474
Y. Yoshida
As the first research project, we performed the in-beam experiments on a fast
diffusion of
57 Fe in metals and semiconductors, where no practical solubility of Fe
impurity exists to perform a conventional absorber experiment. We measured the inbeam Mössbauer spectrum as a function of temperature, which indicated a sudden
area decrease accompanied with a quadrupole relaxation below 50 K, which enable us
to interpret as a local jump process of
57 Fe atom (i.e., cage motion) on interstitial sites
in Al [55] and α-Zr [56, 57]. Subsequently, the line broadenings due to a long-range
interstitial diffusion were also found in the spectra of Fe in Si [58]. Furthermore, we
obtained the in-beam spectra in Alkaline metals [59, 60], Pb [59–61] and Fe [59, 62].
Later, at RIKEN, we applied this technique for the diffusion studies of Fe in solid
Argon [63, 64], and in graphite [65].
Here, we are going to explain another application of this novel technique for
an in-situ experiment on a radiation-enhanced diffusion in pure iron via excess
vacancies distributing in the cascades created by the recoil-implantation of Coulombexcited
57m Fe. In the cascades, there are various defects, such as self-interstitials and
vacancies in the vicinity of the implanted
57m Fe atoms. At higher temperatures, these
defects become mobile even within our short measuring time (400 ns) just after the
implantation. Consequently, the mobile defects could induce the diffusion of the
implanted
57m Fe atom, if the defects would encounter the implanted 57m Fe atom
and would trigger exchange jumps between the 57m Fe atom and the defects. This
process might be observable as a line broadening and a relaxation effect on hyperfine
parameters. There was some evidence for such behaviour [59, 62].
Figures 9.28 shows a Mössbauer spectrum (a) in
56 Fe foil at 5 K, and (b) in natural
Fe foil at 30 K, respectively, and (c) all the spectra measured in
56 Fe foil from 5 to
716 K after Coulomb-Excitation and Recoil-Implantation of
57m Fe. The
56 Fe foil
does not absorb the 14.4 keV-γ-ray resonantly, while the natural Fe foil absorbs the
γ-rays emitting from
57m Fe atoms on substitutional site. This explanation is possible
because the latter contains 2.14%
57 Fe, distributed randomly on substitutional sites
in the
56 Fe matrix. Accordingly, a defect component can appear more pronouncedly
in the spectrum of the natural Fe foil. Indeed, the spectrum in Fig. 9.28(b) consists of
two sextets due to a defect component with a hyperfine field of 35.04 (4) T in addition
to the substitutional one with the hyperfine field of 33.8 T. The result coincides with
the effects observed in the sample of
57 Co-doped-Fe after neutron irradiation at 30 K,
which produces the cascade damages around
57 Co/
57 Fe probes; we found a sextet
component with a larger hyperfine field of 35.7 T.
In Fig. 9.29, we present all the fitting parameters as functions of the measuring
temperature. We obtained those values from one sextet analysis on all the spectra of
as-rolled
56 Fe sample between 5 and 716 K, shown in Fig. 9.28 (c). The resonance
area decreases stepwise at around 300 K and at 650 K, which deviates from the
expected change predicted by the Debye model with a Debye temperature of 470 K,
as in Fig. 9.29(a). The hyperfine fields below 400 K are slightly higher than those
of the laboratory experiment (Fig. 9.29(b)), as can be seen in the difference plot (c).
The difference tends to zero at around 400 K, and increases and finally decreases
again at about 700 K. The centre shift follows the second-order Doppler shift (d). It
Y. Yoshida
As the first research project, we performed the in-beam experiments on a fast
diffusion of
57 Fe in metals and semiconductors, where no practical solubility of Fe
impurity exists to perform a conventional absorber experiment. We measured the inbeam Mössbauer spectrum as a function of temperature, which indicated a sudden
area decrease accompanied with a quadrupole relaxation below 50 K, which enable us
to interpret as a local jump process of
57 Fe atom (i.e., cage motion) on interstitial sites
in Al [55] and α-Zr [56, 57]. Subsequently, the line broadenings due to a long-range
interstitial diffusion were also found in the spectra of Fe in Si [58]. Furthermore, we
obtained the in-beam spectra in Alkaline metals [59, 60], Pb [59–61] and Fe [59, 62].
Later, at RIKEN, we applied this technique for the diffusion studies of Fe in solid
Argon [63, 64], and in graphite [65].
Here, we are going to explain another application of this novel technique for
an in-situ experiment on a radiation-enhanced diffusion in pure iron via excess
vacancies distributing in the cascades created by the recoil-implantation of Coulombexcited
57m Fe. In the cascades, there are various defects, such as self-interstitials and
vacancies in the vicinity of the implanted
57m Fe atoms. At higher temperatures, these
defects become mobile even within our short measuring time (400 ns) just after the
implantation. Consequently, the mobile defects could induce the diffusion of the
implanted
57m Fe atom, if the defects would encounter the implanted 57m Fe atom
and would trigger exchange jumps between the 57m Fe atom and the defects. This
process might be observable as a line broadening and a relaxation effect on hyperfine
parameters. There was some evidence for such behaviour [59, 62].
Figures 9.28 shows a Mössbauer spectrum (a) in
56 Fe foil at 5 K, and (b) in natural
Fe foil at 30 K, respectively, and (c) all the spectra measured in
56 Fe foil from 5 to
716 K after Coulomb-Excitation and Recoil-Implantation of
57m Fe. The
56 Fe foil
does not absorb the 14.4 keV-γ-ray resonantly, while the natural Fe foil absorbs the
γ-rays emitting from
57m Fe atoms on substitutional site. This explanation is possible
because the latter contains 2.14%
57 Fe, distributed randomly on substitutional sites
in the
56 Fe matrix. Accordingly, a defect component can appear more pronouncedly
in the spectrum of the natural Fe foil. Indeed, the spectrum in Fig. 9.28(b) consists of
two sextets due to a defect component with a hyperfine field of 35.04 (4) T in addition
to the substitutional one with the hyperfine field of 33.8 T. The result coincides with
the effects observed in the sample of
57 Co-doped-Fe after neutron irradiation at 30 K,
which produces the cascade damages around
57 Co/
57 Fe probes; we found a sextet
component with a larger hyperfine field of 35.7 T.
In Fig. 9.29, we present all the fitting parameters as functions of the measuring
temperature. We obtained those values from one sextet analysis on all the spectra of
as-rolled
56 Fe sample between 5 and 716 K, shown in Fig. 9.28 (c). The resonance
area decreases stepwise at around 300 K and at 650 K, which deviates from the
expected change predicted by the Debye model with a Debye temperature of 470 K,
as in Fig. 9.29(a). The hyperfine fields below 400 K are slightly higher than those
of the laboratory experiment (Fig. 9.29(b)), as can be seen in the difference plot (c).
The difference tends to zero at around 400 K, and increases and finally decreases
again at about 700 K. The centre shift follows the second-order Doppler shift (d). It