Report No xxxx - Instytut Fizyki JÄ drowej PAN

OPTICAL PUMPING OF 3 He
Katarzyna Cieślar, Katarzyna Suchanek, Mateusz Suchanek, Zbigniew Olejniczak*,
Tadeusz Pałasz, Tomasz Dohnalik
Institute of Physics, Jagiellonian University, Kraków; *Institute of Nuclear Physics, Kraków, Poland
High field thermal polarization achieved in standard MRI systems is of the order
of 10 -6 . Optical pumping techniques enable to create polarization exceeding the thermal levels
by up to six orders of magnitude (hyperpolarization) in two stable noble gas isotopes: 3 He,
129 Xe. As both gases are metabolically inert, they can be used as a source of NMR signal in
the MRI lung imaging. This method, however, generates a non-equilibrium polarization,
which has important consequences to the imaging procedures used.
An optical pumping method called metastability exchange is used to produce the
hyperpolarized 3 He. A glass cell containing the 3 He gas at a low pressure (1 torr) is placed in
a magnetic field of about 30 Gs. As the optical transition between 1 1 S 0 and 2 3 S 1 states is
strictly forbidden, a weak RF discharge is used to populate the metastable 2 3 S 1 state (Fig.1).
3
10 -8 [s]
1 D 2
2 3 P
3
2*10 -8 [s]
3 D
0
2 3 P
2 3 P 1
2 1 P 5*10 -10 [s]
1
2 3 P 10 -7 [s]
2 3 P 2
2
2*10 -2 [s]
1 S 0
8*10 3 [s]
2 3 S 1
1083 nm
METASTABLE
STATE
2 3 S 1
1 1 S 0
Fig.1. Energy levels for 3 He atom.
F =
F =
F =
F =
F =
F =
F =
The absorption of circularly polarized (σ+) laser beam of wavelength λ = 1083nm causes a
transition from 2 3 S 1 , m F = –1/2 to 2 3 P 0 , m F = +1/2 state (Fig.2). After excitation a spontaneous
reemission to both m F = –1/2 and m F = +1/2 sublevels of 2 3 S 1 state takes place. However, the
continuous depletion of the m F = –1/2 sublevel results in higher population of the 2 3 S 1 , m F =
+1/2 state. This is equivalent to the electronic polarization of the atom. Through hyperfine
coupling the nucleus also becomes polarized. Nuclear polarization
of the ground state 3 He atom is achieved via
m = -
metastability exchange collisions in which 2 3 P 0
a non-polarized ground state atom and a polarized
F =
m =
metastable atom take part. The polarization achieved
in that way is about 80%. One can measure the
polarization using NMR or optical methods.
To use the gas in the MRI lung imaging, it is 2 3 S 1
necessary to compress it without losing the
polarization up to atmospheric pressure for which
purpose a compressor is being built.
1083 nm
σ+
F =
Fig.2.Optical pumping.
m = -
m =
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