A non-visible particle detection device includes an optical module capable of converting an ionizing radiation into visible light. The optical module includes has an attachment unit that is configured to removably attach the optical module to the image capturing module of a mobile device. The image capturing module generates a photon digital image based on the photons converted from the ionizing radiation. The mobile device can be implemented with a radiation dose determining module to execute a radiation dose equivalent calculation method. Based on the pixel brightness analysis of the photon digital image, the radiation equivalent dose can be determined. This method sums up the total brightness of all pixels in the images, determines whether the total brightness is smaller than the minimum effective brightness, and determines the radiation equivalent dose when the total brightness is equal to or larger than the minimum effective brightness.