Chemistry Reference
In-Depth Information
Fig. 6.13 Diffusion
coefficients of Fe in Si from
the Mössbauer data (filled
squares) and from other
techniques [
26
]
state into a metastable excited state
57m
Fe by the Coulomb interaction between a
projectile charged particle and
57
Fe nucleus. Subsequently, the excited state
57m
Fe
decays again to its stable ground state within a lifetime T
1/2
= 10
-6
s. The ion
implantation and Mössbauer experiment have to be performed within this timespan.
If the bombarding energy of such particles is carefully selected to be below the
Coulomb barrier, other radioactive nuclei will not be produced, which might
otherwise cause a high background for the Mössbauer experiments. As mentioned in
the previous section Latshaw [
13
,
23
] applied this technique on Fe in Si.
The technique was further optimized in an experimental set-up at the
Hahn-Meitner Institute in Berlin. A sketch of the set-up is shown in Fig.
6.15
[
29
].
A pulsed beam of 110 MeV
40
Ar ions (pulse length *1 ns, repetition
rate * 2.5 MHz) from the VICKSI heavy ion accelerator at the Hahn-Meitner-
Institute in Berlin hits an iron-foil target (thickness 3 mg/cm
2
, 90 % enriched in
57
Fe). More than 90 % of the excited
57
Fe recoils ejected from the target have
angles between 15 and 75 with respect to the beam direction. These recoiling
ions are trapped in a catcher made of the host material under investigation. The
Mössbauer c-radiation is detected in two resonance detectors of the parallel-plate-
avalanche type (PPAC) with stainless-steel foil absorbers (53 % enriched in
57
Fe).
A time resolution of 3 ns allows to measure the 14.4 keV Mössbauer radiation in a
time window of about 380 ns between the beam bursts from the accelerator (AT
400 ns). This technique strongly reduces prompt background radiation. This set-up
used for the experiments described in this section is shown in Fig.
6.16
.
