  In conventional ionization chambers (ICs) using P-10 (Ar+CH4) gas, as the
atomic number (Z) of the ion beams increases in the energy region of 200-300
MeV/u, the Z resolution deteriorates rapidly when Z>70. This degradation is
attributed to substantial energy loss straggling caused by charge state
fluctuation when the beams traverse a gas medium. The energy loss straggling
increases when the beams cannot attain charge state equilibrium in the IC gas.
In this study, a xenon-based gas (Xe+CH4), exhibiting a sufficiently large
charge state changing cross section, was used in the IC to reach charge state
equilibrium. The responses of ICs with P-10 and the xenon-based gases were
examined using 238U beams and cocktail radioactive isotope (RI) beams with
Z=40-90 at the RI Beam Factory (RIBF). For 238U beams at 165-344 MeV/u, the
P-10 gas IC yielded an energy resolution of 1.9-3.0% in full width at half
maximum (FWHM), which proved inadequate for Z identification in the uranium
region. In contrast, the xenon-based gas IC demonstrated a satisfactory energy
resolution of 1.4-1.6%. When using cocktail RI beams, a Z resolution of 1.28
and 0.74 was achieved by the P-10 and the xenon-based gas ICs, respectively,
for beams with Z=84-88 at 200 MeV/u. The contrast in Z resolutions between the
P-10 and the xenon-based gas ICs was effectively elucidated by the energy loss
straggling model, incorporating collisional straggling and straggling due to
charge state changes in the IC gases. The xenon-based gas IC, with more than
3sigma Z separation across a broad Z range (Z=40-90), emerged as a practical
solution for Z identification of heavy ion beams.
