The NPDGamma Experimentat 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

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

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 / shieldingon ceiling,floor,sides

extra coil neededto compensatehigher ceilingflux return

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

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

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 reflectionpreserve phase space

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

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