What type of radioactive suitable to detect a leak in an underground pipe?

Leaks in underground industrial pipes can be found by adding radioactive material to the liquid in the pipe.

A Geiger-Muller tube ( GM tube ) is moved along the ground where the pipe is buried.

The count rate will be found to increase where the pipe is leaking as a larger amount of liquid containing the radioactive source will collect there.

The radioactive source would be gamma* emitter because it can penetrate through the soil (and metal pipe) and be detected at the surface.

The gamma emitter would have a short half-life to minimize the possible danger to environment.

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Remarks:

*Alpha particles are easily absorbed by a few centimetres of air whereas beta particles may be absorbed by a few millimetres of aluminium.