A mineral collector obtained a rare piece of rock that originated from the Jornada del Muerto desert in New Mexico; the rock exhibited a glassy texture. However, a friend of the collector (who is also a radiochemist) was suspicious of the rock on account of the history in that particular region. The radiochemist analyzed the rock in a gamma-laboratory using a high-purity germanium detector. Identify the isotopes affiliated with peaks 18 and explain why the rock is radioactive.