Johannes Gutenberg-Universität
Mainz
Institut für Kernphysik
Study for the use of the polarized 3He
target at the photon beam of MAMI
• Motivation
• Polarized 3He gas target
• Solenoid design and test
• 3He feasibility test
• Summary and outlook
Patricia Aguar Bartolomé
Motivation
 Test
of the GDH sum rule on the
neutron
 Double polarization experiments
 3He gas as neutron target for real
photon beams
• Number of neutrons
2 cm D-butanol target
8.4 1023 nucleons / 2 cm
20 cm 3He gas target (p = 6 bar)
1.3 1022 nucleons / 20 cm
Patricia Aguar Bartolomé
Polarized 3He gas target
 3He
is made of two protons and one neutron

In the ground state the two protons couple together
their spin contribution to the nuclear magnetic moment

3He
spin is carried by the unpaired neutron
3
cancel


He  n
Patricia Aguar Bartolomé
3He
Polarization
Metastability Exchange Optical Pumping
(Colegrove et al., Phys. Rev. 132 (1963) 2561)
 Electronic optical pumping in the 2 3S1
State + transfer of polarization to the
Nucleus by hyperfine coupling
 Metastability collisions
transfer of
Nuclear polarization to the 1 1S0 state
Result
Nuclear polarization of the 1 1S0
ground state
Patricia Aguar Bartolomé
3He
polarization relaxation
The polarization decays with a time constant, T1tot
 1 
Pt   exp   tot 
 T1 
tot
The overall relaxation time, T1 is given by
1
1
1
1
1
1
 dipol  wall  grad  imp  beam
tot
T1
T1
T1
T1
T1
T1
The relaxation is caused by
 Magnetic dipole-dipole interactions between 3He
atoms ( T1dipol)
817
T1dipol h 
p
 Wall relaxation on the inner surface of the cell ( T1wall )
Gas stored in special ironfree glass vessels.
Patricia Aguar Bartolomé
3He
polarization relaxation (cont.)
 Magnetic field gradients ( T1grad )
Transport in homogenous
magnetic field
2
 B 
grad
 p
0
T1 h   
 B 
r 

  17500 cm2 bar h 1 
The required relative field gradient is
 B 
 z   10 3 cm 1
 B 
 0 
T1grad h  300 h
 Interactions with gas impurities ( T imp)
1
must be very clean.
 p  6 bar
; B0  7 G 
The cell and gasses
beam
 Ionization due to the experimental photon beam ( T1
).
Patricia Aguar Bartolomé
Solenoid design
Polarized targets require a magnetic holding field, B0,
to provide a quantization axis.
SOLENOID
The strength and uniformity of the
field play an important role in obtaining
long polarization relaxation times.
•
Depolarization effects due to field
gradients should be smaller than
the relaxation due to surface effects.
Main constraints

Produce magnetic holding field of 7G with a relative field gradient of 10-3 cm-1 for
6 bars in the target cell area.
 Appropriate outer radius to fit inside the Crystal Ball detector.
 Target cell dimensions determine the inner radius.
 The outgoing particles have to pass the coil
minimization of the wire thickness.
Patricia Aguar Bartolomé
Magnetic field calculation
• Numerical calculation of the field and gradients with a finite
element code (FEMM)
• Geometric parameters for the solenoid
Parameter
Value
Longitud solenoide
No vueltas
Corriente
Diámetro interior
Diámetro exterior
Diámetro conductor
80 cm
1457
0.3 A
82.0 mm
83.06 mm
0.53 mm
• Coil wound on a CFK-tube with cross section of 82 x 1 mm2 and 120 cm
length.
• B magnitude determined using the three axis magnetometer MAG-03 MS
(Bartington Instruments Ltd.)
Patricia Aguar Bartolomé
Solenoid test
Magnetic field for a solenoid
L=80cm
7
B(r=0cm)
B(r=1cm)
B(r=2cm)
6
5
-3
1.0x10
dB(r=0cm)
dB(r=1cm)
dB(r=2cm)
-4
8.0x10
3
-4
6.0x10
2
-4
4.0x10
-1
1
z (cm )
B(G)
4
Homogeneity for a solenoid
L=80cm
0
-20
0
20
40
z(cm)
60
80
100
-4
2.0x10
0.0
-4
-2.0x10
Target
Region
-4
-4.0x10
-4
-6.0x10
-4
-8.0x10
-3
-1.0x10
-20
0
20
40
60
80
100
z(cm)
Patricia Aguar Bartolomé
3He
•
feasibility test
Goal: Study the ratio of nuclear scattering events produced on the windows
of the target cell compared to that produced on the gas.
•
Detectors
 Scattering experiment performed with
the 855 MeV polarized photon beam at
MAMI.
 Main detector was CB.
 Vertex detector consisting of two coaxial
MWPCs with cathode readout.
 PID coaxial with the MWPCs used to
identify charged particles.
Patricia Aguar Bartolomé
Experimental setup
20 cm
• Quartz
• 100 cm3
• 0.4 cm wall
• Pressure up 10 bar
• 50 m kapton windows
50 m
• Entrance cell window placed 2 cm downstream
from CB center
• Leak test performed in the whole system
Patricia Aguar Bartolomé
Data analysis and results
Analysis focused on identifying hadronic events from the particles scattered
in the target cell.
Normalized Counts (a.U.)
Z-Vertex from 2 Trajectories
empty target
4
He (7 bar)
(same angular
acceptance)
Nw / Ng = 0.93
NWindows / N4He  0.936554
Experiment is possible!!!
Z (cm)
Patricia Aguar Bartolomé
Summary and outlook
 Conclusions
 A solenoid produced to provide the holding field for the 3He target
was designed and tested
 B


  110 3 cm 1

Z
 First measurements of the homogeneity give values of 
B0 


 3He feasibility test was successfully performed giving a 0.93 ratio which
is in good agreement with the 0.89 theoretical prediction
EXPERIMENT IS POSSIBLE!!!
 Future
 Production of a new cylindrical target cell
aluminosilicate
materials present better relaxation times
 New measurements for the relaxation time in a polarized cell placed
inside the solenoid
Patricia Aguar Bartolomé
3He
Polarization
Spin-exchange optical pumping
(Bouchiat et al., Phys. Rev. Lett. 5 (1960) 373)
 Circularly polarized light at 795 nm
optically pumps an alkali-metal
vapor
optical electron spins
aligned along the quantization axis.
 Spin-exchange collisions between
Rb and the 3He
transfer of
angular momentum from Rb to 3He
nuclei
Patricia Aguar Bartolomé