Final answer:
Three natural sources of background radiation are building materials, internal sources within the human body, and thoron and radon gases. Nuclear medicine and occupational sources are not considered natural sources but rather human-made sources of radiation.
Step-by-step explanation:
Natural background radiation comes from various sources, some of which are present in our day-to-day environment. Among the listed options, three correct sources of natural background radiation include:
Building materials: Natural radioactive elements, like uranium and thorium, and their decay products, are found in the earth and are incorporated into building materials. This contributes to background radiation exposure inside homes, particularly those made of materials like brick or concrete.
Internal sources: Our bodies contain naturally occurring radioactive isotopes, such as potassium-40, which contribute to our internal exposure to background radiation.
Thoron and radon gases: These radioactive gases are by-products of the decay of uranium and thorium found in soil and rocks. They can accumulate in buildings and contribute significantly to background radiation exposure.
While nuclear medicine does involve exposure to radiation, it is not considered a source of natural background radiation but rather an artificial one. Similarly, occupational sources are human-made and related to specific jobs in the radiation or nuclear sectors. Therefore, options 2 and 5 are not correct in the context of natural background radiation.