Electronic Material Properties - und Geowissenschaften ...

Material modifications induced by swift heavy ions in NbTi
Aleksandra Newirkowez 1 , Joachim Brötz 1 , Hartmut Fuess 1 , Reinhard
Neumann 2 , Christina Trautmann 2 , Kay-Obbe Voss 2
1 University of Technology, Darmstadt, Germany; 2 GSI, Darmstadt, Germany
The Facility for Antiproton and Ion Research (FAIR) to be built at GSI will be equipped with
superconducting magnets. Due to the high beam intensities and related beam losses,
radiation damage of the Cu/NbTi superconducting wires used in the magnet coils has to be
considered [1]. Radiation-induced degradation of the superconducting properties of NbTi
alloy was studied in the 1970s and ‘80s. Using projectiles such as protons, neutrons,
deuterons, and O ions, was shown to result in increased electrical resistivity and
decreased critical current, superconducting transition temperature, and upper critical
magnetic field [2]. These irradiations with light and/or low-energy ion beams involve small
electronic energy losses.
Here we are interested in radiation effects in particular in structural changes produced with
swift heavy ions (2.6-GeV U) of much higher electronic energy deposition.
A superconducting wire has typically a diameter of ~1 mm and consists of several
thousand thin NbTi-filaments arranged in bundles embedded in a copper matrix for thermal
stabilization (Fig. 1). The filaments are a few µm thick and are composed of β-NbTi (bcc)
and α-Ti precipitate (hcp) which improve the critical current by flux-pinning. During the
cold-drawing process of the filaments, the β-NbTi matrix develops a
crystallographic texture.
Fig. 1: Optical micrograph of cross-section of multifilamentary wire (dark: NbTi filaments, light: Cu
matrix).
The irradiation of NbTi filaments was performed at the UNILAC (GSI) applying fluences
between 3×10 11 and 5×10 12 ions/cm 2 at a flux of 1 - 2×10 8 ions/cm 2 ·s. Before irradiation,
the copper matrix was dissolved in aq. FeCl3. The filaments were clamped on an Al holder
and exposed to a beam of 2.6-GeV U ions at room temperature. The electronic energy
loss (dE/dx)e for U in NbTi is 53 keV/nm.
Radiation-induced effects were examined using transmission electron microscopy (TEM)
(Philips CM20, 200 keV), x-ray powder diffraction (λMo–Kα1 = 0.70926 Å) in flat-sample
transmission geometry and with four-circle diffractometry (λCo–Kα = 1.78897 Å).
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