Water radiolysis with heavy ions of energies up to 28 GeV—2: Extension of primary yield measurements to very high LET values

Measurements of primary g-values (at ∼10−7 s after the initial ionizing event) of eaq-, OH and H2O2 were extended to the very high linear energy transfer (LET) region (∼700 eV/nm) near the Bragg peak. Heavy ions (4He2+, 12C6+, 20Ne10+, 28Si14+, 40Ar18+ and 56Fe26+) of energies up to 28 GeV were provided by the Heavy Ion Medical Accelerator in Chiba (HIMAC) of the National Institute of Radiological Sciences (NIRS) in Chiba, Japan. Energies of the ions were decreased down to about 10 MeV/u for 4He2+ using an energy absorber made of polymethylmethacrylate (PMMA) plates in order to vary the LET values. Beam was visualized after passing through the energy absorber using agarose gel of aqueous solution containing methyl viologen and sodium formate in order to determine how long ions can penetrate into water. Based on the information of the penetration depth of ions in samples, much attention was paid to dose correction and LET evaluation. The obtained data were plotted as a function of (Zeff/β)2 also instead of LET in order to discuss effects of physical track structures on product yields, resulted in better universality.