TRIAC Progress Report - KEK

higher efficiency for gaseous elements is considered partly due to desorption of the
injected ions on the surface of the plasma chamber. The desorption of the gaseous ions
may lead to the weak dependence of εCB on the ∆V over a rather wider range as shown
in Fig. 2-8. On the other hand, the lower value of εCB for the nongaseous elements
suggests that the adsorption to the surface is not negligible. The improvement of the
beam injection efficiency to the ECR plasma may help increase εCB for the nongaseous
elements, if the ionization efficiency of ECRIS is element independent. The εCB was
found to be independent of the lifetime for radioactive nuclei with lifetimes of a few
seconds, as shown in Table 2-3. This result suggests that the charge breeding time is
shorter than the lifetimes of presently studied isotopes, which is consistent with the
charge breeding time of about 100 ms measured at the KEKCB [2-9]. We suppose that
εCB could be dependent on lifetime for RIBs with lifetimes shorter than a few 100 ms.
2-3-3. Element-dependent charge breeding efficiency
The charge breeding efficiencies are 7.4 % for Xe 20+ and 2.3 % for In 16+ , which are
typical efficiencies for radioactive gaseous and metallic elements with a half-life time of
~ 1 s., as well as for stable ions already shown in Fig. 2-8. The efficiencies for gaseous
elements are observed to be higher than those for metallic ones, usually by a factor of
three as long as the similar masses of the elements are concerned. Because of the
significantly different behavior in the injection, which just depends on if the element is
volatile or not at room temperature as discussed above, one can conclude that the lower
charge breeding efficiencies for metallic ions could be improved by more careful
optimization of the injection. The difference could be attributed to ad- and de-sorption
of the ions on the surface of the plasma chamber during the injection; the gaseous
elements adsorbed on the surface could be more easily desorbed and recycled for further
ionization, as compared to the case of metallic, non-volatile elements that stick to the
surface once adsorbed. On the other hand, the element-dependent ionization efficiency
cannot be completely excluded. Therefore, in order to identify the direction of the
development to obtain higher breeding efficiency, especially for metallic elements, it is
of importance to study how the injected ions, but failed to be re-extracted, are
distributed on the surface of the plasma chamber. The distribution of the injected ions
accumulated on the surface of the plasma chamber would help us to understand when
the injected ions are adsorbed, i.e. at the time of injection or in the course of the
step-by-step ionization in the ECR plasma after being well captured.
We have injected radioactive ions of 111 In into the KEKCB and, after charge
breeding, measured the residual activity to investigate how ions externally injected
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