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Neutron Polarization and polarimetry for precision neutron decay
measurments Tim Chupp
University of Michigan For the Nab/abBA/PANDA collaboration
Outline: The goal: control <J> and measure to <0.1%
Polarization techniques (PSM, 3He) Spin flipping Polarimetry
Outlook
e
p
ν
e
p
ν N+ N-
Asymmetry: __________ = k(E)A Pn A F (1-f) + Afalse N+ - N- N+ + N-
background spin flip efficacy analyzing power neutron polarization–Averaged appropriately
Polarization���
€
d5ΓdEedΩedΩν
= G(Ee ) 1+ a p e ⋅ p ν
Ee Eν
+ b me
Ee
+ A σ n ⋅
p eEe
+ B σ n ⋅
p νEν
+ D σ n ⋅
p e × p ν( )
Ee Eν
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
A 0.1% measurement of A requires a 0.1% measurment of Pn
Polarization
P/A Pn (5Å) Tn P2ρ features PSM(XSM) 99.x% 10% 0.1 Pn~ 1; static He (60%) 80% 30% 0.2 flip P3; P3 varies
neutrons
Spin Flipper
M2
N0(v)
R A(v) TA(v)
M1 polarizer analyzer
€
P(v, r )T(v, r )
A 0.1% measurement of A requires a 0.1% measurment of <Pn>
Crossed Supermirror (XSM)���Kreuz et al.
Thick 3He analyser Pn=0.997±0.001 Schuman PRL 99, 191803 (2007)
0
0.2
0.4
0.6
0.8
1
Reflectivity
θ
No θc !
Si substrate B
θ
R
3He spin filter: n +3He _> 1H + 3H
σ0=5333 b
1.0
0.8
0.6
0.4
0.2
0.0
Tran
smiss
ion
1086420Distance (cm)
0.2 0.4 0.6 0.8 0.9Polarization
€
T± = e−n3σ 0t
v0v(1P3 )
neutrons
Spin Flipper
M2
N0(v)
R
M1 polarizer analyzer
€
P(v, r )T(v, r )
€
A(v, r )TA (v,
r )
3He: continuous polarization by Spin Exchange
• Glass/cell technology (GE180) • Hybrid Spin Exchange • Metastability exchange/compressors • Lasers • Magnetic boxes …
Huge Progress with 3He (NIST-Wisc/ILL/UVa/Jlab…)
Earl Babcock – FRMII (Juelich)
3He Polarizer Neutron-flux Limitations? 0.54
0.53
0.52
0.51
0.50
Pola
rizat
ion
120100806040200Time (in hrs)
Beam
off
4.5
hrs
Beam
off
1.5
hrs
Beam
off
for 0
.5 h
rs
Beam
off
2.5
hrs
Beam
off
5 hr
s
Beam
off
7 hr
s
Rb re
laxa
tion
rate
incr
ease
Lansce data shows effect on PRb
€
PRb =γ opt
γ opt + SΓA
€
ΔΓA ∝ φnSNS/NIST
Also a slow (100s) increase Not fully understood Requires increased laser power (e.g. few times) Double cell – T Gentile
T1>100 h; P3~46% (in progress)
SRb = 10.8 SK = 6
[ion] or [e]
Neutron Beam
3He (60%)
XSM
Neutron density Fluence
Figure of Merit (P2dρn/dλ)
Polarization Summary • Both approaches can be perfected for abBA/PANDA • 3He provides Pα(tof) (Bowman and Pentilla) • Supermirror design for m=3+ guides required • Magnetic field accomodations for spin transport • Rate estimates based on 0.1 of Nab flux (XSM): 75 live days for 0.1% measurement of A
asymmetry
The fast adiabatic spin flipper
€
i dψdt
= − µ ⋅ B ( )ψ = −
µ+B− + µ−B+
2+ µzBz
⎛
⎝ ⎜
⎞
⎠ ⎟ ψ
€
ψ =ψ+
ψ−
⎡
⎣ ⎢
⎤
⎦ ⎥
€
B± =B12e±iω 0t
€
Bz = β0t = β0zt
€
i ddt
ψ+
ψ−
⎡
⎣ ⎢
⎤
⎦ ⎥ =
µzβ0tB12eiω 0t
B12e− iω0t µzβ0t
⎡
⎣
⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥
ψ+
ψ−
⎡
⎣ ⎢
⎤
⎦ ⎥
1.0
0.8
0.6
0.4
0.2
0.0
ψ±
10008006004002000time
z=z’
x’
z=z’
x’
z=z’
x’
z=z’
x’
z=z’
x’
99.9x% “efficient” AND we can measure it!
