2 National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

We first measured the energy loss of charge-frozen ions (i.e., partially stripped ions which maintain their charge states throughout the passage) in a Si crystal under non-channeling conditions. Energy loss measurements of charge-frozen ions with several MeV/u energy have been practically difficult, because of the rapid charge exchange processes even in a very thin foil target unless under channeling conditions, and a high-resolution magnetic spectrograph is often in need to detect the small energy loss compared to the initial projectile energy. In the several 100 MeV/u energy region, however, the charge exchange cross sections are much reduced, which allows us to measure the energy loss of charge-frozen ions in a substantially thick target. We directly measured energy depositions of 390 MeV/u bare and hydrogen-like Ar ions to a totally depleted silicon detector with 4.1μm thickness in coincidence with the charge states of the transmitted ions. In the present case, the escape of energetic binary encountered electrons from the target detector has to be taken into account to evaluate the energy loss of ions. With a correction for this effect by a Monte Carlo simulation, we observed the screening effect in charge-frozen hydrogen-like Ar ions, and have obtained the effective charge of Z_eff=17.3.