462
Y. Yoshida
process of the residual Austenite phase. This method appears, therefore, to be very
sensitive to detect the kinetics in Fe steels for low concentrations of carbon, as low
as 0.5 at. %.
This thermal scanning shows that the zero-velocity counts of the 14.4 keV-γ-rays
emitted from
57 Co-in-Fe source strongly depend on the number of Fe atoms in pure
α-Fe, i.e., in a region without any perturbations from carbon impurities. The onresonance absorption in the sample increases, and therefore the transmission counts
decrease when the carbon clustering and the carbide formation processes gradually
proceed during the isochronal annealing. On the other hand, the counts from
57 Co-inRh source increase when the number of Fe atoms in the Austenite phase decreases.
Finally, the thermal scan technique convinced us that the zero velocity counts would
be sensitive enough to detect small changes due to the defect’s recovery stages in
pure Fe.
9.3.2 Thermal Scanning Studies of 57 Co-doped-Fe Irradiated
by Neutrons and Electrons at Low Temperature
We investigated the recovery process of the excess point defects introduced by energetic particle irradiation at low temperature, to clarify the kinetics of the point defects
in pure Fe. Mössbauer spectroscopy appears to be ideal for identifying the “defect
components” through the hyperfine interactions. Their concentrations are, however,
expected to be too small to be detected as a “defect component” in a Mössbauer
spectrum. The thermal scanning technique [38, 39] must, therefore, enable us to
study the kinetics of the recovery process of point defects in pure Fe irradiated at
low temperature as a function of isochronal annealing temperature [39].
An ultrahigh pure-iron foil with a resistivity ratio of R 300K /R 4.2 K (RRR H ), 5000,
was irradiated at 30 K by fast neutrons with a total fluence of 5 × 10
16 n/cm
2 , and
the Mössbauer spectra were measured at 77 K using a stainless-steel absorber. The
left-hand side of Fig. 9.16 show the spectra as irradiated, annealed at 110 K, 150 K,
180 K, 230 K, and 280 K, respectively, while we plot their successive subtractions
on the right-hand side. Although there appears to be no defect component as a
well-resolved satellite in the spectrum after the neutron irradiation, we obtained the
average hyperfine field of 35.7 T from a fitting analysis with one sextet, the value
of which is considerably higher than that of pure iron, i.e., 33.8 T at 77 K. This
change can be found in the subtraction between the spectrum after the annealing at
110 K and the spectrum as-irradiated. The red lines in the subtraction plot indicate
the positions of the outermost sextet of Fe corresponding to the hyperfine splitting
of 33.8 T. The result means that a defect component with a larger hyperfine field
was formed presumably in the cascades by the neutron irradiation. The spectra and
also the subtractions obtained from further annealing treatments indicate only small
changes.
Y. Yoshida
process of the residual Austenite phase. This method appears, therefore, to be very
sensitive to detect the kinetics in Fe steels for low concentrations of carbon, as low
as 0.5 at. %.
This thermal scanning shows that the zero-velocity counts of the 14.4 keV-γ-rays
emitted from
57 Co-in-Fe source strongly depend on the number of Fe atoms in pure
α-Fe, i.e., in a region without any perturbations from carbon impurities. The onresonance absorption in the sample increases, and therefore the transmission counts
decrease when the carbon clustering and the carbide formation processes gradually
proceed during the isochronal annealing. On the other hand, the counts from
57 Co-inRh source increase when the number of Fe atoms in the Austenite phase decreases.
Finally, the thermal scan technique convinced us that the zero velocity counts would
be sensitive enough to detect small changes due to the defect’s recovery stages in
pure Fe.
9.3.2 Thermal Scanning Studies of 57 Co-doped-Fe Irradiated
by Neutrons and Electrons at Low Temperature
We investigated the recovery process of the excess point defects introduced by energetic particle irradiation at low temperature, to clarify the kinetics of the point defects
in pure Fe. Mössbauer spectroscopy appears to be ideal for identifying the “defect
components” through the hyperfine interactions. Their concentrations are, however,
expected to be too small to be detected as a “defect component” in a Mössbauer
spectrum. The thermal scanning technique [38, 39] must, therefore, enable us to
study the kinetics of the recovery process of point defects in pure Fe irradiated at
low temperature as a function of isochronal annealing temperature [39].
An ultrahigh pure-iron foil with a resistivity ratio of R 300K /R 4.2 K (RRR H ), 5000,
was irradiated at 30 K by fast neutrons with a total fluence of 5 × 10
16 n/cm
2 , and
the Mössbauer spectra were measured at 77 K using a stainless-steel absorber. The
left-hand side of Fig. 9.16 show the spectra as irradiated, annealed at 110 K, 150 K,
180 K, 230 K, and 280 K, respectively, while we plot their successive subtractions
on the right-hand side. Although there appears to be no defect component as a
well-resolved satellite in the spectrum after the neutron irradiation, we obtained the
average hyperfine field of 35.7 T from a fitting analysis with one sextet, the value
of which is considerably higher than that of pure iron, i.e., 33.8 T at 77 K. This
change can be found in the subtraction between the spectrum after the annealing at
110 K and the spectrum as-irradiated. The red lines in the subtraction plot indicate
the positions of the outermost sextet of Fe corresponding to the hyperfine splitting
of 33.8 T. The result means that a defect component with a larger hyperfine field
was formed presumably in the cascades by the neutron irradiation. The spectra and
also the subtractions obtained from further annealing treatments indicate only small
changes.