RF ON
Spatial gradient RF
AFP of 3He Neutron Polarimetry – thick 3He
1.0
0.8
0.6
0.4
0.2
Anal
yzin
g Po
wer
8642n3tσa
1.00
0.98
0.96
0.94
0.92
0.90
Anal
yzin
g Po
wer
8.07.57.06.56.05.55.0n3tσa
λ=5 Å
Opaque Analyzer: For n3tσa large enough (1-A) small enough (Zimmer) Characterize Polarizer AND Flipper
40%
60%
neutrons P(v) TP(v)
Spin Flipper
N0(v)
R A(v) TA(v)
X
Detector
polarizer analyzer Detector
3He Spin Filter Spin Flipper
n+p ___> d + γ (@LANSCE) 3He and Pn test:
Booboo is the Cell
GE-180 Glass (boron free)
12 cm
Flipper
FP-12 P(v) TP(v)
M3
N0(v) R A(v) TA(v)
M2 M1
Experiment
M1= N0ε1+B1 M2= N0T1TP ε2+B2
M2(0)/M2(out) = T0 M2(P)/M2(0) = cosh αPPP
(Coulter et al. NIMA 288, 463 (1989)
TA/P=T0cosh αA/PPA/P T0=exp(-αA/P- αgl-…)
P/A=tanh αA/PPA/P
2.0
1.5
1.0
0.5
0.0
Sig
nal (
arb
units
)
642Wavelength (Å)
M2 Cell Out M2 Polarized M2 Unpolarized
normalized to M1 Bragg edges (Al)
Pulsed Beam Neutron Polarimetry
npdgamma FP12
Flipper
FP-12 P(v) TP(v)
M3
N0(v) R A(v) TA(v)
M2 M1
Experiment
M1= N0ε1+B1 M2= N0T1TP ε2+B2
TA/P=T0cosh αA/PPA/P T0=exp(-αA/P- αgl-…)
P/A=tanh αA/PPA/P
Pulsed Beam Neutron Polarimetry
npdgamma FP12
Offsets/Backgrounds
0.6
0.4
0.2
M2(
0)/M
2(ou
t)
642wavelength(Å)
-4-2024
x10-3
2.5
2.0
1.5
1.0
M2(
P)/M
2(0)
642wavelength (Å)
-4-2024
x10-3
Residual (10-3)
Residual (10-3)
coshαPPP _> Pn=tanhαPPP
expαP
Demonstrates understanding at 10-3 level; also LANSCE FP12 available
Thick 3He analyser Pn=0.997±0.001 (0.0005 reasonable)
Polarimetry Summary • Use opaque 3He and pulsed/chopped beam: measure , • Pulsed beam (SNS): measure continuously
• Chopped beam (NIST): monitor continuously – calibrate monitor
• Neutron flux-3He issues not limiting • Statistics not limiting • Systematics: backgrounds (beta delayed neutrons), 3He AFP losses
• LANSCE FP12 test beam available
€
P(v, r )
€
R(v, r )
~10-4 at LANSCE Should be ~constant vs tof at SNS
T. Soldner
Community 3He resources • NIST/Wisconsin/MunichORNL/SNS/LANL/Mich/
Indiana/Hamilton: Gentile, Walker, Babcock, Tong… • PLUS neutron scattering AND electron scattering communities
Cell production, lasers, optical pumping research, 3He in beam
SNS FP13: Apr 1, 2011 (M. Musgrave, N. Fomin et al.)
Flipper off
Flipper on
Summary���• Polarizer: 99.xx % polarization; 10% transmission • Flipper: 99.xx % effiecient • 0.1% measurement of <Pn>, spin flipper: CONSERVATIVE • Community has expertise and resources
Segmented Si detector
decay volume (field rB,DV·B0)
0 kV
0 kV
-30 kV
TOF region (field rB·B0)
Supermirror (3He)
AFP Spin
Flipper
3He Polarimeter
AFP