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Owing to the potentially therapeutic enhancement of delayed particles in treating malignant diseases by radioactive 9 C-ion beam, LET spectra at different penetration depths for a 9 C beam with 5% momentum spread, produced in the secondary beam line (SBL) at HIMAC, were measured with a multi-wire parallel-plate proportional counter. To compare these LET spectra with those of a therapeutic 12 C beam under similar conditions, the 12 C beam was replaced with an 11 C beam, yielded in the SBL as well and having almost the same range as that of the 9 C beam. The LET spectra of the 9 C beam and its counterpart, i.e. the 11 C beam, at various depths were compared, especially around the Bragg peak regions. The results show that nearby the Bragg peak lower LET components decreased in the LET spectra of the 9 C beam while extra components between the LET peak caused by the primary beam and the lower components due to the fragments could be observed. These additional contributions in the LET spectra could be attributed to parts of the emitted particles from the radioactive 9 C ions with suitable conditions regarding the LET counter. Integrating these LET spectra in different manners, depth-dose and dose-averaged LET distributions were obtained for the 9 C and 11 C beams, forming the basic data sets for further studies. In general, the depth-dose distributions of the 9 C and 11 C beams are comparative, i.e. almost the same peak-to-plateau ratio. The ratio for the 9 C beam, however, has room to increase due to the geometric structure limitation of the present detector. The doseaveraged LETs along the beam penetration are always lower for the 9 C beam than for the 11 C beam except at the falloff region beyond the Bragg peak. Applying the present depth-dose and dose-averaged LET data sets as well as

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