1. The NPDGamma Experiment at the SNS FnPB
Christopher Crawford
University of Kentucky
for the NPDGamma Collaboration
DNP Fall Meeting

2. 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

3. Layout of experimental setup at the FnPB
CsI Detector Array
Supermirror
polarizer
Liquid H2 Target
H2 Vent Line
H2 Manifold Enclosure
Magnetic Shielding
FNPB guide
Magnetic Field Coils
Beam Stop

4. LH2 target gas manifold and vent line

5. LH2 Target Improvements
reduce backgrounds: thinner Al entrance window

6. 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

7. 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 / shielding on ceiling,floor,sides
extra coil needed to compensate higher ceiling flux return
The NPDGamma magnetic field is low (10 Gauss) but occupies a large volume. The shielding enclosure is lined with 0.25”-0.75” of 1010 steel to act as a flux return. This also serves to provide magnetic shielding and to confine stray magnetic fields. We used commercial magnetic FEA software (OPERA3D) to model our magnetic field arrangement.

8. Stray magnetic fields
Facility requirements call for magnetic field to be less than 50 mGauss at the boundary of adjacent beamlines E
90.8
152.2
348
359.2
Coils
Magnetic shield Z
182.68
Concrete wall 1 X
331.65
788.72
303.83
30.5 A B
440.72
133
225 2
FP 12 side
FP 14 side F

9. Neutron beam chopper design: opening angles
Figure of merit: P2N
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

10. 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

11. 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 reflection preserve phase space

12. 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£1010 n/s
4x improvement in P2N

13. 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

14. 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