Biomedical Engineering Reference
In-Depth Information
The first study of noble gases implanted into UDD by ion
bombardment was performed by mass spectrometric analysis of
noble gases released during stepped pyrolysis up to 1600°C [79]
using a protocol similar to the one used in studies of the MND [78].
The measured temperature profile of noble gas (He, Ar, Kr, and Xe)
released during pyrolysis of UDD has a bimodal character (for Ar, Kr,
and Xe) with main peaks in the ranges 200-700 and 1200-1500°C,
as shown in Fig. 6.9.
The main features of such profiles for all noble gases released
from UDD closely resemble those obtained for the diamond residual
extracted from Orgueil meteorite [78]. The most striking result is
that with a single implantation event we obtain the same double-
peak release pattern found for presolar diamonds. It is to be noted,
however, that in the latter case the bimodal character of the curves
was attributed to the presence of two distinct components of trapped
noble gases, namely, P3-component (isotopically normal) released
at low temperatures and HL (exotic) released at high temperatures
[78]. Our results clearly showed that both the low-temperature (LT)
and the high-temperature (HT) peaks may arise simultaneously
in the same substance during “one-component” ion implantation
event.
It was found in addition that the trapped gases are fractionated
favoring the heavy isotopes relative to the starting composition and
the fractionalization factor increased with the temperature of the
pyrolysis step [79]. The isotope fractionalization factors reached 0.8
(Ar) and 2.5 (Xe) per mass unit at high temperatures.
These results provided strong evidence that ion implantation
is a viable mechanism for trapping of noble gases by interstellar
diamond grains. The observed bimodal character of the release
profiles and the isotopic fractionalization during pyrolysis have been
used to reconstruct the possible scenario of implantation events in
interstellar media [79] and to interpret the noble gas compositions
in meteoritic nanodiamonds [83].
C step as a
rule) in the closed vacuum chamber (static mode) is rather time
consuming and does not allow to resolve the fine structure of
temperature profiles of noble gas release from nanodiamonds. We
have therefore developed a new approach based on TDMS method,
that allow the continuous measurements of noble gas release during
linear heating of nanodiamonds [81].
The standard procedure of stepped pyrolysis (100
o
