Brain tumors can be located by using an isotope of

Analyze the question: The question asks which radioactive isotope is used in medicine to locate brain tumors (a diagnostic imaging use of radioisotopes).

Concept: Radioisotopes are used as tracers in medicine. Each one is chosen for its half-life and the organ it accumulates in or can be imaged in:

• Carbon-14: radiocarbon dating of once-living materials (wood, fossils, bones).
• Iodine-131: thyroid gland. It is used to diagnose and treat thyroid disorders because the thyroid absorbs iodine.
• Technetium-99m: the most widely used diagnostic medical isotope. It emits gamma rays, has a short half-life of about 6 hours, and is used in scans of the brain, bones, heart, and other organs. Brain scans with it show tumors and blood-flow problems.
• Uranium-235: nuclear fuel in reactors and atomic bombs.
• Uranium-238: dating of very old rocks and the age of the Earth, and it is also used in producing plutonium.

Solution:

1. Eliminate A (dating) and D and E (nuclear fuel and rock dating), since none are used for imaging tumors.
2. Iodine-131 is mainly linked to the thyroid, though it has had limited use in other contexts. It is not the standard answer for brain tumors.
3. Technetium-99 (technetium-99m) is the standard isotope for brain imaging and locating tumors.

Answer: C. technetium 99

Tip: Remember the pairing "Iodine → Thyroid" and "Technetium → Tumor scans" (brain, bone). Also: Carbon-14 = dating, Uranium = nuclear energy or rock dating.