Abstract: A portable radiation dosimeter unit for continuously measuring exposure to radioactive decay products in ambient air. The unit is constructed as a unitary assembly suitable for mounting on a portable battery casing, the assembly housing a constant flow air pump for maintaining a constant flow of air through a filter, a detection unit and electronic circuitry connected to the detection unit for processing, counting and storing pulses generated by the detection of radioactive particles from decay products entrapped by the filter. The filter is readily replaceable, and in use is clamped in position by a clamp assembly pivotally mounted on a front wall of the unit, the clamp assembly providing an internal air passage forming part of the air flow path and positioned to receive the air which is drawn through the filter.