An environmental engineer injects a radioactive tracer into a buried pipeline to detect the precise location of a suspected underground leak. An external radiation detector is swept along the ground surface above the pipeline to measure the count rate of the radiation emitted.
Explain why a source that emits gamma radiation is used for this investigation rather than one that emits alpha radiation.
The radioactive isotope used in the tracer has a half-life of 4 hours4\text{ hours}4 hours. At the start of the investigation, a reference sample of the tracer has a detected count rate of 800 counts per second800\text{ counts per second}800 counts per second. Calculate the count rate of this reference sample after 20 hours20\text{ hours}20 hours, assuming background radiation is negligible. Show your working.
Explain why it is important that the radioactive isotope used for this environmental test has a short half-life of only a few hours, rather than a long half-life of several years.