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

4-39
Relationship between Internuclear Distance and Charge
State of Constituent Ions Resulting from Foil-induced
Dissociation of C2 + ions
A. Chiba a) , Y. Saitoh a) , K. Narumi b) , Y. Takahashi b) , K. Yamada a) and T. Kaneko c)
a) Department of Advanced Radiation Technology, TARRI, JAEA, b) Advanced Science
Research Center, JAEA, c) Department of Applied Physics, Okayama University of Science
When a swift cluster ion penetrates a solid, the
interactions between cluster constituent ions and electrons in
matter give rise to the characteristic phenomena which never
appear under a monatomic-ion irradiation due to the
interference resulting from the spatiotemporal correlation
among the closely spaced ions within the cluster, that is to
say “vicinage effect”. Since Brandt et al. 1) showed
experimentally and theoretically that the energy loss per
atom of a swift hydrogen cluster was different from that of a
proton at the same velocity, the characteristic phenomena
caused by the penetration of a heavy-atom cluster also have
been frequently observed not only for the energy loss but
also for the average charge state of constituent ions and for
the secondly-particle emission from the target surface.
These vicinage effects had been presented also theoretically
2)
by the several authors , and they have indicated that
analysis of the charge state of the constituent ions inside
matter is significant in the clarification of vicinage effect,
because the interaction forces such as stopping power, wake
force and Coulomb repulsion depend on the charge state of
individual constituents in the cluster. The charge-state
measurements of N2 + constituents penetrated to the various
foil-thicknesses indicated that the vicinage effect in charge
state gradually decreased with increasing internuclear
3)
distance due to the mutual Coulomb repulsion . This fact
means that the internuclear distance is extremely significant
parameter for the precise evaluation in vicinage effect.
Therefore, a measurement of the divergence angular
distributions classified into the charge-state combination of
constituent ions resulting from foil-induced dissociation of
6-MeV C2 + ions was carried out in order to investigate the
relationship between the internuclear distance and the charge
state of individual constituent ions.
The magnetically selected C2 + ion beam extracted from
the tandem accelerator in the TIARA was pulsed and
collimated by a beam chopper composed of a switching
deflection plates and a pair of 0.1-mm-diam aperture, and
penetrated an amorphous carbon foil of 0.5-μg/cm 2 thickness.
The dissociated constituent ions from C2 + emerging from the
foil were deflected depending on their charge state by an
electric field applied to the deflection plates, and were
detected by a micro-channel plate (MCP) equipped with
fluorescent screen placed at 1.4 m downstream from the foil.
The pictures of the luminance points on the MCP taken by a
CMOS camera were saved event-by-event to the computer
at the rate of approximately 50 frames per second. The
divergence angle and the charge state of constituents were
determined from an arrangement of a pair of luminance
JAEA-Review 2010-065
- 163 -
points on the MCP. An overall resolution of the divergence
angle was less than 1.4 × 10 -4 rad (FWHM).
In this experimental condition, the kinetic energy of
constituent ions would be unaffected almost by the elastic
collision with target nuclei. Therefore, when the incident
spatial orientation of C2 + projectile at the foil surface is
perpendicular to the beam direction, the divergence angle of
these constituents would correspond to a peak value (θmax) in
the angular distribution geometrically because the electronic
perturbation due to a wake force is negligible. Figure 1
shows the relationship between the charge-state combination
+
qi × qj and θmax for 6-MeV C2 projectiles, where qi and qj are the charge state of dissociated i- and j-ion. The
measured θmax in each angular distribution classified into the
charge state combination of constituent ions cannot be
reproduced by the simple model calculation, which takes
into account the interaction through a screened Coulomb
repulsion. This consequence suggests that the final
charge-state combination of constituents emerging from the
foil depends on the initial internuclear-distance which is
fluctuated due to a vibration excitation of C2 + ion, if this
disagreement is resulting from an assumption that the initial
internuclear-distance of all C2 + ions is an average value in
this calculation.
References
1) W. Brandt et al., Phys. Rev. Lett. 33 (1974) 1325.
2) For example, T. Kaneko, Phys. Rev. A 66 (2002) 52901.
3) D. Maor et al., Phys. Rev. A 32 (1985) 105.
θ max [mrad]
12
9
6
3
3 6 9 12 15
Charge-state combination q X q i j
Fig. 1 Charge-state combinations qi × q j of dissociated ions
resulting from impact of 6-MeV C2 + ions on 0.5-μg/cm 2
carbon foil, as a function of θmax. The dashed and solid
lines are fits to the experimental and calculated data,
respectively.