Xenon-gas ionization chamber for improving high-Z beam particle identification*

RI beams are used in the study of exotic nuclei. The RI Beam Factory (RIBF) at RIKEN is capable of producing a variety of RI beams of 200-300 MeV/nucleon. The in-flight RI beam separator BigRIPS at RIBF has successfully provided RI beams in the Z<70 regions. Recently, the BigRIPS separator started to supply heavier RI beams with Z>70; however, one of the problems was the need for more Z resolution for particle identification.

The insufficient Z resolution is due to the increased energy loss straggling due to charge fluctuations. The Z is estimated from the particle velocity and the energy loss of the ions. The energy loss is measured using an ionization chamber (IC) at BigRIPS. The conventional IC gas, P-10 (argon 90% and methane 10%), causes a large energy loss straggling due to insufficient charge state changes.

To solve this problem, we switched from the conventional P-10 gas to xenon-based gas (xenon 70% and methane 30%) to increase the number of charge state changes. According to the charge fluctuation cross-section, the xenon-based gas increases the number of charge state changes by approximately one order of magnitude compared to P-10 gas. By reducing the energy loss straggling caused by the charge fluctuation, the Z resolution is expected to improve.

The Z resolutions of the xenon-based gas IC and the P-10 gas IC were evaluated using a cocktail beam with Z=40-90 at 200-250 MeV/u. The results of the xenon-based gas IC showed a Z resolution comparable to that of the P-10 gas IC at Z<70 and a remarkable improvement in Z resolution at Z>70. The Z resolution (FWHM) at Z=84-88 was 1.3 for the P-10 gas and 0.8 for the xenon-based gas, which is a 1.7-fold improvement over the P-10 gas. The simulation of the charge fluctuation explained this improvement. In conclusion, the xenon-based gas IC was highly effective for Z identification. The separation and supply of heavy RI beams using the xenon-based gas IC have already been started at RIBF.