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We designed a ridge filter to obtain a uniform depth-dose distribution as well as to deliver high linear energy transfer along the depth of a target for 36MeV proton beam. Aluminum was chosen as the material of the filter to reduce the radioactivity induced by proton irradiation. The designed ridge filter has a continuous cross-sectional line shape of ridges so that the smoothly varying depth-dose distribution can be maintained before the distal fall-off for lower proton energy than 36MeV. The height of the ridge is 6 mm, its period is also 6 mm and the minimum thickness is 0.3 mm. A Monte Carlo simulation code, MCNPX 2.5.0., was used to calculate the dose distributions. The width of the calculated uniform dose region was 11 mm for 36MeV proton beam in a water-equivalent sample.
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