Transcript The NPDGamma Experiment at the SNS FnPB

Moving the NPDGamma
Experiment to the SNS FnPB
Christopher Crawford
University of Kentucky
2007-11-08
• overview of NPDGamma
• transition to the SNS
• expected results
Madison
Spencer
Overview of NPDG experiment

3He

beam monitors

RF spin flipper

LH2 target

CsI detectors
polarizer
fp ¼ 5£10-7
A = -0.11 fp ¼ 5 £ 10-8
N / 1 + Pn A cos  + Pn APC sin 
A = 10%
for N ¼ 5 £ 1017 events
3He
neutron polarizer
 n + 3He  3H + p cross section is highly spin-dependent
J=0 = 5333 b /0
n
+
n
p
p
n
n
p
+
p
J=1 ¼ 0
 10 G holding field determines the polarization angle
rG < 1 mG/cm to avoid Stern-Gerlach steering
Steps to polarize neutrons:
1.
Optically pump Rb vapor
with circular polarized laser
2.
Polarize 3He atoms via
spin-exchange collisions
3.
Polarize 3He nuclei via
the hyperfine interaction
4.
Polarize neutrons by spindependent transmission
P3 = 57 %
16L liquid para-hydrogen target

30 cm long  1 interaction length

99.97% para  1% depolarization

super-cooled to reduce bubbles

SAFETY !!
p
E =
15 meV
p
para-H2
p
p
ortho-H2
15 meV
ortho
 (b)
para
capture
En (meV)
CsI(Tl) Detector Array

4 rings of 12 detectors each
•
15 x 15 x 15 cm3 each

VPD’s insensitive to B field

detection efficiency: 95%

current-mode operation
•
•
5 x 107 gammas/pulse
counting statistics limited
NPDGamma setup at LANSCE (LANL)
Spallation Neutron Source (SNS)
Oak Ridge National Laboratory, Tennessee
spallation sources: LANL, SNS
pulsed -> TOF -> energy
LH2 moderator: cold neutrons
thermal equilibrium in ~30 interactions
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 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
Neutron beam chopper design: opening angles
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