A nanodosimeter device (15) for detecting positive ions induced in a sensitive gas volume by a radiation field of primary particle, comprising an ionization chamber (10) for holding the sensitive gas volume to be irradiated by the radiation field of primary particles; an ion counter system connected to the ionization chamber (10) for detecting the positive ions which pass through the aperture opening and arrive at the ion counter (12) at an arrival time; a particle tracking system for position-sensitive detection of the primary particles passing through the sensitive gas volume; and a data acquisition system capable of coordinating the readout of all data signals and of performing data analysis correlating the arrival time of the positive ions detected by the ion counter system relative to the position sensitive data of primary particles detected by the particle tracking system. The invention further includes the use of the nanodosimeter for method of calibrating radiation exposure with damage to a nucleic acid within a sample. A volume of tissue-equivalent gas is radiated with a radiation field to induce positive ions. The resulting positive ions are measured and compared with a determination of presence or extent of damage resulting from irradiating a nucleic acid sample with an equivalent dose of radiation.