Shielding
H2 Manifold Enclosure
Layout of experimental setup at the FnPB
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LH2 target gas manifold and vent line
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LH2 Target Improvements
reduce backgrounds: thinner Al entrance window
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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
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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
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Stray magnetic fields
E
F
90.8
152.2
348
359.2
Coils
Magnetic shield
Z
182.68
Concrete wall
Concrete wall 1
X331.65
788.72
303.83
30.5
30.5
A
B
440.72
133
225
2
FP 12 side
FP 14 side
Facility requirements call for magnetic field to be less than 50 mGauss at the boundary of adjacent beamlines
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Neutron beam chopper design: opening angles
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
Figure of merit: P2N
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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
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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
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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
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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
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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