Speaker
Mr
Ilhan Engin
(Forschungszentrum Jülich)
Description
Polarized $^3$He is of particular importance for fundamental research since the spins of the two protons
are oriented anti-parallel so that the nuclear spin is basically carried by the unpaired neutron. That
is why polarized $^3$He $^1$ can be used, for example, as an effective polarized neutron target for studying
the neutron structure by scattering with polarized electrons $^2$. For many experiments in nuclear and
particle physics, like experiments with stored particle beams, the use of polarized $^3$He ion beams
would be advantageous. $^3$He gas can be polarized for long time durations at standard conditions.
However, building a spin-polarized $^3$He ion source for nuclear and particle physics experiments with
high degrees of polarization is extremely challenging. Until now, only a few approaches could be
accomplished - but not with the desired particle currents or an adequate beam polarization $^3$. At
Brookhaven National Lab's Relativistic Heavy Ion Collider (Rhic) attempts are now being made
to develop a polarized $^3$He ion beam source $^4$. An unsolved question in the context of laser-driven
ion acceleration is the in
uence of the strong laser and plasma fields on the spin polarization of the
particle beams. Two scenarios are possible here: either the magnetic fields of the incoming laser
beam or the induced plasma change the spin direction of the accelerated particles, or the spins are
too inert so that the short laser pulse has no effct on the spin alignment of a pre-polarized target,
and the polarization is conserved. In the latter case, the polarization could be conserved during
laser-acceleration processes, and also laser-induced polarized nuclear fusion with increased energy
gains seems to be feasible: due to the use of polarized fuel, the cross-sections for nuclear fusion
reactions may be enhanced which leads to higher energy yields compared to the case of unpolarized
fuel. While the above mentioned first scenario (polarization creation by laser-particle interaction)
has already been investigated with conventional foil targets by spin-dependent hadronic proton
scattering off silicon nuclei 5, for the second one (polarization conservation during laser-plasma
interaction) pre-polarized $^3$He gas can be used as production target. The relaxation rate of the
polarization degree of $^3$He is depending on several conditions, e.g. gas pressure or magnetic field
gradients. Also the absence of one electron in the atomic shell leads to a rapid decrease of the
polarization degree: the interaction time $\tau_{HF}$ for the coupling of the nuclear spins with the spin
of the remaining electron is about 0.2 ns (GHz energy level). Thus, a full ionization of the pre-
polarized $^3$He has to be accomplished within a few picoseconds. This can be easily achieved with
currently available laser intensities.
$^1$ K. Krimmer, M. Distler, W. Heil et al., A highly polarized 3He target for the electron beam at MAMI,
Nucl. Instr. Meth. Phys. Res. A, vol. 611, issue 1, p. 18, 2009
$^2$ M.Tanaka, Review from experimentalists: The role of polarized $^3$He in the next century, Nucl. In-
str. Meth. Phys. Res. Sec. A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 402, no. 2-3,
p. 492-498, 1998
$^3$ D.O. Findley et al., A polarized $^3$He$^+$ ion source, Nucl. Instr. Meth., vol. 71, issue 2, pp. 125-132, 1969
W.E. Burcham et al., A source of polarized $^3$He ions, Nucl. Instr. Meth., vol. 116, issue 1, pp. 1-7, 1974$\\$
R.J. Slobodrian et al., New method for the production of polarized $^3$He ions based on the 23S1 state of $^3$He, Nucl. Instr. Meth., vol. 185, issue 1-3, pp. 581-583, 1981
$^4$ J.Maxwell, R. Milner, C. Epstein, Development of a polarized $^3$He ion source for RHIC, Physics of Particles
and Nuclei, vol. 45, no. 1, p. 301-302, 2014
$^5$ N. Raab et al., Polarization measurement of laser-accelerated protons, Phys. Plasmas, vol. 21, 023104, 201
