Final answer:
For leak checking, effective tracers like radioactive isotopes (e.g., tritium) are often used because they can be easily detected, helping locate leaks in various applications such as underground pipes or cryogenic systems.
Step-by-step explanation:
When it is necessary to use a trace gas for leak checking, it is essential to choose a gas that will be detected easily if it escapes. In many cases, a particular radioactive isotope known as a tracer is used for this purpose. Radioactive tracers such as tritium (H-3) are effective because they can be easily detected using instruments like Geiger counters due to their radioactivity. In industrial applications such as checking for leaks in underground pipes, tritium-containing water is sent through the pipes. Any escaped tritium, which becomes present in the surrounding ground, can be readily detected indicating the presence of a leak. Similarly, in scientific research, gas like helium can be used in conjunction with a mass spectrometer to detect leaks in cryogenic systems.