Abstract: A detector member having the property of forming damage tracks along the paths traversed by impinging alpha particles is provided with at least two opposing detection surfaces exposed on respective opposite sides of the detector member. The opposing detection surfaces enable the detector member to form damage tracks by alpha particles impinging either of the detection surfaces which are simultaneously exposed to irradiation. Conventional cellulosic film such as cellulose nitrate can be employed with both sides of the film serving as detection surfaces. The dual-surfaced film can be framed and packaged in a compact configuration convenient to use in homes or offices. The damage tracks from both of the irradiated surfaces can be simultaneously counted using preferred spark counting techniques.