The Model 940-702 Shielded Duct Monitor, when used with the Model 943-25T Beta or Model 943-36 Gamma Scintillation Detector and a Model 942A Universal Digital Ratemeter (UDR) or Model 960 Digital Radiation Processor Controller, provides a radiation monitoring system for applications where the direct measurement of the radioactivity in a duct is needed. The Model 940-702 is a duct mounted, single channel detector shield, suitable for the monitoring of airborne effluents for noble gasses. The sampler is a shielded mounting fixture that provides a fixed sampling geometry for the detector. Since the active volume is fixed, the noble gas concentration measured by the detector is independent of the duct flow rate. The shield is manufactured using a 0.25 inch thick steel weldment, filled with two (2) inches of virgin lead. The lead provides shielding of the detector from background radiation. The weldment is designed to retain the lead shielding and to provide a stable mounting base for the detector. The detector is accessed for calibration or replacement via a hinged door located at the rear of the shield. The door is lead filled to minimize the induced ambient background radiation. The shield is designed for floor or platform mounting, and is bolted directly to the duct. The sensitive end of the detector protrudes approximately one (1) inch from the gasketed shield, and is inserted directly into the duct. An o-ring sealed mounting flange prevents the effluent, or ambient air, from exiting or entering the duct. A junction box is mounted on the top of the shield. The junction box provides a waterproof enclosure for connecting the detector to the control room readout cable. The junction box also contains a solenoid operated check source mechanism. The check source provides a method to verify detector operation from the control room readout device.