Abstract: Fail-safe methods for deactivating the pulsed neutron generator (PNG) of a logging tool are described herein, as are logging tools configured to execute the fail-safe methods. The fail-safe methods deactivate the PNG if the logging tool is disposed in air outside of a borehole. Measurements taken using one or more gamma ray detectors of the logging tool are used to calculate a value for a parameter that is indicative of the tool being disposed in an air environment. Examples of such parameters include ratios of capture gamma rays and burst gamma rays. The disclosed methods operate without reference to sensors and/or control from outside the tool. The methods do not inadvertently deactivate the tool when it encounters an air-filled borehole.