The use of semiconductor materials for radiation detection has been under investigation for many years. Cadmium sulfide, germanium, and silicon all have been tried. Cadmium sulfide dosimeters have been successfully employed at dose rates similar to those associated with therapy (1, 2). These have made use of the photoconductance properties of the crystal. Pulse operation of germanium and silicon surface barrier counters was originally accomplished at low temperatures (3, 4), and more recently at room temperature by the Oak Ridge group (5). P-N junction detectors made of silicon with a shallow phosphorus diffused junction have been described (6,7). These units have proved suitable for the detection of heavy charged particles but not for use with beta or gamma radiations. Because of the shallow depletion region and the low atomic number of silicon, energy deposited by these radiations in the charge-sensitive region was insufficient to produce pulses above the noise level. In some instances where high resistivity material (> 10,000 ohm-cm.) was available, beta rays could be measured if a sufficiently low noise amplifier was used. Recently a unit made from gallium arsenide, having a zinc diffused junction, has been successfully employed as an x-ray dosimeter (8). In order to use the P-N junction semiconductor as a detector for high-energy charged particles and for x- and gamma rays, one has to increase the energy deposited within the charge-sensitive region. This may be accomplished in two ways: first, by increasing the atomic number of the semiconductor material; second, by increasing the depth of the charge-sensitive region. We have chosen to attack this problem by the latter approach. Silicon has been successfully diffused with a lithium impurity, and by ion-drift technics a sensitive volume has been established with a width of about 1 mm. In comparison with the usual 10- to 100-micron width established when silicon is diffused with phosphorus (9). Lithium-diffused silicon P-N junction devices have shown very good sensitivity to beta ray, gamma ray, and mixed emitters. The following table gives approximate sensitivity values derived experimentally with the use of dry sources having approximate area of 1 sq. cm. and a detector of 3 mm. diameter. Values given are those recorded when the electronics was adjusted to give a zero background level. With a Co 60 needle (1 cm. × 1 mm. diam.) the sensitivity was 250 cpm/mr/hr. when a background of about 60 cpm was allowed. This background was mainly due to noise pulses generated in the cables and the electronics. The use of a low noise amplifier should improve the signal-to-noise ratio, thereby allowing use of the detector at higher sensitivity settings. Sensitivity can also be gained through the use of larger area detectors.