Development of spin-polarized slow positron beam using a new 68Ge−68Ga/GaNpositron source and its application for positron annihilation spectroscopy
M. Maekawa, Y. Fukaya, H. Zhang, H. Li and A. Kawasuso
R.G. for Spin-Polarized Positron Beam, JAEA

Pv (1  cos ),c2here v is the positron speed, c is the speed of light and θ is anopen angle in a specific direction [3]. This equation suggeststhat positron sources with a higher energy end-point are betterfor generating highly spin-polarized positron beams.
The 68Ge−68Ga positron source has relatively long half-lifeand higher energy end-point (288 days and 1.9 MeV). Inaddition, higher cross section (0.465×10−24 cm2 at Eproton = 20MeV) of nuclear reaction 69Ga(p, 2n)68Ge allows the source
production using ion beam. Figure 1 shows schematic diagramsof the source capsule. High-purity metal 69Ga should be used asa target material, however, metal Ga is molten during irradiation
because of its low melting point (29 oC). Such molten Ga reacts
with the other metal components of the source capsule andeventually destroys the capsule. To avoid this problem, wepropose GaN as a target material, although the amount of 69Ga(p,2n)68Ge is decreased. To avoid metallization during irradiation,the GaN target was placed in a carbon tray. The target wasdirectly irradiated with 20 MeV proton beam with a maximum
current of 3.5 μA using the cyclotron at the Takasaki Advanced
Radiation Research Institute. The efficiency of the source
production was estimated to be 0.5 MBq/μA/h from the gamma
ray energy spectrum. This was in good agreement with theresults from our simulation. The irradiation was carried out over
400 h in total.
The positrons emitted from the GaN target were moderated
by a tungsten mesh moderator. The slow positrons extracted bya grid electronode were transported to the sample position by anelectrostatic system in order to avoid the depolarization during
the beam transportation.
Consequently, a positron beam with a diameter of 5 mm and aflux of 5×103 e+/s was obtained with the positron energy of
E=5~15 keV. The spin polarization of the positron beam wasdetermined to be 47 ± 8% from the magnetic quenching of thepositronium in fused silica [4]. Thus, it is important to use
positron sources with higher energy end-points to generatehighly spin-polarized positron beam.
The field-reversal asymmetry of the DBAR spectra for
ferromagnetic Fe was measured using the positron beam.
Figure 2 shows the differential DBAR spectra in a magnetic
field of 0.55 T [4]. The differential DBAR spectra [N↑(p) andN↓(p)] were obtained by altering the field polarity. The finite

differential intensity means that there is a field-reversal
asymmetry, which arises from the enhanced annihilation
between the spin-up positrons and spin-down 3d unpaired
electrons [5]. This also shows that our positron beam wassufficiently spin-polarized.
In summary, we produced a 68Ge−68Ga/GaN source byirradiating a GaN target with proton beam and a spin-polarized
positron beam was generated using this source. Measurementsof the DBAR spectra of fused silica and ferromagnetic Fe inmagnetic field demonstrated that the beam was sufficiently spinpolarized.
This work was supported by JSPS KAKENHI Grant No.
24310072.
(a)M1 bolt(b)GaN target

Fix ring

8mm

GaN target
Carbon tray
Proton absorber(tungsten)Pedestal

Fig.1 (a) Schematic and (b) photograph of the source
capsule.
0.0002N(p)-N(p) (arb.units)

Positrons emitted from radioisotopes are longitudinally spinpolarized due to the parity non-conservation in the weakinteraction. Positron annihilation studies have shown that theferromagnetic band structure can be investigated using theDoppler broadening of annihilation radiation (DBAR) techniquewith spin-polarized positrons [1,2]. Considering the fact thatvarious novel spin phenomena have recently been found tooccur at surfaces/interfaces and in thin films, spin-polarized
positron beams is useful in spintronics studies. We developed ahighly spin-polarized beam from a new 68Ge−68Ga source.
The spin polarization of positrons emitted from a positronsource P is given by

0.00010.0000-0.0001-0.0002-30 -20 -10010 -3 20
Electron momentum p (10 m0c)

30

Fig.2 Differential DBAR spectra of Fe sample obtained in
the external magnet field of 0.55 T.
References[1] A. Kawasuso et al., Phys. Rev. B 83 100406(R) (2011).
[2] D. W. Gidley et al., Phys. Rev. Lett. 49 1779 (1982).
[3] L. A. Page, Rev. Mod. Phys. 31 759 (1959).
[4] M. Maekawa et al., Nucl. Instr. Methods B 308 9 (2013).
[5] T. W. Mihalisin et al., Phys. Lett. 21 610 (1966).