Biomedical Engineering Reference
In-Depth Information
8.2.3
Ionization of molecules
The situation for molecules is less well established, as illustrated in Fig.
8.3
for
simple molecules such as CO
2
,N
2
,H
2
OandO
2
. Even so, interesting differences
emerge in comparison with atoms (e.g. significant formation of Ps at several hundred
eV and ionization-excitation) which may provide useful insights into the interaction
of positrons with larger molecular systems. As with atoms, the contribution this
process makes to
Q
i
is high. Convergence among different experiments has not
yet been achieved and theoretical results are absent with the exception of H
2
Ofor
which one high-energy calculation exists [
29
]. Both determinations of
Q
Ps
for O
2
display a distinctive early peak followed by a second peak at higher energies. The
local minimum between these peaks is thought to arise from the coupling between
Ps formation and excitation of O
2
Q
i
to the Schumann-Runge band, as present in
shown in the inset [
30
].
It has been recently found [
37
] that significant ionization-excitation occurs
during positron-impact ionization of CO
2
and N
2
(see Fig.
8.4
). For both these
a
b
4
6
N
2
3
CO
2
5
2
3
1
4
0
3
2
1
10
100
2
1
1
0
0
10
100
1000
10
100
1000
Energy (eV)
Energy (eV)
c
d
3.5
2.0
H
2
O
O
2
2
3.0
1.5
1
2.5
0
2.0
10
1.0
1.5
1.0
0.5
0.5
0.0
0.0
10
100
10
100
Energy (eV)
Energy (eV)
Q
Ps
for CO
2
:[
31
](
M
O
Fig. 8.3
(a)
measurements of
), [
7
](
ı
), the upper (
)andlower(
) limits
of [
32
] and the early work of [
33
](
Q
Ps
for N
2
:
—[
31
],
—[
34
], ˘—[
33
]. Inset: [
31
] compared those of [
34
] renormalized to demonstrate
energy dependence similarity.
(c)
˘
).
(b)
three experimental determinations of
Q
Ps
for H
2
O: [
7
](
O
), [
9
](
), calculation of [
29
] (solid line).
(d)
measurements of
Q
Ps
for O
2
:[
34
](
ı
)and[
33
](
). Inset shows local minimum in
Q
i
(
ı
—[
35
]) for this target coinciding with peak in the excitation cross-section [
36
]
