Which portable survey instrument utilizes a gas detector for radiation detection?

The choice of Geiger-Muller instruments as the correct answer is based on their design and functionality. Geiger-Muller (GM) instruments operate by employing a gas-filled tube, known as a Geiger-Muller tube, which serves as the detector for ionizing radiation. When radiation passes through the tube, it ionizes the gas inside, leading to the creation of charged particles. This ionization event is then amplified and converted into an audible click or a measurable electrical signal, providing a readout that indicates the presence of radiation.

In contrast, ionization chamber survey instruments primarily function through a different mechanism, measuring the amount of charge collected as created by ionization events over a specific volume. Scintillation survey instruments detect radiation by using materials that emit flashes of light when they are struck by radiation, while radiation shielding instruments are not primarily designed for detection but rather to attenuate or block radiation exposure. Therefore, the distinct mechanism of action that Geiger-Muller instruments use, which revolves around a gas-filled tube for detecting radiation, confirms why this option is the correct one.