The present invention is a method and apparatus for operating boron-coated straw detectors in sealed mode, without the need for a continuous flow of gas. Sealed-mode operation is necessary when using the boron-coated straw detectors in the field, where access to a continuous flow of the required gas mixture is not practical. Also, sealed-mode operation is necessary when the straw detectors are used as portable instruments, that must be moved from one location to the next swiftly, or that must be operated while in motion.