Transcript The NPDGamma Experiment at the SNS FnPB

The NPDGamma Experiment
at 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
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
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 / shielding
on ceiling,floor,sides
 extra coil needed
to compensate
higher ceiling
flux return
Stray magnetic fields

acility requirements call for magnetic field
F
to be less than 50 mGauss at the
boundary of adjacent beamlines
788.72
348
30.5
A
E
1
Concrete wall
133
C
o
i
l
s
90.8
152.2
Z
182.68
303.83
225
331.65
X
Concrete wall
Magnetic shield
B
359.2
2
30.5
F
440.72
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 reflection
preserve phase space
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
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