Manual: http://www.orau.org/ptp/Library/cdv/victor720-3a.pdf

The CD V-720 is a high-range radiological survey meter that can discriminate between Beta and Gamma Radiation. The range of the CD V-720 is 0-500 Roentgens per hour. The sliding cover on the bottom of the meter case blocks Beta particles when closed allowing only Gamma radiation to pass through the detector. When the cover is opened this allows Beta particles to pass through the very thin metal membrane on the bottom of the CD V-720's Ion Chamber resulting in detection of beta radiation.