A semiconductor detector is described which responds to the atomic hydrogen in an atomic beam but is insensitive to molecular hydrogen. The hydrogen flux is measured through the change in conductivity of the semiconductor material which occurs when the hydrogen is chemisorbed. The atomic flux from a rf hydrogen discharge is used to determine the sensitivity of the detector. The minimum detectable signal is ∼109hydrogen atoms mm−2 s−1. The detector has a response time of less than 2 ms.