Commercial-off-the-shelf (COTS) Silicon diodes are widely used for treatment plan verification and characterization of narrow beams in conventional radiotherapy using photons . Their main advantage is high sensitivity with a factor of approximately 2 · 104 compared to air filled ionisation chambers of the same volume. In this paper, a detector based on a low-cost BPW-34 photodiode is developed and charac- terized for the use in proton and carbon ion beams. The BPW-34 photodiode has a well defined sensitive volume of 2.7 × 2.7 mm2 × 10 μm which is suitable in situations where a high spatial resolution is needed. The detector system consists of a silicon photodiode with a retail price less than 1 Euro encapsulated in a polyoxymethylene housing, connected to a dosimetry elec- trometer. In MV photons beams the output factors, the temperature behavior and the noise properties are similar to commercially available diode detectors used for radiotherapy. In carbon ion and proton beams the detector reproduces well dose distributions such as beam profiles and depth dose curves. Quenching in the order of a few percent was observed with increased LET. In addition, a decreasing sensitivity is seen with total absorbed dose, which was not observed in MV photon beams. In the case of carbon ions the sensitivity stabilizes after approximately 600 Gy, whereas in proton beams a stabilization after 200 Gy is reported in the literature . These results are comparable to earlier experiments in proton beams [2, 3] This low-cost inhouse made detectors based on a commercially available silicon diode was suc- cessfully tested at the Heidelberg Ion Beam Therapy Center for the characterization carbon ion beams.