JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Simultaneous Measurement of Secondary-electron
Emission and Coulomb Explosion Imaging for
250-keV/u C2 + Ions Bombarded to Thin Carbon Foils
Y. Takahashi a) , K. Narumi a) , A. Chiba b) , Y. Saitoh b) , K. Yamada b) ,
N. Ishikawa a) , H. Sugai a) c, a)
and Y. Maeda
a) Advanced Science Research Center, JAEA, b) Department of Advanced Radiation Technology,
TARRI, JAEA, c) Department of Energy Science and Technology, Kyoto University
The origin of vicinage effect on secondary-electron
emission induced by swift molecular ions or cluster ions is
still an unresolved question although many studies have
been performed. The secondary-electron emission under a
swift monoatomic ion impact has been accounted for by a
three-step process: production (excitation) of scattered
electrons by the incident ion, transport of the scattered
electrons in the solid, and finally, transmission of the
scattered electrons through the surface barrier. It has been
pointed out that the mechanism of the vicinage effect is
mainly due to disturbance in the transport process by an
1)
electric potential induced by fragment ions . Under this
mechanism, it is expected that the potential can strongly
affect the transport process, and such influence may depend
on the relative position of trajectories of fragment ions. In
this study, we have measured the correlation between
secondary-electron emission and relative position of
fragment ions obtained from Coulomb explosion imaging
for C 2 + ions bombarded to thin carbon foils.
The swift C + and C 2 + ions with the energies of 250 keV/u
(3.2 v 0) and self-supporting amorphous carbon foils of 1.4,
2.8, 14.1 μg/cm 2 (70-700 Å) were used. The secondary-
electron yield γ n in the forward and backward directions
from a carbon foil were measured simultaneously by two
microchannel-plate (MCP) detectors. Fragment ions with
different charge states transmitted through a foil were
separated by electrostatic deflection plates, and detected as
luminous points by an MCP detector equipped with a
fluorescent screen. The orientational angle ϕ between the
internuclear vector of a pair of fragment ions after the
penetration of the foil and the beam direction was derived
from the relative positions of the two luminous points. The
γ 2(ϕ) as a function of the orientational angle divided into
three angular regions was obtained from the distribution of
the number of secondary electrons for each orientational
angle. We also measured γ 1 using C + ions with the same
velocity. The orientation dependence of the vicinage effect
was evaluated from the ratio of the secondary-electron yield
R 2(ϕ) = γ 2(ϕ)/2γ 1. Appearance of the vicinage effect is
presented as R 2(ϕ) ≠ 1.
Figure 1 shows γ 2(ϕ) in the backward and forward
directions for the three foils with different thickness for a
pair of outgoing C 3+ - C 3+ ions. The forward yield was
larger when the internuclear vector of a pair of fragment
ions was parallel to the beam direction than when it was
perpendicular for the thinnest foil. No orientation
JAEA-Review 2010-065
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dependence was observed for the other thicker foils in the
forward direction and for all foils in the backward direction.
The ratio R 2(ϕ) for ϕ = 0-30º and ϕ = 60-90º in the forward
direction plotted as a function of the calculated internuclear
distance between the fragment ions at the exit of the target is
shown in Fig. 2. It indicates that the dependence vanishes
at the internuclear distance of ~1.6 Å, although the vicinage
effect in the transport process has been observed even for
~70 Å in this velocity 2) . Hence, the present result indicates
that there is little effect of the orientation in the transport
process, and the orientation dependence observed in the
forward direction for the thinnest foil could be attributed to
the production process of electrons. No orientation
dependence of the vicinage effect in the transport process
was observed for the region of foil thickness used in this
study. Therefore, it is conceivable that a response distance
of the electric potentials induced by fragment ions is much
larger than the transport length of electrons.
References
1) H. Arai et al., J. Phys. Soc. Jpn., 78 No. 10 (2009)
104301.
2) Y. Takahashi et al., Europhys. Lett. 88 (2009) 63001.
2()
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28
26
24
22
20
18
16
0 30 60 90 0 30 60 90
Orientational Angle [deg]
R 2()= 2() / 2 1
14.1g/cm 2
1.00
0.95
0.90
0.85
0.80
(a)
Backward (C 3+ - C 3+ )
2.8g/cm 2
1.4g/cm 2
Forward (C 3+ - C 3+ )
0.75
0.70
0-30deg
60-90deg
0 1 2 3 4 5 6 7
Internuclear Distance [Å]
Fig. 2 Ratio R 2(ϕ) = γ 2(ϕ)/2γ 1 in the forward direction as
a function of the internuclear distance.
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53
51
49
47
45
43
41
Forward (C 3+ - C 3+ )
14.1g/cm 2
2.8g/cm 2
1.4g/cm 2
Fig. 1 γ 2(ϕ) for (a) backward and for (b) forward emission
for a pair of outgoing C 3+ - C 3+ ions from three foils.
(b)